MethaneemissionsinlivestockandricesystemsMethaneemissionsinlivestockandricesystemsSources,quantication,mitigationandmetricsFOODANDAGRICULTUREORGANIZATIONOFTHEUNITEDNATIONSRome,2023Requiredcitation:FAO.2023.Methaneemissionsinlivestockandricesystems–Sources,quantification,mitigationandmetrics.Rome.https://doi.org/10.4060/cc7607enThedesignationsemployedandthepresentationofmaterialinthisinformationproductdonotimplytheexpressionofanyopinionwhatsoeveronthepartoftheFoodandAgricultureOrganizationoftheUnitedNations(FAO)concerningthelegalordevelopmentstatusofanycountry,territory,cityorareaorofitsauthorities,orconcerningthedelimitationofitsfrontiersorboundaries.Thementionofspecificcompaniesorproductsofmanufacturers,whetherornotthesehavebeenpatented,doesnotimplythatthesehavebeenendorsedorrecommendedbyFAOinpreferencetoothersofasimilarnaturethatarenotmentioned.ISBN978-92-5-138148-9©FAO,2023Somerightsreserved.ThisworkismadeavailableundertheCreativeCommonsAttribution-NonCommercial-ShareAlike3.0IGOlicence(CCBY-NC-SA3.0IGO;https://creativecommons.org/licenses/by-nc-sa/3.0/igo/legalcode).Underthetermsofthislicence,thisworkmaybecopied,redistributedandadaptedfornon-commercialpurposes,providedthattheworkisappropriatelycited.Inanyuseofthiswork,thereshouldbenosuggestionthatFAOendorsesanyspecificorganization,productsorservices.TheuseoftheFAOlogoisnotpermitted.Iftheworkisadapted,thenitmustbelicensedunderthesameorequivalentCreativeCommonslicence.Ifatranslationofthisworkiscreated,itmustincludethefollowingdisclaimeralongwiththerequiredcitation:“ThistranslationwasnotcreatedbytheFoodandAgricultureOrganizationoftheUnitedNations(FAO).FAOisnotresponsibleforthecontentoraccuracyofthistranslation.Theoriginal[Language]editionshallbetheauthoritativeedition.”DisputesarisingunderthelicencethatcannotbesettledamicablywillberesolvedbymediationandarbitrationasdescribedinArticle8ofthelicenceexceptasotherwiseprovidedherein.TheapplicablemediationruleswillbethemediationrulesoftheWorldIntellectualPropertyOrganizationhttp://www.wipo.int/amc/en/mediation/rulesandanyarbitrationwillbeconductedinaccordancewiththeArbitrationRulesoftheUnitedNationsCommissiononInternationalTradeLaw(UNCITRAL).Third-partymaterials.Userswishingtoreusematerialfromthisworkthatisattributedtoathirdparty,suchastables,figuresorimages,areresponsiblefordeterminingwhetherpermissionisneededforthatreuseandforobtainingpermissionfromthecopyrightholder.Theriskofclaimsresultingfrominfringementofanythird-party-ownedcomponentintheworkrestssolelywiththeuser.Sales,rightsandlicensing.FAOinformationproductsareavailableontheFAOwebsite(www.fao.org/publications)andcanbepurchasedthroughpublications-sales@fao.org.Requestsforcommercialuseshouldbesubmittedvia:www.fao.org/contact-us/licence-request.Queriesregardingrightsandlicensingshouldbesubmittedto:copyright@fao.org.Coverphotograph:©MarkStebnickionPexelsContentsForewordxviAcknowledgementsxviiiDevelopmentprocessofthemethanereportxixMulti-stepreviewprocessxxiiiTheLivestockEnvironmentalAssessmentandPerformance(LEAP)PartnershipxxivAbbreviationsxxviExecutivesummaryxxxINTRODUCTION1PART1SOURCESANDSINKSOFMETHANEEMISSIONSINAGRICULTURE31.SOURCESOFMETHANE51.1Ruminantlivestockandentericmethanogenesis51.2Biochemistryofmethaneproductioninmicrobialanaerobicecosystems61.2.1Rumenmethanogenesis61.2.2Manure91.2.3Soil111.3Methaneemissionsduringthestorageofmanure141.4Methaneemissionsfollowingtheapplicationofmanure141.5Trade-offbetweenGHGandothergaseousemissions151.6Spatio-temporalvariationinmethaneemissions151.7Contributionofhumanfoodandanimalfeedwastetomethaneemissions161.8Anaerobicdigestion171.8.1Leakageofmethanefromanaerobicdigestionfacilities182.METHANESINKS192.1Soilmethanesink192.1.1Factorsaffectingthesoilmethanesinkcapacity192.1.2Landmanagementeffectsonthesoilmethanesink20PART2QUANTIFICATIONOFMETHANEEMISSIONS233.MEASUREMENT253.1Animal-basedtechniques253.1.1Gasexchangetechnique273.1.2Tracertechnique293.1.3Open-pathlasertechnique303.1.4Invitrotechniques31iii3.2Facility-basedtechniques313.2.1Manurestorages313.2.2Soilfluxes343.3Large-scaletechniques373.3.1Aircrafts373.3.2Satelliteanddroneimagery373.4Uncertainties384.ESTIMATION404.1Bottom-upapproaches404.1.1Modellingtoestimateentericmethane404.1.2Modellingtoestimatemanuremethane444.1.3Soil/cropmodelling444.2Top-downapproaches474.2.1Comparisonbetweenbottom-upandtop-downapproaches48PART3MITIGATIONOFMETHANEEMISSIONS495.MITIGATIONSTRATEGIESFORMETHANEEMISSIONS515.1Animalbreedingandmanagement:Increasedanimalproduction525.1.1Description525.1.2Modeofaction525.1.3Efficacy525.1.4Potentialtocombinewithothermitigationstrategies535.1.5Effectsonotheremissions535.1.6Productivityandthequalityofmeat,milk,manure,crop,andair535.1.7Safetyandhealthaspects535.1.8Adoptionpotential535.1.9Researchrequired535.2Animalbreedingandmanagement:Selectionforlowmethane-producinganimals545.2.1Description545.2.2Modeofaction545.2.3Efficacy545.2.4Potentialtocombinewithothermitigationstrategies545.2.5Effectsonotheremissions545.2.6Productivityandthequalityofmeat,milk,manure,crop,andair545.2.7Safetyandhealthaspects555.2.8Adoptionpotential555.2.9Researchrequired555.3Animalbreedingandmanagement:Improvedfeedefficiency555.3.1Description555.3.2Modeofaction565.3.3Efficacy565.3.4Potentialtocombinewithothermitigationstrategies565.3.5Effectsonotheremissions565.3.6Productivityandthequalityofmeat,milk,manure,crop,andair565.3.7Safetyandhealthaspects56iv5.3.8Adoptionpotential565.3.9Researchrequired565.4Animalbreedingandmanagement:Improvedanimalhealth575.4.1Description575.4.2Modeofaction575.4.3Efficacy575.4.4Potentialtocombinewithothermitigationstrategies575.4.5Effectsonotheremissions575.4.6Productivityandthequalityofmeat,milk,manure,crop,andair585.4.7Safetyandhealthaspects585.4.8Adoptionpotential585.4.9Researchrequired585.5Animalbreedingandmanagement:Improvedanimalreproduction585.5.1Description585.5.2Modeofaction595.5.3Efficacy595.5.4Potentialtocombinewithothermitigationstrategies595.5.5Effectsonotheremissions595.5.6Productivityandthequalityofmeat,milk,manure,crop,andair595.5.7Safetyandhealthaspects595.5.8Adoptionpotential595.5.9Researchrequired605.6Feedmanagement,dietformulationandprecisionfeeding:Increasedfeedinglevel605.6.1Description605.6.2Modeofaction605.6.3Efficacy605.6.4Potentialtocombinewithothermitigationstrategies615.6.5Effectsonotheremissions615.6.6Productivityandthequalityofmeat,milk,manure,crop,andair615.6.7Safetyandhealthaspects615.6.8Adoptionpotential615.6.9Researchrequired625.7Feedmanagement,dietformulationandprecisionfeeding:Decreasedforagetoconcentrateratio625.7.1Description625.7.2Modeofaction625.7.3Efficacy625.7.4Potentialtocombinewithothermitigationstrategies635.7.5Effectsonotheremissions635.7.6Productivityandthequalityofmeat,milk,manure,crop,andair635.7.7Safetyandhealthaspects635.7.8Adoptionpotential645.7.9Researchrequired645.8Feedmanagement,dietformulationandprecisionfeeding:Starchconcentratesourcesandprocessing645.8.1Description645.8.2Modeofaction645.8.3Efficacy65v5.8.4Potentialtocombinewithothermitigationstrategies655.8.5Effectsonotheremissions655.8.6Productivityandthequalityofmeat,milk,manure,crop,andair655.8.7Safetyandhealthaspects665.8.8Adoptionpotential665.8.9Researchrequired665.9Feedmanagement,dietformulationandprecisionfeeding:Supplementationoflipids665.9.1Description665.9.2Modeofaction665.9.3Efficacy675.9.4Potentialtocombinewithothermitigationstrategies685.9.5Effectsonotheremissions685.9.6Productivityandthequalityofmeat,milk,manure,crop,andair685.9.7Safetyandhealthaspects685.9.8Adoptionpotential685.9.9Researchrequired695.10Forages:Foragestorageandprocessing695.10.1Description695.10.2Modeofaction695.10.3Efficacy695.10.4Potentialtocombinewithothermitigationstrategies705.10.5Effectsonotheremissions705.10.6Productivityandthequalityofmeat,milk,manure,crop,andair705.10.7Safetyandhealthaspects705.10.8Adoptionpotential705.10.9Researchrequired705.11Forages:Increasedforagedigestibility715.11.1Description715.11.2Modeofaction715.11.3Efficacy715.11.4Potentialtocombinewithothermitigationstrategies715.11.5Effectsonotheremissions725.11.6Productivityandthequalityofmeat,milk,manure,crop,andair725.11.7Safetyandhealthaspects725.11.8Adoptionpotential725.11.9Researchrequired725.12Forages:Perenniallegumes725.12.1Description725.12.2Modeofaction735.12.3Efficacy735.12.4Potentialtocombinewithothermitigationstrategies735.12.5Effectsonotheremissions735.12.6Productivityandthequalityofmeat,milk,manure,crop,andair745.12.7Safetyandhealthaspects745.12.8Adoptionpotential745.12.9Researchrequired74vi5.13Forages:High-starchforages755.13.1Description755.13.2Modeofaction755.13.3Efficacy755.13.4Potentialtocombinewithothermitigationstrategies755.13.5Effectsonotheremissions755.13.6Productivityandthequalityofmeat,milk,manure,crop,andair765.13.7Safetyandhealthaspects765.13.8Adoptionpotential765.13.9Researchrequired765.14Forages:High-sugargrasses775.14.1Description775.14.2Modeofaction775.14.3Efficacy775.14.4Potentialtocombinewithothermitigationstrategies775.14.5Effectsonotheremissions775.14.6Productivityandthequalityofmeat,milk,manure,crop,andair785.14.7Safetyandhealthaspects785.14.8Adoptionpotential785.14.9Researchrequired785.15Forages:Pasturesandgrazingmanagement795.15.1Description795.15.2Modeofaction795.15.3Efficacy795.15.4Potentialtocombinewithothermitigationstrategies805.15.5Effectsonotheremissions805.15.6Productivityandthequalityofmeat,milk,manure,crop,andair805.15.7Safetyandhealthaspects805.15.8Adoptionpotential805.15.9Researchrequired815.16Rumenmanipulation:Ionophores815.16.1Description815.16.2Modeofaction815.16.3Efficacy815.16.4Potentialtocombinewithothermitigationstrategies825.16.5Effectsonotheremissions825.16.6Productivityandthequalityofmeat,milk,manure,crop,andair825.16.7Safetyandhealthaspects825.16.8Adoptionpotential835.16.9Researchrequired835.17Rumenmanipulation:Chemicalinhibitorsofmethaneproduction835.17.1Description835.17.2Modeofaction835.17.3Efficacy835.17.4Potentialtocombinewithothermitigationstrategies845.17.5Effectsonotheremissions845.17.6Productivityandthequalityofmeat,milk,manure,crop,andair845.17.7Safetyandhealthaspects84vii5.17.8Adoptionpotential845.17.9Researchrequired855.18Rumenmanipulation:3-nitrooxypropanol(3-NOP)855.18.1Description855.18.2Modeofaction855.18.3Efficacy855.18.4Potentialtocombinewithothermitigationstrategies865.18.5Effectsonotheremissions865.18.6Productivityandthequalityofmeat,milk,manure,crop,andair875.18.7Safetyandhealthaspects875.18.8Adoptionpotential885.18.9Researchrequired885.19Rumenmanipulation:Immunizationagainstmethanogens895.19.1Description895.19.2Modeofaction895.19.3Efficacy895.19.4Potentialtocombinewithothermitigationstrategies895.19.5Effectsonotheremissions895.19.6Productivityandthequalityofmeat,milk,manure,crop,andair895.19.7Safetyandhealthaspects895.19.8Adoptionpotential905.19.9Researchrequired905.20Rumenmanipulation:Bromoform-containingseaweeds(Asparagopsissp.)905.20.1Description905.20.2Modeofaction915.20.3Efficacy915.20.4Potentialtocombinewithothermitigationstrategies915.20.5Effectsonotheremissions915.20.6Productivityandthequalityofmeat,milk,manure,crop,andair915.20.7Safetyandhealthaspects925.20.8Adoptionpotential925.20.9Researchrequired935.21Rumenmanipulation:Otherseaweeds935.21.1Description935.21.2Modeofaction935.21.3Efficacy935.21.4Potentialtocombinewithothermitigationstrategies935.21.5Effectsonotheremissions945.21.6Productivityandthequalityofmeat,milk,manure,crop,andair945.21.7Safetyandhealthaspects945.21.8Adoptionpotential945.21.9Researchrequired955.22Rumenmanipulation:Defaunation955.22.1Description955.22.2Modeofaction955.22.3Efficacy965.22.4Potentialtocombinewithothermitigationstrategies965.22.5Effectsonotheremissions96viii5.22.6Productivityandthequalityofmeat,milk,manure,crop,andair965.22.7Safetyandhealthaspects975.22.8Adoptionpotential975.22.9Researchrequired975.23Rumenmanipulation:Alternativeelectronacceptors975.23.1Description975.23.2Modeofaction975.23.3Efficacy985.23.4Potentialtocombinewithothermitigationstrategies995.23.5Effectsonotheremissions995.23.6Productivityandthequalityofmeat,milk,manure,crop,andair1005.23.7Safetyandhealthaspects1005.23.8Adoptionpotential1005.23.9Researchrequired1015.24Rumenmanipulation:Essentialoils1015.24.1Description1015.24.2Modeofaction1015.24.3Efficacy1025.24.4Potentialtocombinewithothermitigationstrategies1035.24.5Effectsonotheremissions1035.24.6Productivityandthequalityofmeat,milk,manure,crop,andair1035.24.7Safetyandhealthaspects1035.24.8Adoptionpotential1045.24.9Researchrequired1045.25Rumenmanipulation:Tanninextracts1045.25.1Description1045.25.2Modeofaction1045.25.3Efficacy1045.25.4Potentialtocombinewithothermitigationstrategies1055.25.5Effectsonotheremissions1055.25.6Productivityandthequalityofmeat,milk,manure,crop,andair1065.25.7Safetyandhealthaspects1065.25.8Adoptionpotential1065.25.9Researchrequired1075.26Rumenmanipulation:Saponins1075.26.1Description1075.26.2Modeofaction1075.26.3Efficacy1075.26.4Potentialtocombinewithothermitigationstrategies1085.26.5Effectsonotheremissions1085.26.6Productivityandthequalityofmeat,milk,manure,crop,andair1085.26.7Safetyandhealthaspects1085.26.8Adoptionpotential1095.26.9Researchrequired1095.27Rumenmanipulation:Biochar1095.27.1Description1095.27.2Modeofaction1095.27.3Efficacy109ix5.27.4Potentialtocombinewithothermitigationstrategies1105.27.5Effectsonotheremissions1105.27.6Productivityandthequalityofmeat,milk,manure,crop,andair1105.27.7Safetyandhealthaspects1105.27.8Adoptionpotential1105.27.9Researchrequired1105.28Rumenmanipulation:Direct-fedmicrobials1115.28.1Description1115.28.2Modeofaction1115.28.3Efficacy1115.28.4Potentialtocombinewithothermitigationstrategies1125.28.5Effectsonotheremissions1125.28.6Productivityandthequalityofmeat,milk,manure,crop,andair1125.28.7Safetyandhealthaspects1125.28.8Adoptionpotential1125.28.9Researchrequired1135.29Rumenmanipulation:Earlylifeinterventions1135.29.1Description1135.29.2Modeofaction1135.29.3Efficacy1135.29.4Potentialtocombinewithothermitigationstrategies1145.29.5Effectsonotheremissions1145.29.6Productivityandthequalityofmeat,milk,manure,crop,andair1145.29.7Safetyandhealthaspects1155.29.8Adoptionpotential1155.29.9Researchrequired1155.30Rumenmanipulation:Phageandlyticenzymesactiveagainstmethanogens1155.30.1Description1155.30.2Modeofaction1165.30.3Efficacy1165.30.4Potentialtocombinewithothermitigationstrategies1165.30.5Effectsonotheremissions1165.30.6Productivityandthequalityofmeat,milk,manure,crop,andair1165.30.7Safetyandhealthaspects1165.30.8Adoptionpotential1165.30.9Researchrequired1175.31Summarytables1176.MITIGATIONSTRATEGIESFORMETHANEEMISSIONSFROMANIMALHOUSING,MANUREMANAGEMENTANDLANDAPPLICATION1256.1Biogascollectionandutilization1276.1.1Description1276.1.2Modeofaction1276.1.3Efficacy1276.1.4Potentialtocombinewithothermitigationstrategies1276.1.5Effectsonotheremissions1276.1.6Productivityandthequalityofmeat,milk,manure,crop,andair1276.1.7Safetyandhealthaspects127x6.1.8Adoptionpotential1286.1.9Researchrequired1286.2Decreasedmanurestoragetemperature1286.2.1Description1286.2.2Modeofaction1286.2.3Efficacy1286.2.4Potentialtocombinewithothermitigationstrategies1286.2.5Effectsonotheremissions1286.2.6Productivityandthequalityofmeat,milk,manure,crop,andair1286.2.7Safetyandhealthaspects1286.2.8Adoptionpotential1286.2.9Researchrequired1296.3Manureacidificationthroughdietarymeasures1296.3.1Description1296.3.2Modeofaction1296.3.3Efficacy1296.3.4Potentialtocombinewithothermitigationstrategies1296.3.5Effectsonotheremissions1306.3.6Productivityandthequalityofmeat,milk,manure,crop,andair1306.3.7Safetyandhealthaspects1306.3.8Adoptionpotential1306.3.9Researchrequired1306.4Manureacidificationthroughdirectamendment1306.4.1Description1306.4.2Modeofaction1306.4.3Efficacy1306.4.4Potentialtocombinewithothermitigationstrategies1306.4.5Effectsonotheremissions1306.4.6Productivityandthequalityofmeat,milk,manure,crop,andair1316.4.7Safetyandhealthaspects1316.4.8Adoptionpotential1316.4.9Researchrequired1316.5Methaneinhibitors1316.5.1Description1316.5.2Modeofaction1316.5.3Efficacy1316.5.4Potentialtocombinewithothermitigationstrategies1316.5.5Effectsonotheremissions1326.5.6Productivityandthequalityofmeat,milk,manure,crop,andair1326.5.7Safetyandhealthaspects1326.5.8Adoptionpotential1326.5.9Researchrequired1326.6Decreasedmanurestorageinterval1326.6.1Description1326.6.2Modeofaction1326.6.3Efficacy1326.6.4Potentialtocombinewithothermitigationstrategies1326.6.5Effectsonotheremissions132xi6.6.6Productivityandthequalityofmeat,milk,manure,crop,andair1326.6.7Safetyandhealthaspects1336.6.8Adoptionpotential1336.6.9Researchrequired1336.7Solid–liquidseparation1336.7.1Description1336.7.2Modeofaction1336.7.3Efficacy1336.7.4Potentialtocombinewithothermitigationstrategies1336.7.5Effectsonotheremissions1336.7.6Productivityandthequalityofmeat,milk,manure,crop,andair1336.7.7Safetyandhealthaspects1336.7.8Adoptionpotential1346.7.9Researchrequired1346.8Manurecomposting/aeration1346.8.1Description1346.8.2Modeofaction1346.8.3Efficacy1346.8.4Potentialtocombinewithothermitigationstrategies1346.8.5Effectsonotheremissions1346.8.6Productivityandthequalityofmeat,milk,manure,crop,andair1346.8.7Safetyandhealthaspects1356.8.8Adoptionpotential1356.8.9Researchrequired1356.9Biofiltersandscrubbers1356.9.1Description1356.9.2Modeofaction1356.9.3Efficacy1356.9.4Potentialtocombinewithothermitigationstrategies1356.9.5Effectsonotheremissions1356.9.6Productivityandthequalityofmeat,milk,manure,crop,andair1356.9.7Safetyandhealthaspects1356.9.8Adoptionpotential1356.9.9Researchrequired1366.10Manureincorporationandinjection1366.10.1Description1366.10.2Modeofaction1366.10.3Efficacy1366.10.4Potentialtocombinewithothermitigationstrategies1366.10.5Effectsonotheremissions1366.10.6Productivityandthequalityofmeat,milk,manure,crop,andair1366.10.7Safetyandhealthaspects1376.10.8Adoptionpotential1376.10.9Researchrequired1376.11Manureapplicationtiming1376.11.1Description1376.11.2Modeofaction1376.11.3Efficacy137xii6.11.4Potentialtocombinewithothermitigationstrategies1376.11.5Effectsonotheremissions1376.11.6Productivityandthequalityofmeat,milk,manure,crop,andair1386.11.7Safetyandhealthaspects1386.11.8Adoptionpotential1386.11.9Researchrequired1386.12Nutritionalstrategies1386.12.1Description1386.12.2Modeofaction1386.12.3Efficacy1386.12.4Potentialtocombinewithothermitigationstrategies1386.12.5Effectsonotheremissions1386.12.6Productivityandthequalityofmeat,milk,manure,crop,andair1386.12.7Safetyandhealthaspects1386.12.8Adoptionpotential1396.12.9Researchrequired1396.13Grazingpractices–Productionsystem1397.MITIGATIONOFMETHANEEMISSIONFROMRICEPADDIES1407.1Watermanagement1407.2Organicamendments1417.3Fertilizerandotheramendments1427.4Plantingmethodsandcropmanagementpackages1427.5Selecting/breedingricevarieties1437.6Reducingmethanefromstrawburning1437.7Choiceofoptions1447.8Newlyemergingtechnologies1458.CROSS-CUTTINGMETHANEMITIGATION1468.1Generalguidancefortakinganintegratedapproachtomethanemitigationstrategies1468.2LCAscenarioanalysisforintensivesystems1498.3LCAscenarioanalysisforlessintensivesystems150PART4METRICSFORQUANTIFYINGTHEIMPACTOFMETHANEEMISSIONS1539.INTRODUCTION1559.1Contextanddefinitions1579.1.1KeyprinciplesofGHGemissionmetrics1579.1.2Pulse-emissionmetrics1599.1.3Step-pulsemetrics1619.1.4Keydifferencesbetweenstep-pulseandpulse-metrics1659.1.5Timehorizon/endpointformetrics1689.1.6Discountratesconsideration1689.1.7Non-radiativeforcingimpacts1699.2TheuseofGHGmetricsinimpactandmitigationapplications1709.2.1Lifecycleassessmentandcarbonfootprinting171xiii9.2.2Cost-benefitassessmentofclimatechangemitigation1729.2.3Cost-effectivenessofdifferentmitigationoptions1749.2.4Overallemissionreductionpolicyandtheroleofagriculture1759.2.5Cross-sectorcomparisons1789.2.6AggregationofdifferentGHGsforreportingandaccounting1809.2.7Biogenicmethane–Implicationsformetrics1809.3Climatetargetsandrelatedissues1819.3.1TheParisAgreement1829.3.2Climateneutrality1849.3.3Methaneabatementandsustainableagriculture1889.3.4Equityconsiderations1889.4Metricselectionguide1909.4.1Pointstoconsider1909.4.2Examples1969.4.3SummaryofkeyfeaturesandlimitationsofGWP,GWPandGTP203CONCLUSION205REFERENCES207APPENDIX307DETAILSOFCASESTUDIES309TABLES1.Characteristicsofdifferenttechniquesusedtomeasuremethane262.Summaryofentericmethanemitigationstrategiesforconfinedruminant(beef,dairyorother)systems1193.Summaryofentericmethanemitigationstrategiesforextensivepastoral/ranchingsystems(beef,dairyorother)basedongrazingwithoutsupplementation1214.Summaryofentericmethanemitigationstrategiesformixedgrazingwithsupplementationofconcentrates,by-productsandconservedforages1235.Mitigationstrategiesformethaneemissionsfromanimalhousing,manurestorageandlandapplication1266.GWPvaluesfromtheIPCC’sSixthAssessmentReport(AR6)1607.GTPvaluesbasedonformulaefromtheIPCC’sSixthAssessmentReport(AR6)1618.GWPvaluesformethaneacrossthedifferenthistoricalIPCCreports1839.AnnualemissionsassociatedwiththefarminExample119610.Changeinannualemissionsfromusingthefeedadditivecomparedtothecontrolfarm,aggregatedusingGWP,GTPandGWP197A1.Absoluteemissionswhenusingthefeedadditive,relativetonoemissions,aggregatedusingGWP,GTPandGWP310xivFIGURES1.Mainbiochemicalpathwaysinrumenfermentation72.Simplifiedschemeofthemainpathwaysofanaerobicdigestion103.Methanedynamicsinfloodedricesoil134.Aschematicflowchartofcurrenttechniquesusedtodeterminemethaneemissionsattheanimal,facilityandlarge-scalelevels255.SystemboundaryofthelifecycleassessmentfortheCalifornianmilkproduction1516.Thecause–effectchainfromemissionstoclimatechangeimpacts1577.Differenteffectsonradiativeforcingandtemperaturechangeoronegigatonne(Gt)ofCO2,CH4andN2Opulseemissions1588.Anillustrationofhowrising(left),constant(middle)andfalling(right)emissionsofCO2(red)andCH4(blue)affectlevelsofglobalwarming1639.CumulativeCO2-equivalentemissionsofmethaneareshown,calculatedusingdifferentmetrics,fortwomitigationscenariosnamedSSP4-6.0(panela)andSSP1-2.6(panelb)16410.ContributionstoglobalwarmingfromglobalnetCO2emissionsandglobalCH4emissionsfromlivestock,inapathwaythatlimitsglobalwarmingto1.5degreeswithlimitedovershoot16611.Sectoralcontributiontoannualtotalgreenhousegasemissionsin2010weightedbythreedifferentgreenhousegasmetrics,GWP100,GWP20andGTP10017912.Modelledglobaltemperatureanomaliesfrom1850to2015forallanthropogenicemissions17913.Scenarioscomputedusingtheaggregatedcarboncycle,atmosphericchemistryandclimate(ACC2)model19914.CO2eqandCO2-weemissionsfromthethreefarmscalculatedusingGWP100andGWP202A1.AdditionalresultsforExample1311A2.DetailedresultsforExample1(evaluationofemissionmetricsinrepresentingthebenefitsofusingafeedadditive)312A3.DetailedresultsforExample2(illustratingthepathdependencyofstep-pulsemetricsinrepresentingtheimpactofthreefarmerswithdifferenthistoricalemissions)313xvForewordMethaneisashort-livedgaswithanatmosphericlifetimeofaroundadecade,whereasthedominantgreenhousegas,carbondioxide,affectstheglobalclimateforhundredsofyears.Accordingtothe2021SixthAssessmentReportoftheIntergovernmentalPanelonClimateChange,methaneemissionsfromanthro-pogenicactivitiescurrentlycontributeabout0.5°Ctoobservedglobalwarming.Reducingmethanelevelshasbeenidentifiedasacrucial–andrapid-steptowardsslowingdownglobalwarming.Mostanthropogenicmethaneemissionsfromagrifoodsystemsresultfromtheentericfermentationofruminantlivestockandtheanaerobicdigestionofanimalmanureaswellasotherorganicwastes,whichinvolvecomplexmetabolicinteractionsbetweenmicrobialgroups.Byjoiningoureffortstoreducemethaneemissionsfromlivestockandricesystems,agrifoodsystemswillcontributetotheGlobalMethanePledge,anon-bindinginitiativesignedby150countriesatthe26thUNClimateConferencein2021.Curbingmethaneemissionsisanintegralpartofthestrategiesaimedatlimitingtheglobaltemperatureincreasetowellbelow2°Candpreferablyto1.5°Cabovethepreindustriallevel,inlinewiththeParisAgreementandwithSustainableDevelopmentGoal13onclimateaction.Thisgoalalsochimeswiththecalltoraiseambitionsforthemitigationcommitmentsandtargetsoutlinedinnation-allydeterminedcontributions.Forthefirsttime,FAO,throughtheLivestockEnvironmentalAssessmentandPerformancePartnership(LEAP),providesacomprehensivepictureandrobustanalysisofmethaneemissionsinlivestockandricesystems.DevelopedbytheFAOLEAPPartnershiptechnicaladvisorygrouponmethane,amultidisciplinaryteamcomposedof54internationalscientistsandexperts,thisreportanalysesthesourcesandsinksofmethanerelatedtolivestockaswellasriceproductionsystems,summa-rizesexistingtechnicalandinnovativemitigationsolutions,andevaluatesmetricstoquantifytheimpactsofmethaneemissionsontheclimate.Thegroupanalysedawiderangeofscientificpaperstooffervaluableinsightsandprovidescientificevidencethatpolicymakersandstakeholders–includingthepublic,theprivatesector,non-stateentitiesandproducers’organizations–canusetodesignandimplementtechnicalmitigationstrategiesandformulatepolicyframeworkstoenhanceclimateactionsinthecontextoflivestockandricesystems.ReducinggreenhousegasemissionsisanimportantcomponentoftheFAOStrategyonClimateChangeandoftheOrganization’sStrategicFramework2022-2031foundedonbetterproduction,betterlife,betternutritionandbetterenviron-ment.ThisreportcontributestoabetterenvironmentandsupportsMembersininte-gratingspecificmethanemitigationinterventionsandtargetsintonationalclimateactionsasrequestedatthefirstsessionoftheSub-CommitteeonLivestockofFAO’sCommitteeonAgriculture(COAG)(https://www.fao.org/3/ni966en/ni966en.pdf,paragraph25).xviIhopethattheresultsandrecommendationsofthisreportbolstertheeffortsofcountriesandstakeholderscommittedtoreducingmethaneemissionsand,indoingso,movesustowardsmoreefficient,inclusive,resilient,low-emissionandsustainableagrifoodsystems.MariaHelenaSemedoDeputyDirector-GeneralxviiAcknowledgementsThisreportwaspreparedbythetechnicaladvisorygrouponmethane(MethaneTAG)oftheFAOLivestockEnvironmentalAssessmentandPerformancePartnership(FAOLEAPPartnership).TheSteeringCommitteeofFAOLEAPPartnershipprovidedtheoverallguidanceforthedevelopmentofthisreport.ThePartnershipwasco-chairedbyTimMcAllister,AgricultureandAgri-FoodCanada(2021),HenningSteinfeld,FAO(untilJuly2022),HsinHuang,InternationalMeatSecretariat(2022),ThanawatTiensin,DirectorofFAOAnimalProductionandHealthDivision(sinceJanuary2023),JulieAdamchick,WorldWildlifeFoundation(sinceFebruary2023).TheSecretariatoftheFAOLEAPPartnershipwascoordinatedbyTimRobinson,FAO(untilJune2022)andAimableUwizeye,FAO(fromAugust2022).CamilloDeCamillis,FAO(untilApril2022)andXiangyuSong,FAO(fromDecember2022)wereresponsiblefortheday-to-daymanagementoftheFAOLEAPPartnership.xviiiDevelopmentprocessofthemethanereportTheMethaneTAGwascomposedof54internationalexpertsinanimalsciences,climatesciences,physics,plantsciences,soilsciencesandenvironmentalsciences.TheMethaneTAGwasestablishedinDecember2020andmetvirtuallyinthecourseofseveralmeetings.Itwasco-ledbyErmiasKebreab(Co-Chair,UniversityofCalifornia,Davis,UnitedStatesofAmerica),MichelleCain(Co-Chair,CranfieldEnvironmentCentre,CranfieldUniversity,UnitedKingdomofGreatBritainandNorthernIreland)andJunMurase(Co-Chair,GraduateSchoolofBioagriculturalSciences,NagoyaUniversity,Japan),withthetechnicalsupportofAimableUwizeye(CoordinatorofFAOLEAPSecretariat,Italy).PART1.SOURCESANDSINKSOFMETHANEEMISSIONSINAGRICULTUREDavidKenny(Teagasc,AnimalandGrasslandResearchandInnovationCentre,Ireland)ledtheresearchandwritingofthispart.Itwasco-authoredbyEmilioM.Ungerfeld(CentroRegionaldeInvestigaciónCarillanca,InstitutodeInvestigacionesAgropecuarias[INIA],Chile),ClementinaÁlvarez(DepartmentofResearch,TINESA,Norway),MélyndaHassouna(NationalInstituteforAgriculture,FoodandEnvironment[INRAE],InstitutAgroRennes,France),RogerioM.Mauricio(DepartmentofBioengineering,FederalUniversityofSãoJoãodel-Rei,Brazil),PhilippeBecquet(InternationalFeedIndustryFederation,Germany),AdibeL.Abdalla(CenterforNuclearEnergyinAgriculture,UniversityofSãoPaulo,Brazil),DiptiPitta(UniversityofPennsylvania,UnitedStates),Jean-BaptisteDollé(IDELE,France),MariaPazTieri(FONTAGRO,Spain),MichaëlMathot(AgriculturalSystemsUnit,WalloonAgriculturalResearchCentre,Belgium),BrianG.McConkey(VirescoSolutionsInc.,Canada),AlexandreBerndt(EmbrapaSoutheastLivestock,Brazil),JuliánChará(CenterforResearchonSustainableAgriculture[CIPAV],Colombia)andJunMurase(NagoyaUniversity,Japan).PART2.QUANTIFICATIONOFMETHANEEMISSIONSLuisO.Tedeschi(DepartmentofAnimalSciences,TexasA&MUniversity,UnitedStates)ledthewritingofthispart.Itwasco-authoredbyAdibeL.Abdalla(CenterforNuclearEnergyinAgriculture,UniversityofSãoPaulo,Brazil),ClementinaÁlvarez(DepartmentofResearch,TINESA,Norway),SamuelW.Anuga(EuropeanUniversityInstitute,Italy),JacoboArango(InternationalCenterforTropicalAgriculture[CIAT],Colombia),KarenA.Beauchemin(AgricultureandAgri-FoodCanada,LethbridgeResearchandDevelopmentCentre,Canada),EmilioM.Ungerfeld(CentroRegionaldeInvestigaciónCarillanca,InstitutodeInvestigacionesAgropecuarias[INIA],Chile),PhilippeBecquet(InternationalFeedIndustryFederation,Germany),AlexandreBerndt(EmbrapaSoutheastLivestock,Brazil),RobertBurns(UniversityofTennessee,Knoxville,UnitedStates),CamilloDeCamillis(AnimalProductionandHealthDivision,FAO,Italy),JuliánCharáxix(CenterforResearchonSustainableAgriculture[CIPAV],Colombia),JavierM.Echazarreta(CentroCarnes–InstitutoNacionaldeTecnologíaIndustrial[INTI],Argentina),MélyndaHassouna(NationalResearchInstituteforAgriculture,FoodandEnvironment(INRAE),InstitutAgroRennes,France),DavidKenny(Teagasc,AnimalandGrasslandResearchandInnovationCentre,Ireland),MichaëlMathot(AgriculturalSystemsUnit,WalloonAgriculturalResearchCentre,Belgium),RogerioM.Mauricio(DepartmentofBioengineering,FederalUniversityofSãoJoãodel-Rei,Brazil),ReinerWassmann(Independentresearcher,previouslyworkingfortheInternationalRiceResearchInstitute),VinisaSaynes(LandandWaterDivision,FAO,Italy),ShelbyC.McClelland(AnimalProductionandHealthDivision,FAO,Italy,SoilandCropSciences,SchoolofIntegrativePlantScience,CornellUniversity,UnitedStates),MutianNiu(InstituteofAgriculturalSciences,ETHZürich,Switzerland),AliceAnyangoOnyango(MazingiraCenter,InternationalLivestockResearchInstitute[ILRI]andDepartmentofChemistry,MasenoUniversity,Kenya),RanjanParajuli(EcoEngineers,UniversityofArkansas,UnitedStates),LuizG.RibeiroPereira(Embrapa,Brazil),AgustíndelPrado(BasqueCentreforClimateChange[BC3]andIkerbasque,BasqueFoundationforScience,Spain),MariaPazTieri(FONTAGRO,Spain),AimableUwizeye(AnimalProductionandHealthDivision,FAO,Italy),JunMurase(NagoyaUniversity,Japan),andErmiasKebreab(DepartmentofAnimalScience,UniversityofCalifornia,Davis,UnitedStates).Part1andPart2weresummarizedandpublishedinapeer-reviewedjournal:Tedeschietal.2022.Quantificationofmethaneemittedbyruminants:Areviewofmethods.JournalofAnimalScience,100(7):1-22,https://doi.org/10.1093/jas/skac197.PART3.MITIGATIONOFMETHANEEMISSIONSKarenA.Beauchemin(AgricultureandAgri-FoodCanada,LethbridgeResearchandDevelopmentCentre,Canada)andEmilioM.Ungerfeld(CentroRegionaldeInvestigaciónCarillanca,InstitutodeInvestigacionesAgropecuarias[INIA],Chile)ledthedevelopmentofthispart.Itwasco-authoredbyAdibeL.Abdalla(CenterforNuclearEnergyinAgriculture,UniversityofSãoPaulo,Brazil),ClementinaÁlvarez(DepartmentofResearch,TINESA,Norway),ClaudiaArndt(InternationalLivestockResearchInstitute[IRLI],Kenya),PhilippeBecquet(InternationalFeedIndustryFederation,Germany),ChaoukiBenchaar(SherbrookeResearchandDevelopmentCentre,AgricultureandAgri-FoodCanada,Canada),AlexandreBerndt(EmbrapaSoutheastLivestock,Brazil),AgustíndelPrado(BasqueCentreforClimateChange[BC3],Ikerbasque,BasqueFoundationforScience,Spain),DavidKenny(Teagasc,AnimalandGrasslandResearchandInnovationCentre,Ireland),JohnLynch(UniversityofOxford,UnitedKingdom),RogerioM.Mauricio(DepartmentofBioengineering,FederalUniversityofSãoJoãodel-Rei,Brazil),TimA.McAllister(AgricultureandAgri-FoodCanada,LethbridgeResearchandDevelopmentCentre,Canada),MutianNiu(InstituteofAgriculturalSciences,ETHZürich,Switzerland),WalterOyhantçabal(FacultaddeAgronomía,UniversidaddelaRepública,Uruguay),AndyReisinger(Independentconsultant,NewZealand),SaheedA.Salami(MootralLtd,UnitedKingdom),LaurenceShalloo(Teagasc,AnimalandGrasslandResearchandInnovationDepartment,Ireland),YanSun(CargillInc.,UnitedStates),MariaPazTieri(FONTAGRO,Spain),JuanM.Tricarico(InnovationCenterforU.S.Dairy,UnitedStates),AimableUwizeye(AnimalProductionandHealthDivision,xxFAO,Italy),CamilloDeCamillis(AnimalProductionandHealthDivision,FAO,Italy),MartialBernoux(OfficeofClimateChange,BiodiversityandEnvironment,FAO,Italy),TimothyRobinson(AnimalProductionandHealthDivision,FAO,Italy),JunMurase(NagoyaUniversity,Japan),andErmiasKebreab(DepartmentofAnimalScience,UniversityofCalifornia,Davis,UnitedStates).Part3wassummarizedandpublished:Beaucheminetal.2022.Invitedreview:Currententericmethanemitigationoptions.JournalofDairyScience,105(12):9297-9326.https://doi.org/10.3168/jds.2022-22091.PART4.METRICSFORQUANTIFYINGTHEIMPACTOFMETHANEEMISSIONSThedevelopmentofthischapterwasledbyMichelleCain(CranfieldUniversity,UnitedKingdom),withsectionsco-ledbyJohnLynch(UniversityofOxford,UnitedKingdom)forSection9,WilliamJ.Collins(DepartmentofMeteorology,UniversityofReading,UnitedKingdom)forSection9.1,MikoKirschbaum(ManaakiWhenua–LandcareResearch,NewZealand)forSection9.2,BradRidoutt(CommonwealthScientificandIndustrialResearchOrganisation[CSIRO],AgricultureandFood,AustraliaandDepartmentofAgriculturalEconomics,UniversityoftheFreeState,Bloemfontein,SouthAfrica)andAgustíndelPrado(BasqueCentreforClimateChange[BC3]andIkerbasque,BasqueFoundationforScience,Spain)forSection9.3,whileKatsumasaTanaka(LaboratoiredesSciencesduClimatetdel’Environnement[LSCE],FranceandNationalInstituteforEnvironmentalStudies[NIES],Japan)co-ledSection9.4.Thispartwasco-authoredbyJuanM.Tricarico(InnovationCenterforU.S.Dairy,UnitedStates),AdibeL.Abdalla(CenterforNuclearEnergyinAgriculture,UniversityofSãoPaulo,Brazil),AlexandreBerndt(EmbrapaSoutheastLivestock,Brazil).JavierM.Echazarreta(CentroCarnes–InstitutoNacionaldeTecnologíaIndustrial[INTI],Argentina)ClementinaÁlvarez(DepartmentofResearch,TINESA,Norway),LuizG.Ribeiro(Embrapa,Brazil),LuisO.Tedeschi(DepartmentofAnimalSciences,TexasA&MUniversity,UnitedStates),EmilioM.Ungerfeld(CentroRegionaldeInvestigaciónCarillanca,InstitutodeInvestigacionesAgropecuarias[INIA],Chile),ErmiasKebreab(UniversityofCalifornia,Davis,UnitedStates),MunavarZhumanova(CenterforGlobalChangeandEarthObservations,MichiganStateUniversity,UnitedStates),BrianG.McConkey(VirescoSolutionsInc.,Canada),KarenA.Beauchemin(AgricultureandAgri-FoodCanada,LethbridgeResearchandDevelopmentCentre,Canada),AndyReisinger(Independentconsultant,NewZealand),AnnaFlysjö(ArlaFoods,Sweden),DiptiPitta(UniversityofPennsylvania,UnitedStates),Jean-BaptisteDollé(IDELE,France),JuliánChará(CenterforResearchonSustainableAgriculture[CIPAV],Colombia),MariaPazTieri(FONTAGRO,Spain),FrankMitloehner(UniversityofCalifornia,Davis,UnitedStates),SamuelWenifaAnuga(EuropeanUniversityInstitute,Italy),SaheedA.Salami(MootralLtd,UnitedKingdom),AndréMazzetto(AgResearch,NewZealand),ClaudiaArndt(InternationalLivestockResearchInstitute[IRLI],Kenya),ChaoukiBenchaar(AgricultureandAgri-FoodCanada,Canada),JacoboArango(InternationalCenterforTropicalAgriculture[CIAT],Colombia),JoeriRogelj(CentreforEnvironmentalPolicy,ImperialCollegeLondon,UnitedKindom),MutianNiu(InstituteofAgriculturalSciences,ETHZürich,Switzerland),StephanPfister(ETHZürich,Switzerland),Carl-FriedrichSchleussner(HumboldtUniversitätxxizuBerlin,Germany),WalterH.Oyhantçabal(Cironi,MinistryofLivestock,AgricultureandFisheries,Uruguay),RanjanParajuli(EcoEngineers,UniversityofArkansas,UnitedStates),DavidKenny(Teagasc,AnimalandGrasslandResearchandInnovationDepartment,Ireland),JacobP.Muhondwa(ArdhiUniversity,Tanzania),MélyndaHassouna(NationalResearchInstituteforAgriculture,FoodandEnvironment[INRAE],InstitutAgroRennes,France),HongminDong(ChineseAcademyofAgriculturalSciences[CAAS],China),JunMurase(NagoyaUniversity,Japan),TimMcAllister(AgricultureandAgri-FoodCanada,Canada),MichaëlMathot(AgriculturalSystemsUnit,WalloonAgriculturalResearchCentre,Belgium),PhilippeBecquet(InternationalFeedIndustryFederation,Germany),RobertT.Burns(UniversityofTennessee,Knoxville,UnitedStates),RogerioM.Mauricio(DepartmentofBioengineering,FederalUniversityofSãoJoãodel-Rei,Brazil),StephenWiedemann(IntegrityAg&Environment,Australia),AliceAnyangoOnyango(InternationalLivestockResearchInstitute,Kenya),LaurenceShalloo(Teagasc,AnimalandGrasslandResearchandInnovationDepartment,Ireland),YanSun(Cargill,UnitedStates),andAimableUwizeye(AnimalProductionandHealthDivision,FAO,Italy).ThedevelopmentofPart4wasalsobasedonthescopinganalysisbyMarc-AndreeWolfandAimableUwizeye,Evaluationofclimatechangemetricsformethaneemissionsfromtheagrifoodlivestocksystems(unpublished).xxiiMulti-stepreviewprocessThereportbenefittedfromthetwo-stepreviewprocesses:technical(January2022)andpublicreviews(fromOctober2022toJanuary2023).FAOisgratefultothefollowingtechnicalreviewersandorganizationsinparticular:•ClaudiaArndt(InternationalLivestockResearchInstitute,Kenya);•AndreBannink(WageningenUniversityandResearch,KingdomoftheNetherlands);•OlivierBoucher(InstitutPierre-SimonLaplace,SorbonneUniversity,France);•AlexandradeAthayde(InternationalFeedIndustryFederation,Germany);•PabloManzano(InternationalUnionforConservationofNature,Spain);•BrunoNotarnicola(UniversityofBariAldoMoro,Italy);•NicoPeiren(InternationalDairyFederation,Belgium);•CarlosAlbertoRamírezRestrepo(CREco-efficientAgricultureConsultancy,Australia);•MaríaSánchezMainar(InternationalDairyFederation,Belgium);•SabineVanCauwenberghe(DSM,Switzerland);•RonaldVargas(LandandWaterDivision,FAO,Italy);and•TheFAOLEAPSecretariat.FAOisalsogratefultothefollowingscientistsandorganizationsthatreviewedthisreportduringthepublicreview:•UshaAmaranathan(ZestBiotech,NewZealand);•MichaelBinder(Evonik,Germany);•AntonyDelavois(EuropeanSpaceAgency,France);•BaishaliDutta(GroupeAGÉCO,Canada);•AlisonEagle(EnvironmentalDefenseFund,UnitedStates);•BillGrayson(MorecambeBayConservationGrazingCompany,UnitedKingdom);•KriteeKritee(EnvironmentalDefenseFund,UnitedStates);•FranciscoNorris(ZELPLtd,UnitedKingdom);•MinistryforPrimaryIndustries,NewZealandrepresentedbyJennyReid;•PeriRosenstein(EnvironmentalDefenseFund,UnitedStates);•TianyiSun(EnvironmentalDefenseFund,UnitedStates);•BartTas(MootralLtd,UnitedKingdom);•JohnTauzel(EnvironmentalDefenseFund,UnitedStates);•PaulLovatt-Smith(UnitedKingdom);•SabineVanCauwenberghe(DSM,Switzerland);and•KimViggoWeiby(TINESA,Norway).xxiiiTheLivestockEnvironmentalAssessmentandPerformance(LEAP)PartnershipTheFAOLEAPisamulti-stakeholderinitiativelaunchedinJuly2012toimprovetheenvironmentalperformanceoflivestocksupplychains.HostedbytheFoodandAgricultureOrganizationoftheUnitedNations,thepartnershipbringstogethertheprivatesector,governments,academia,civilsocietyrepresentativesandleadingexperts,whohaveadirectinterestinthedevelopmentofscience-based,transparentandpragmaticguidancetomeasureandimprovetheenvironmentalperformanceoflivestockproducts.TheFAOLEAPPartnershipprovidesstate-of-the-artmethodsandmetricstoassessenvironmentalimpactsandbenchmarkperformanceacrosslivestocksupplychains.1THESTEERINGCOMMITTEEOFTHEFAOLEAPPARTNERSHIPFAOisverygratefulforallthevaluablecontributionsprovidedatvariouslevelsbytheFAOLEAPpartners.ParticulargratitudegoestothefollowingcountriesthatcontinuetosupportthePartnershipthroughfundingandoftencontributionsinkind:Australia,Brazil,Canada,China,CostaRica,France,Hungary,Ireland,Kenya,KingdomoftheNetherlands,NewZealand,Switzerland,UnitedStatesandUruguay.Thefollowinginternationalentitiesandcompaniesfromtheprivatesectoralsoprovidedfundingand/orin-kindcontributions:theInternationalFeedIndustryFederation(IFIF),theInternationalMeatSecretariat(IMS),theInternationalDairyFederation(IDF),theInternationalPoultryCouncil(IPC),theInternationalEggCommission(IEC),theWorldRenderersOrganization(WRO),theWorldFarmers’Organisation(WFO),theInternationalWoolTextileOrganisation(IWTO),theEUvegetableoilandproteinmealindustryassociation(FEDIOL),DSMNutritionalProductsAG,Evonik,andNovusInternational.FAOisalsogratefultothecivilsocietyorganizationsandnon-governmentalorganizationsthatprovidedin-kindcontributions:WorldWildlifeFund(WWF),WorldVisionInternational,WorldAllianceofMobileIndigenousPeoples(WAMIP),theInternationalPlanningCommitteeforFoodSovereignty(IPC),theInternationalUnionforConservationofNature(IUCN),theInternationalCooperativeAlliance(ICA),VétérinairesSansFrontières(VSF),theInternationalOrganizationforStandardization(ISO),BusinessforSocialResponsibility(BSR),andtheBill&MelindaGatesFoundation.THELEAPSECRETARIATFAOLEAPSecretariathascoordinatedandfacilitatedtheworkofthisTAG.TheSecretariatguidedandcontributedtothecontentdevelopment,aswellasensuringcoherencewithotherexistingguidelines.TheLEAPSecretariat,hostedatFAO,iscomposedofAimableUwizeye(FAOTechnicalOfficerandCoordinatorof1MorebackgroundinformationonthePartnershipcanbefoundatwww.fao.org/partnerships/leap/enxxivtheSecretariatsinceAugust2022),CamilloDeCamillis(PartnershipManageruntilApril2022),XiangyuSong(PartnershipManagersinceDecember2022),MonicaRulli(TechnicalSpecialist),MaríaSoledadFernándezGonzález(CommunicationSpecialistuntilJanuary2021),EmmieWachira(ProgrammeandOutreachSpecialistuntilDecember2021),SaraGiuliani(ProgrammeandOutreachSpecialistsinceMarch2022),TimRobinson(SecretariatCoordinatoruntilJuly2022),HenningSteinfeld(PartnershipCo-ChairuntilJuly2022),ThanawatTiensin(PartnershipCo-chairsinceJanuary2023).ADDITIONALCONTRIBUTIONSProfessionaleditingandproofreadingweredonebyAgnieszkaGratza.SaraGiulianilookedafterthecommunicationandpublicationmanagement.ClaudiaCiarlantini(FAO)andEnricoMasciwereresponsibleforthedesignandlayoutofthispublication.AdministrativesupportwasprovidedbyEvaMaríaPardoNavarroandIsabelBurgos.xxvAbbreviationsADanaerobicdigestionAFOLUagriculture,forestryandotherlanduseAR4FourthAssessmentReportoftheIntergovernmentalPanelonClimateChangeAR5FifthAssessmentReportoftheIntergovernmentalPanelonClimateChangeAR6SixthAssessmentReportoftheIntergovernmentalPanelonClimateChangeATPadenosinetriphosphateAWDalternativewettinganddryingBATbestavailabletechnologyBCMbromochloromethaneBESbromoethanesulfonateCFcharacterizationfactorCGTPcombinedglobaltemperaturechangepotentialCO2eqcarbondioxideequivalentCPcrudeproteinCTcondensedtanninsDMdrymatterDMIdrymatterintakeDNDCDeNitrification-DeCompositionEFemissionfactorFAOFoodandAgricultureOrganizationoftheUnitedNationsGCPglobalcostpotentialGDamPglobaldamagepotentialxxviGEgrossenergyGEIgrossenergyintakeGHGgreenhousegasGISgeographicinformationsystemGITgastrointestinaltractGTPglobaltemperaturepotentialGWPglobalwarmingpotentialGWPGWP-starHThydrolysabletanninsIAMintegratedassessmentmodelIDFInternationalDairyFederationIRRIInternationalRiceResearchInstituteIgGimmunoglobulinGIPCCIntergovernmentalPanelonClimateChangeISOInternationalOrganizationforStandardizationJRCJointResearchCentreoftheEuropeanCommissionLCAlifecycleassessmentLEAPFAOLivestockEnvironmentalAssessmentandPerformancePartnershipLMDlasermethanedetectorLUClandusechangeMCFAmedium-chainfattyacidsMCRmethyl-coenzymeMreductaseNASEMNationalAcademiesofSciences,Engineering,andMedicineNDFneutraldetergentfibreNOPA3-nitrooxypropionicacidxxviiOMorganicmatterOMDorganicmatterdigestedPUFApolyunsaturatedfattyacidsPGPRplantgrowth-promotingrhizobacteriaSETACSocietyofEnvironmentalToxicologyandChemistrySPSsilvopastoralsystemSRIsystemofriceintensificationTAGtechnicaladvisorygroupUNEPUnitedNationsEnvironmentProgrammeUNFCCCUnitedNationsFrameworkConventiononClimateChangeVFAvolatilefattyacidVRventilationrateWSCwatersolublecarbohydratesYmmethaneconversionfactor(percent)CHEMICALELEMENTSANDFORMULAE3-NOP3-nitrooxypropanolCcarbonCH4methaneClchlorineCO2carbondioxideFeSiron(II)sulphideH+proton,cationicformofatomichydrogenH2hydrogenxxviiiNnitrogenNAD+oxidizednicotinamideadeninedinucleotideNADHreducednicotinamideadeninedinucleotideNH3ammoniaN2OnitrousoxideO3ozoneOHhydroxylSF6sulphurhexafluoride,tracergasUNITSdegreeCelsiusgigatonne,metricunitequivalentto1billion(109)tonnes°CmegaJouleGtmegatonne,metricunitequivalentto1million(106)tonnesMJnanometreMtpartspermillionnmteragrsam,metricunitequivalentto1million(106)tonnesPpmWattTgWxxixExecutivesummaryThereportcontainsfourparts,addressing1.thesourcesandsinksofmethane(CH4)emissionsinagriculture;2.thequantificationofCH4emissions;3.themiti-gationofCH4emissionsand4.themetricsforquantifyingtheimpactofCH4emissions.ThemajorityofCH4emissionsfromtheagriculturalsectorareacon-sequenceofmicrobial-mediatedentericfermentationprocessesinruminantlive-stock,whichmakeupabout30percentofglobalanthropogenicCH4emissions.Anaerobicdigestionofanimalmanureandotherorganicwastes,whichinvolvescomplexmetabolicinteractionsbetweenmicrobialgroups,contributestoabout4.5percentoftheworld’santhropogenicCH4emissions.Ricepaddies,meanwhile,areestimatedtocontribute8percentoftotalhuman-causedCH4emissions.GlobalCH4emissionsarelargelyoffsetbytheatmosphericandsoilCH4sinks.Theatmo-sphericsinkoccursthroughthechemicaldegradationofCH4byhydroxyl(OH)andchlorine(Cl)radicalsinthetroposphereandstratosphereandisresponsiblefor90to96percentoftheglobalCH4sink.Thesoilaccountsforabout4to10percentoftheCH4degraded.TheoceanactsasasmallCH4sinkforatmosphericCH4.Methaneisashort-livedgaswhichhasanatmosphericlifetimeofaroundadecade,whereasthedominantgreenhousegas(GHG),carbondioxide,affectstheclimateforhundredsofyears,ifnotlonger.Becauseofthisdifferenceintheirrespectivelifetimes,theGHGemissionmetricsusedtocompareCH4withCO2(carbondioxide)varydependingonwhattimeframetheyconsider.Thisisnotanissuefornitrousoxide(N2O),forexample,asitslifetimeextendsbeyondacenturyandmetricstypicallycomparetimeframesofacenturyorless.TheappropriatequantificationofGHGemissions,specificallyCH4,hasraisedquestionsabouthowGHGemissioninventoriesarereportedand,perhapsmoreimportantly,howbesttomitigateCH4emissions.Thisreviewdocumentsexistingmethodsandmetho-dologiestomeasureandestimateCH4emissionsfromruminantanimalsandthemanureproducedthereinaccordingtovariousscalesandconditions.MeasurementsofCH4havefrequentlybeenconductedinresearchsettingsusingclassicalmethodologiesdevelopedforbioenergeticpurposes,suchasgasexchangetechniques(respirationchambers,headboxes).Whileveryprecise,thesetechniquesarelimitedtoresearchsettingsastheyareexpensive,labour-intensiveandappli-cableonlytoafewanimals.Head-stalls,asexemplifiedbytheGreenFeedsystem,havebeenusedtomeasureexpiredCH4forindividualanimalshousedaloneoringroups,inconfinementorgrazing.Thistechniquerequiresfrequentanimalvisita-tionoverthediurnalmeasurementperiodandanadequatenumberofcollectiondays.ThetracergastechniquecanbeusedtomeasureCH4fromindividualanimalshousedoutdoors,aslowbackgroundconcentrationsofmethanemakeiteasiertodetectemissions.Micrometeorologicaltechniques,suchasopen-pathlasers,canmeasureCH4emissionsoverlargerareasandfrommanyanimals,butlimitationsexist,includingtheneedtomeasureovermoreextensiveperiods.ThemeasurementofCH4emissionsfrommanuredependsonthetypeofstor-age,animalhousing,CH4concentrationinsideandoutsidetheboundariesofthetargetarea,andventilationrate(VR),whichislikelythevariablethatcontributesxxxthemosttomeasurementuncertainty.Chamber(open/closed)andmicrometeo-rologicalmethodsareusedtocollectCH4fluxesfromricepaddysoilsinsitu.Forlarge-scaleareas,aircraft,dronesandsatelliteshavebeenusedinassociationwiththetracerfluxmethod,inversemodelling,imagery,andlightdetectionandranging(LiDAR),butresearchislaggingbehindinvalidatingthesemethods.Bottom-upapproachestoestimatingCH4emissionsrelyonempiricalormechanisticmodel-lingtoquantifythecontributionofindividualsources(entericandmanure).Incontrast,top-downapproachesestimatetheamountofCH4intheatmosphereusingspatialandtemporalmodelstoaccountfortransportationfromanemittertoanobservationpoint.Whilethesetwoestimationmethodsrarelyagree,inpracticetheyhelpidentifyknowledgegapsandresearchrequirements.TheSixthAssessmentReportoftheIntergovernmentalPanelofExpertsonClimateChangefoundthatmethaneemissionsfromallhumanactivitiescontributedabout0.5°Ctothepresentobservedwarming.DecreasingtheemissionsofentericCH4fromruminantproductionisakeyelementofthestrategiesdesignedtolimittheglobaltemperatureincreaseto1.5°C.ResearchintheareaofentericCH4mitigationhasgrownexponentiallyinthelasttwodecades,andvariousstrategiesforentericCH4abatementhavebeeninvestigated:productionintensification,dietarymanipula-tion(includingsupplementationandprocessingofconcentratesandlipids,manage-mentofforageandpastures),rumenmanipulation(supplementationofionophores,3-nitrooxypropanol,macroalgae,alternativeelectronacceptorsandphytochemicals),andtheselectionoflowCH4-producinganimals.OtherentericCH4mitigationstrat-egies,althoughatlessadvancedstagesofresearch,arerapidlydeveloping.ThereportdiscussesandanalysesthecurrentlyavailableentericCH4mitigationstrategieswithanemphasisonopportunitiesandbarrierstotheirimplementationinconfinedandpartialgrazingproductionsystems,aswellasinextensiveandfullygrazingproductionsystems.ForeachentericCH4mitigationstrategy,thereportdiscussesitseffectivenessindecreasingthetotalCH4emissionsandemissionscal-culatedonaperanimalproductbasis,safetyaspects,impactsontheemissionsofotherGHGs,andothereconomic,regulatoryandsocietalissuesthatarekeytoimplementation.Mostresearchhasbeenconductedwithconfinedanimals,andcon-siderablymoreresearchisneededtodevelop,adaptandevaluateanti-methanogenicstrategiesforgrazingsystems.Ingeneral,fewoptionsarecurrentlyavailableforextensiveproductionsystemsthatdonotusefeedsupplements.FurtherresearchisneededtodevelopentericCH4mitigationstrategiesthatarelocallyapplicable.Thereisalackofinformationrequiredtocalculatecarbonfootprintsofinterven-tionsonaregionalbasis,whichwouldmakeitpossibletoevaluatetheimpactofmitigationstrategiesonnetGHGemissions.EconomicallyaffordableentericCH4mitigationsolutionsarealsoinshortsupply.Severalagriculturalpractices,includingwatermanagement,organicamendment,fertilizermanagementandcropmanage-ment,canmitigateCH4emissionsfromricepaddies.LocallyappropriateoptionsthatconsiderriceyieldsandtheriskofotherGHGemissions,likeN2O,shouldbeadopted.Asuccessfulimplementationofsafeandeffectiveanti-methanogenicstrategieswilldependondeliverymechanismsandadequatetechnicalsupportforproducersbutalsoonconsumerinvolvementandacceptance.Itcallsforaholisticapproachandbuy-inatalllevelsofthesupplychain.Part4ofthisreviewfocusesonmetricsusedtoquantifytheimpactofmethaneemissions,andthemitigationthereof.TheprimarypurposeofaGHGemissionxxximetricistoprovideinformationonhowdifferentGHGemissions(oremissionreductions)contributetoclimatechangeandtheresultingimpacts.ThemetriccanthenbeusedtoaggregatedifferentGHGemissionsintoatotal“CO2-equivalent”emission.EachGHGemissionmetricisdefinedbyagivenclimateimpact(e.g.tem-perature,radiativeforcing)overagiventime.Ametricthatestablishesequivalenceasregardsonekeymeasureoftheclimatesystem’sresponsetoemissionsdoesnotimplyequivalencewithrespecttootherkeymeasures.Themostappropriatemetricdependsonthepolicyobjectives(i.e.whataspectofclimatechangedoesthepolicyfocusonandoverwhichtimehorizon).ThemostcommonGHGemissionmetricistheglobalwarmingpotential(GWP)integratedover100years,whichisusedforreportingnationalemissionsinventoriestotheUNFCCC.ThecontextinwhichGHGmetricsareusedisreviewed,alongsidetechnicaldescriptionsofpulse-emissionmetricsandstep-pulsemetrics,includingadiscus-sionoftimehorizons,discountratesandnon-radiativeforcingimpacts.Pulse-emissionmetricscompare1kgofagasto1kgofanothergas,usuallyoverachosentimeframe(e.g.a100-yearperiodaftertheemission)orataparticulartimeinthefuture(e.g.50yearsaftertheemissionoccurs).Forshort-livedpollutantslikeCH4,thismeansthatthechoiceoftimehorizonhasalargeimpactontheirmetricvalue.TheGWPandtheglobaltemperaturepotential(GTP)arepulse-emissionmetrics.Takingadifferentapproachtocomparinggases,step-pulsemetricsaccountspecifi-callyfortheeffectsofsustainedshort-livedemissions.TheequivalenceisbasedonworkingbackwardsfromthetemperatureorradiativeforcingoutcomeofatimeseriesofCH4emissions,andapproximatingwhatCO2emissionswouldleadtothesametemperatureorradiativeforcingoutcome.Thestep-pulsemetricsexploredinthisreportaretheGWP(GWP-star)andthecombinedglobaltemperaturechangepotential(CGTP).Pulse-emissionmetricsprimarilyprovideinformationaboutthefutureclimateimpacts(asdefinedbythespecificmetric)thatwouldbecausedbyanextraunitofemissionofagivengas,comparedtohavingnoemission.Inthisreview,wecalltheseimpacts“marginal”,e.g.“marginalwarming”.Incontrast,step-pulsemetricshaveprimarilybeenusedtoshowthechangeintemperatureovertime,causedbyaparticularemissionspathway,relativetowarmingatareferencedateresultingfrompreviousemissions.Wecalltheseimpacts“additional”sincethereferencedate,e.g.“additionalwarming”.Itisimportanttonotethatthereisnopurelyscientificoruniversalbasistodeterminemetricchoice.Theuseofmetricsinassessmentsofimpactandmitigationisreviewed.Metricsareneededifpotentialtrade-offswithemissionsofotherGHGsaretobeevaluated,ortocomparedifferentsectorsoremitterswithavarietyofGHGsbeingemitted.IfareductiontargetforCH4aloneisunderconsideration,ametricwouldnotberequiredtotrackprogress,althoughitcouldbeusedtoevaluateorjustifythelevelofambition.Metricsusedinthelifecycleassessment(LCA)shouldbechosentomatchtheuser’simpactobjectives,whichcouldencompassavarietyofenviron-mentalimpactobjectives.Cost-benefitandcost-effectivenessassessmentaredis-cussed,andthisincludesarangeofdifferentmetricswhichfactorinassociatedcosts.IrrespectiveofthemethodusedtoaggregatedifferentGHGs,reportingtheemissionsofindividualGHGsisrecommendedtoensureclarityandtransparency.Usingarangeofmetricscanhelptotestthesensitivityofclimatechangeimpactassessmentstothechoiceofmetric.xxxiiMetricsareusedwithinawiderpolicyframeworkrelatingtoclimateactionandsustainabledevelopment.Hencethekeythemesrelevantformetricsoutlinedinthefinalpartofthereport,whichdiscussestheParisAgreement,differingdefinitionsofclimateneutralityandthecomplexitythisintroduces,sustainableagricultureandequityconsiderations,amongotherissues.Theseareofcrucialimportancewhenapplyingmetricsandmakingdecisionsabouttargetsandclimateaction.xxxiiiIntroductionAchievingthesustainabilityofagrifoodsystemsisurgentandtheglobalcommunityisexpectingeachsectoroftheeconomytoundertakethenecessarytransformativeactions.Sustainabilityremainsachallengefortheagrifoodsystemssectorbecauseofthesheervolumeoffood,andlivestockproductsinparticular,producedtomeetthenutritionalneedsofagrowingpopulationinthecontextofclimatechangeandotherenvironmentalimpacts.In2017,agrifoodsystems,includingagriculture,forestryandotherlanduse(AFOLU),wereresponsiblefor23percentoftotalanthropogenicgreenhousegas(GHG)emissionsglobally,assessedusingaglobalwarmingpotential(GWP)fora100-yearhorizon(IPCC,2019b).Livestocksupplychainsaloneplayanimportantroleinclimatechange,representing14.5percentofhumaninducedGHGemissions.TheshareofthelivestocksectorinGHGemissionsisregion-specificanddependsonthemagnitudeofothereconomicsectors,abovealltheenergysector.Forinstance,theUSEnvironmentalProtectionAgency(EPA)reportsthat,althoughagrifoodsystemsareresponsiblefor9to10percentoftotalGHGemissions,live-stockcontributeslessthan4percentofdirectemissions(Dillonetal.,2021;Tedeschi,2022).MostoftheemissionsfromAFOLUareintheformofmethane(CH4)origi-natingfromlivestocksystems(entericfermentationandmanuremanagementsys-tems,32percent)andfloodedpaddyriceproduction(8percent)(UNEPandCCAC,2020).AccordingtoFAOSTAT(2017),theglobalruminantpopulationincreasedby66percentfrom1960to2017,whereasthepopulationofnon-ruminantsincreasedevenmorerapidlyby435percentoverthesameperiod.Bothruminantandnon-ruminantpopulationsareprojectedtofurtherincrease,whichwillonlyexacerbateGHGemissions,inparticularCH4,fromlivestocksystems(FAO,2018b).Meatandmilkfromruminantlivestockprovideanimportantsourceofproteinandothernutri-entsdestinedforhumanconsumption.Althoughruminantshaveauniqueadvantageofbeingabletoconsumeforagesandgrazeonlandsnotsuitableforarablecropping,2to12percentofthegrossenergy(GE)consumedisconvertedtoentericCH4duringruminaldigestion.MoreCH4isalsoemittedinmanuremanagementsystems.Over150countriesandsupportershaveendorsedtheGlobalMethanePledge(www.globalmethanepledge.org),avoluntarycommitmentinitiatedbytheEuropeanUnionandtheUnitedStatesofAmericatocollectivelydecreaseCH4emissionsby30percentfrom2020levelsby2030.ReducingCH4by30percentwouldeliminateover0.2°Cofaverageglobaltemperatureincreaseby2050.DuetotherelativelyshortlifeofCH4intheatmosphereanditshighglobalwarmingpotential,decreasingCH4emissionsisseenasarapidwayofhelpingtolimitglobalwarmingto1.5°Cabovepreindustriallevels.TheFAOLivestockEnvironmentalAssessmentandPerformancePartnership(FAOLEAPPartnership)commissionedthisreport,whichwasdevelopedbyaninternationalgroupofscientistsandexpertsworkingonthesourcesandsinksofCH4,thequantificationofCH4emissions,andrelatedmitigationsandclimatemet-rics.ThereportaimstoprovideacomprehensivereviewandanalysisofCH4sourcesandsinksinagrifoodsystems,existingmitigationsolutionsandthosethatareatanexperimentalstage,andmetricsusedtoquantifytheimpactsofCH4emissions1Methaneemissionsinlivestockandricesystemsonclimate.InthecontextoftheGlobalMethanePledgeandtheParisAgreementgoals,thisreportprovidescomprehensivescientificinformationthatcanbeusedbydifferentstakeholders–includingthepublic,theprivatesector,non-stateentitiesandproducers’organizations–todesignandimplementtechnicalmitiga-tionstrategiesandprogrammesaimedatcuttingCH4emissionsinlivestockandricesystems.Italsocontainsusefulinformationdesignedtofacilitatepolicyworkandenhancenationalclimateactions.ThereportcomplementsthepreviousFAOLEAPguidelineswithdetailedinformationneededtoconductmitigationscenarioanalysis,usingthehighesttierfromtheIPCCguidelines.Thiswillcontinuouslyimprovetheaccuracy,transparency,consistency,comparabilityandcompletenessoftheinventoryofgreenhousegases,includingCH4,aswellasthemonitoringofmitigationprogrammesinlivestock.Thereportisdividedintofourparts:•Part1:Sourcesandsinksofmethaneemissionsfromfoodandagriculture•Part2:Quantificationofmethaneemissions•Part3:Mitigationofmethaneemissions•Part4:MetricsforquantifyingtheimpactofmethaneemissionsPart2waspublishedasTedeschietal.2022.Quantificationofmethaneemit-tedbyruminants:Areviewofmethods.JournalofAnimalScience,100(7):1-22,https://doi.org/10.1093/jas/skac197.Part3waspublishedasBeaucheminetal.2022.Invitedreview:Currententericmethanemitigationoptions.JournalofDairyScience,105(12):9297-9326.https://doi.org/10.3168/jds.2022-22091.2PART1Sourcesandsinksofmethaneemissionsfromfoodandagriculture1.SourcesofmethaneTheprovisionofqualityhumanfoodinthecontextofagrowingworldpopulationandtheneedforsustainablefoodproductionsystemsisamajorchallenge.Indeed,by2050theglobaldemandforanimalproductsisprojectedtoincreaseby60to70percent,withdevelopingcountriesaccountingforthemajorityofthisincrease(Makkar,2018).Globalwarmingasaconsequenceofanthropogenicemissionsofgreenhousegases(GHGs)hasbecomeamajorchallengetohumanityinrecentyears.AgriculturallyderivedGHGemissions,andinparticularmethane(CH4),primarilyresultfromentericfermentationofruminantlivestockand,toalesserextent,storageofmanure.Thelivestocksectoristhelargestland-usesystemonearth,occupyingbetween30(Herreroetal.,2013)and60percent(Manzano,2015)oftheworld’sice-freesurface.Livestocksupplychainsareestimatedtoaccountfor14.5percentoftotalhuman-inducedGHGemissions(Gerberetal.,2013a),anditisestimatedthatabout80percentoftheGHGemissionsfromlivestockand90percentofCH4emissionsisderivedfromruminantlivestock(Scholtz,NeserandMakgahlela,2020).Theworldruminantpopulationincreasedby66percentfrom1960to2017,whereasthepopulationofnon-ruminantshasincreasedevenmorerapidlyby435percentoverthesameperiod(FAOSTAT,2017).Bothruminantandnon-ruminantpopulationsareprojectedtofurtherincrease,whichwillfurtherexacerbateGHGemissionsfromanimalagriculture.Meatandmilkfromruminantlivestockpro-videanimportantsourceofproteinandothernutrientsforhumanconsumption.Althoughruminantshaveauniqueadvantageofbeingabletoconsumeforagesandgrazeonlandsnotsuitableforarablecropping,2to12percentofthegrossenergy(GE)consumedisconvertedtoentericCH4duringruminaldigestion,depend-ingonthetypeoffeedoffered,contributingtoapproximately6percentofglobalanthropogenicGHGemissions(Beaucheminetal.,2020).1.1RUMINANTLIVESTOCKANDENTERICMETHANOGENESISThemajorityofCH4emissionsfromtheagriculturalsectorareaconsequenceofmicrobial-mediatedentericfermentativeprocessesinruminantlivestock.Amongruminants,thehighestdailyemittersonaperanimalbasisarecattle,followedbysheep,thengoatsandbuffalo,whichhavesimilaremissions(Seijanetal.,2011).EmissionsofGHG,includingCH4,producedbybothlargeherbivorousnon-ruminantsandthesizeablepopulationofsmallfarmanimals,suchasswine,remainsubstantial(Patra,2014).Indeed,Claussetal.(2020)statedthatCH4emissionsfromsomenon-ruminants,whenexpressedintermsofintensity,remaincompa-rabletothoseofruminants.Misiukiewiczetal.(2021)recentlyproducedacompre-hensivereviewofmethanogenslivinginthegastrointestinaltract(GIT)ofvariousnon-ruminants,suchasswine,horses,donkeys,rabbitsandpoultry.EntericCH4emissionscanvarysubstantiallybetweenanimals,evenwithinthesamespecies,andthereisincreasingevidencethathostgeneticsplayanimportantroleinthis(seeSection1.2).Notwithstanding,factorssuchasthechemicalcom-positionofthediet,theleveloffeedingabovemaintenance,andtheinclusionofcertainfeedadditiveshaveagreaterinfluenceonindividualanimalemissionsthan5Methaneemissionsinlivestockandricesystemsgeneticmakeupperse.Indeed,themanipulationofenvironmentalfactorscanbeharnessedasmitigationstrategiestoreduceCH4emissions(seePart3).Therumenisacomplexecosystemcomposedofbacteria,fungi,protozoa,archaeaandbacteriophages,allofwhichcontributetodietaryenergyharvestingandresultantnutrientsupplytothehost(Abbottetal.,2020).Thesemicrobesinteractcloselytobreakdownstructuralplantcarbohydratesthatcannotbedigestedbyhumansandotheranimals,whileprovidingmetabolicenergytothehostand,inthecaseofarchaea,producingCH4(Huwsetal.,2018).MethaneisproducedfromreleasedhydrogenbeingutilizedtoreduceCO2bymethanogens,whichbelongtothedomainArchaea.Methaneproducedintherumenaccountsforupto90percentofruminantentericCH4emissions,whereasmicrobialfermentationinthelargeintestineaccountsfortheremainderofemissions.Largeintestinefermentationisalsoacharacteristicofnon-ruminantssuchasswineandhind-gutfermenterssuchashorses,whichalsoproduceCH4buttoamuchlesserextent.1.2BIOCHEMISTRYOFMETHANEPRODUCTIONINMICROBIALANAEROBICECOSYSTEMSInanaerobicenvironmentswithlowoxygenconcentrationsandlimitedmineralelectronacceptors,fermentationcanprovideGibbsenergytogenerateadenosinetriphosphate(ATP)necessaryformicrobialmaintenanceandgrowth.Fermentationisanincompleteoxidationandcarboncompoundsformedintheprocessaretheultimateelectronacceptors(Ungerfeld,2020).Thefollowingsectionfocusesonthreeanaerobicmicrobialecosystems,inwhichCH4isaprincipalelectronsink:therumen,manureandricesoils.1.2.1RumenmethanogenesisMethaneproductionisaubiquitous,apparentlyunavoidablesideeffectoffermen-tativefibredigestionbysymbioticmicrobiotainmammalianherbivores(Claussetal.,2020).Structuralandnon-structuralcarbohydratesarethemainsourceofenergyandcarbonforruminants.Intherumen,polymerssuchascellulose,hemi-celluloseandstarcharedigestedbyacomplexconsortiumofbacteria,protozoaandfungi,andtheresultingmonomersaremetabolizedintovolatilefattyacids(VFA;mainlyacetate,propionateandbutyrate),CO2andCH4asthemainfinalproductsoffermentation,withdihydrogen,formate,lactateandsuccinateasimportantelec-troncarrierintermediates(Figure1;RussellandWallace,1997;Ungerfeld,2020).Bluearrowssignaltransformationsofcarboncompounds.NotethatsmallerflowsofVFAinterconversionalsoexist(seesummarybyUngerfeldandKohn[2006]andmorerecentworkbyMarkantonatos,GreenandVarga[2008],Markantonatosetal.[2009],MarkantonatosandVarga[2017],Nolanetal.[2014]andGleason,BeckettandWhite[2022]).Purplearrowspointtosemi-reactionsofcofactorreduction.RedarrowsindicatedihydrogenproductionviaoxidationofreducedferredoxinsbyprototypicalorEchhydrogenases,orviaflavin-basedelectronconfurcation(UngerfeldandHackmann,2020).FormatemaybeformedinsteadofCO2anddihydrogenifpyruvateoxidativedecarboxylationiscatalyzedbypyruvate-formatelyases(RussellandWallace,1997).Lastly,theincorporationofmetabolichydrogeninreducedcofac-torsordihydrogen(orformate,notshown)intomethanogenesis,propionatefor-mationviarandomizing(succinate)andnon-randomizing(acrylate)pathways,andbutyrateformationaremarkedwithyellowarrows.Metabolichydrogeninreduced6SourcesandsinksofmethaneemissionsfromfoodandagricultureFigure1Mainbiochemicalpathwaysinrumenfermentation½HexoseNAD+NADH+H+PyruvateFdred2-+2H+H2CO2FdoxCH4SuccinateHS-CoALactateCO2Acetyl-CoAPropionateNADH½ButyrateAcetateSource:AdaptedfromRussell,J.B.&Wallace,R.J.1997.Energy-yieldingandenergy-consumingreactions.In:P.N.Hobson&C.S.Stewart,eds.Therumenmicrobialecosystem,pp.246–282.London,BlackieAcademic&Professional.https://doi.org/10.1007/978-94-009-1453-7_6andUngerfeld,E.M.2020.Metabolichydrogenflowsinrumenfermentation:Principlesandpossibilitiesofinterventions.FrontiersinMicrobiology,11:589.https://doi.org/10.3389/fmicb.2020.00589nicotinamideadeninedinucleotide(NADH)ataparticularpointintimemayormaynothavebeenpartofthedihydrogenpoolpriortoitsincorporationintoaparticu-larpathway,thereforeNADHshownasadirectelectrondonorinpropionateandbutyrateformationmayormaynothavebeenformedthroughreductionofoxidizednicotinamideadeninedinucleotide(NAD+)withH2inbifurcationwithferredoxin.Aswithotheranaerobicmicrobialecosystems,syntrophicinteractionsinthemicrobialcommunityarekeytorumenmetabolism.Centraltorumenfermentationisthetransferofmetabolichydrogen,inparticularastheinterspeciesdihydrogentransfer.Inglycolysisandpyruvateoxidativedecarboxylation,electronsaretrans-ferredtooxidizedcofactors(mainlyNAD+andoxidizedferredoxin;UngerfeldandHackmann,2020).Theresultingreducedcofactorsmustbereoxidizedforfermen-tationtocontinue(Wolin,MillerandStewart,1997).Cofactorreoxidationoccursmostlythroughhydrogen-evolvinghydrogenaseswhichtransferelectronstopro-tonstoformdihydrogen(Frey,2002)andformate(RussellandWallace,1997).Greeningetal.(2019)showedthepivotalroleinrumenfermentationofflavin-basedelectronconfurcationandbifurcation(BuckelandThauer,2013,2018a,2018b)intheformationandincorporationofdihydrogen.Dihydrogendoesnotaccumulateintherumenbecauseitistransferredtomethanogensandotherhydrogenotrophicmicroorganisms.Methanogensuti-lizedihydrogentoreduceCO2toCH4,whichisthemainelectronsinkinrumenfermentation.Theconsumptionofdihydrogenbymethanogenesisandotherdihy-drogen-incorporatingpathwayskeepsdihydrogenconcentrationlowandthermo-dynamicallyfavoursreoxidationofreducedcofactors,andthusallowsfermentation7Methaneemissionsinlivestockandricesystemstocontinue(Wolin,MillerandStewart,1997).Elegantexperimentsdemonstratinghowpureculturesofrumenmicroorganismsstoppedordecreasedtheproduc-tionofrumenfermentationintermediatessuchasdihydrogen,formateandetha-nolasfinalfermentationproducts,whencocultivatedwithmethanogensorotherhydrogenotrophs,illustratedtheroleoftheinterspeciestransferofdihydrogeninshapingrumenfermentation(e.g.Marvin-Sikkemaetal.,1990).Thecloseproxim-itybetweenhydrogenogensandhydrogenotrophsfavoursthekineticsofdihydro-gentransferanditsrapidutilization,asinmicrobialbiofilms(Leng,2014)andinprotozoal-methanogensymbiosis(Newboldetal.,2015).Methaneisgenerallythemostimportant,butnottheonlyelectronsinkinrumenfermentation.Propionateformationfromcarbohydratesviatherandomizingandnon-randomizingpathwaysresultsinanetuptakeofmetabolichydrogen.Butyrateformationfromcarbohydratesreleasesmetabolichydrogen,althoughtherearetworeactionsincorporatingmetabolichydrogenintheconversionofacetyl-CoAtobutyrate:thereductionsofacetoacetyl-CoAtoβ-hydroxybutyryl-CoAandofcro-tonyl-CoAtobutyryl-CoA(UngerfeldandHackmann,2020).Microbialbiomass,amorereducedthanfermentedsubstrate,constitutesanotherelectronsink.Mineralelectronacceptorssuchasnitrateandsulfatethermodynamicallyoutcompetemetha-nogenesis,yettheiravailabilityinmostdietslimitsmetabolichydrogenincorpora-tionintheirreduction(Ungerfeld,2020).Reductiveacetogenesis–thereductionofCO2withdihydrogentoacetateandwater–wasconsideredthermodynami-callyunfeasibleintherumen(UngerfeldandKohn,2006),andyetmorerecentfindingshaverevealedittobeaminorelectronsink(Raju,2016).Thepresenceofgenes(Denmanetal.,2015)andtranscripts(Greeningetal.,2019)ofhydrogenasesinvolvedinreductiveacetogenesishasalsobeenreportedtooccurintherumen.MostrumenCH4isproducedthroughthereductionofCO2withdihydrogen(Hungate,1967),inwhichformateisthesecondelectrondonorinimportance(Hungateetal.,1970).FormatemustbeoxidizedtoCO2anddihydrogenbyarchaeaorbacte-riabeforedihydrogenreleasedfromformateoxidationservesasanelectrondonorformethanogenesis(Thaueretal.,2008).Apartfromhydrogenotrophicmethanogenesis,methylotrophicmethanogenesisalsousesassubstratesmethanol,methylaminesandmethylatedsulphurcompounds,whichcanaccumulateintherumenfollowingtheingestionofsomediets,forexamplethosecontainingpectin(Söllingeretal.,2018).Theproductionofacetateand,toalesserextent,butyrateleadtothenetreleaseofmet-abolichydrogenandtheresultingformationofdihydrogen.Acetateproductionisthusassociatedwithmethanogenesis.Thereplacementofroughageswithconcentratestypi-callydecreasesCH4formedperunitoffermentedorganicmatter(OM)–notnecessarilythetotalamountofCH4produced,astheintakeofrumen-fermentedOMoftenincreaseswhenfeedingconcentrates–andshiftsrumenfermentationfromacetatetopropionate.AmechanismexplainingthisfermentationshifthasbeenproposedbyJanssen(2010),drawingontheMonodmodelofmicrobialgrowth,whichrelatesactualandtheoreticalmaximalmicrobialgrowthratetotheconcentrationofthesubstratemostlimitingforthegrowthofmicroorganisms–dihydrogeninthecaseofmostrumenmethanogens,sincetheyarehydrogenotrophs.Asconcentratesformanincreasingpartofaruminant’sdiet,thepassagerateincreases,resultinginanincreasedgrowthrateofmethanogensthatarenotwashedoutandcontinuetoproduceCH4.Agreatergrowthrateinturnleadstoanelevateddihydrogenconcentration,accordingtotheMonodfunction.Likewise,feedingruminantsconcentratesgenerallycausesrapidfermentationandrumenpHdecreases,8Sourcesandsinksofmethaneemissionsfromfoodandagricultureresultingindecreasedtheoreticalmaximalgrowthratesofmethanogens.Consequently,thereisanincreaseintheconcentrationofdihydrogen,whichinturnthermodynami-callyinhibitsacetateandfavourspropionateproduction(Janssen,2010).Similarly,whenmethanogenesisisinhibitedbychemicalcompounds,themaxi-malgrowthrateofmethanogensdecreasesanddihydrogenaccumulates(Janssen,2010).Inthisregard,itisworthbearinginmindthattheinhibitionofmethano-genesisisnotanisolatedinterventioninrumenfermentationandthatitwillhaveprofoundconsequencesontheflowsofmetabolichydrogen.WhenstrategiestomitigateCH4emissionsthroughtheuseofchemicalinhibitorsareconsidered,inhibitingCH4productionshouldthereforenotbeviewedasthesoleobjectiveoftheintervention;anotherapproach,onewhichwouldredirectmetabolichydrogentowardspathwaysthatcouldbenefitthenutritionofthehostruminantanimal,mightbesought.Forexample,dependingonthetypeofphysicochemicalcontrol,itmaybepossibletochannelpartofdihydrogentypicallyaccumulatingwhenmetha-nogenesisisinhibitedtoVFAproductionbyaddingelectronacceptorsthatareintermediatesofVFAformationorspecificmicrobialadditives(Ungerfeld,2020).1.2.2ManureAnaerobicdigestion(AD)ofanimalmanureandotherorganicwastesintoCO2andCH4alsoinvolvescomplexmetabolicinteractionsbetweenmicrobialgroups.Whilethephysicochemicalprinciplescontrollingbothsystemsarethesame,con-ditionssuchastemperature,fractionaloutflowratesandtypesofsubstratesdif-fer,resultinginsomedifferences.Anaerobicdigestionstartswiththehydrolysisofcomplexcarbohydratessuchascelluloseandhemicelluloseintomonosaccharides(Figure2).MonosaccharidesarethenfermentedtoVFAandalcohols,whicharesubsequentlyoxidizedtoacetate,CO2anddihydrogen.Finally,acetateandmethyl-containingone-carboncompoundsarereducedtoCH4byacetoclasticandmethy-lotrophicmethanogensrespectively,andCO2isreducedwithdihydrogenorfor-matetoCH4byhydrogenotrophicmethanogens.AcetateisalsooxidizedtoCO2anddihydrogen,whichserveassubstratesforhydrogenotrophicmethanogenesis.Ifpresent,sulfateandnitratealsoserveaselectronacceptors(Alvaradoetal.,2014;Ferry,2015)andthermodynamicallyoutcompetemethanogenesis,providedtheyarepresentathighconcentration.Hydrolysisofcarbohydratesisarelativelyslowprocesscarriedoutbyaverydiversegroupofbacteria.Inbiodigestersfedwithcattlemanure,bacteriafer-mentinghydrolysedmonomerspredominantlybelongtothegeneraClostridium,EubacteriumandBacteroides(Alvaradoetal.,2014).Biochemicalreactionsinanaer-obicdegradationareclosetothermodynamicequilibrium,andsyntrophyiscrucialtokeepconcentrationsofreactionproductslowandchemicalprocessesthermody-namicallyfeasible(Schink,2002).ImbalancebetweenfermentationandsyntrophycanresultinincreasedconcentrationofVFAandacidification,whichinhibitsfer-mentation.FortheoxidationofVFAlongerthantwocarbonstoacetatetobether-modynamicallyfeasible,theconcentrationofdihydrogenhastobekeptverylow,whichrequiresfunctionalmethanogenesis(Schink,2002;Ferry,2011;Alvaradoetal.,2014).Alowconcentrationofdihydrogen(aswellasapHoflessthan7andhightemperatures)isalsonecessaryforbacterialhomoacetogenstodissociateacetateintoCO2anddihydrogen,insteadofconductingthereverseprocess,reduc-tiveacetogenesis(Thaueretal.,2008).9MethaneemissionsinlivestockandricesystemsFigure2SimplifiedschemeofthemainpathwaysofanaerobicdigestionPolysaccharidesMonosaccharidesAlcoholsCO2+H2Volatilefattyacids≥3CCH4AcetateSource:AdaptedfromFerry,J.G.2015.AcetatemetabolisminanaerobesfromthedomainArchaea.Life,5(2):1454–1471.https://doi.org/10.3390/life5021454Thestabilityofanaerobicdigestionisthereforesensitivetoitslaststep,methanogen-esis.Methanogensarelessdiversethanothermicrobialgroupsandhighlyspecialized.MethanogenordersMethanobacteriales,MethanomicrobialesandMethanosarcinalesarefoundinanaerobicdigesters.MethanobacterialesandMethanomicrobialesusedihy-drogenasanelectrondonor,alongwithformate,ethanolandisopropanolinsomespe-cies.ExceptforthegenusMethanosphaera(Methanobacteriales),MethanobacterialesandMethanomicrobialescannotuseacetateasasubstrateformethanogenesis.Methanosarcinalescanalsousemethanol,methylaminesandothermethylatedcom-pounds,withthefamilyMethanotrichaceae(formerlyMethanosaetaceae)includingacetoclasticmethanogens(Alvaradoetal.,2014;Conrad,2020a).Thedifferentone-carbonreductionpathwaysinmethanogenesisfromvarioussubstrateshavetheirlaststepincommon,thereductionofmethyl-coenzymeMtoCH4.Inthecaseofacetoclasticmethanogenesis,themethylgroupinacetyl-CoAistransferredtocoenzymeMbymethyltetrahydromethanopterinormethyltetrahy-drosarcinapterin.Thecarbonylgroupinacetyl-CoAdonatesviacoenzymeBtheelectronpairnecessaryfordemethylatingmethyl-coenzymeMandproducingCH4(Ferry,1999,2015).GenerationofATPinmethanogenesisiscoupledwithtransmembraneelectro-chemicalgradients.MethanogenspossessingcytochromescangeneratemoreATPpermoleofCH4producedthanthosewhichdonot.However,methanogenswithcytochromeshaveagreaterhydrogenthresholdandtheycannotgrowataverylowhydrogenconcentration(Thaueretal.,2008).SynthropicmethanogenicfermentationofVFAtoCH4isassociatedwithaGibbsenergyvalueofclosetozero,whichallowsforlittleATPgeneratedforanabolicprocessesandveryslowmicrobialgrowthrates(Schink,2002).Thisexplainswhyacetateandlong-chainVFAarenotmetabolizedto10SourcesandsinksofmethaneemissionsfromfoodandagricultureCH4intherumen,asturnoverratesaremuchfasterintherumenthanisthecaseforanaerobicdigesters,whererumenorganismsneedtogenerateATPatahigheryieldandachieveafasterrateofgrowthtomatchrumenoutflowrates.1.2.3SoilRiceisamajorcropforthehumanpopulationglobally,oneforwhichthereisanincreasingdemand.ItisthereforeimportanttounderstandthemechanismsbehindCH4productionandoxidationinricesoilstodeviseCH4mitigationstrategiesforthecultivationofthiscrop(Liesack,SchnellandRevsbech,2000).Knowinghowtocontrolmajorflowsofcarbonandmetabolichydrogencanhelpdesignmoreappropriateinter-ventionsandcultivationpracticesaimedatmitigatingtheemissionsofCH4fromsoils.Theavailabilityofoxygeninthesoilisgreatlyaffectedbythedegreeofsoilwatercontentorsaturationlevel.Thepresenceofoxygenandotherelectronacceptors,car-bonsubstrates,water,redoxpotentialandpHhasanimpactonmethaneproductioninsoils.Thissectionwillfocusmainlyonricefieldsoils,whichareseasonallyflooded,alternatingoxidizingandreducingconditions.Ricepaddiesareestimatedtocontri-buteto5percentoftotalanthropogenicCH4emissions(Knief,2019).SimilartootheranaerobicenvironmentsinwhichCH4isapredominantelectronsink,anaerobicdegradationinsoilsisconductedbyacomplexmicrobialcommunityofferment-ingbacteriaandmethanogenicarchaea(Conrad,2020b);apartfrommethanogenicarchaea,soilfungihavealsobeenreportedtoproduceCH4frommethioninemetabo-lism(Knief,2019).Inaddition,asinotheranaerobicenvironments,thedegradationofpolymers,mainlypolysaccharidessuchascelluloseandhemicellulose,isthefirststeptoreleasingfermentablemonomers.Ricestrawisploughedunderthesoilafterharvest,thussettingthedegradationofpolysaccharidesinmotion(Liesack,SchnellandRevsbech,2000).Between80and90percentofricestrawisdegradedwithinthefirstgrowthseason(Conrad,2020a).TheamountofCH4emittedfromricestraw–eitheraftersoilincorporationorfromopenfieldburning–willdependonthechosentypeofricemanagementonthefarm(seeSection7).Moreover,OMprovidedbytherootsofriceplantsisalwaystheprimarycarbonsourceofCH4producedinricefieldsoil(Kimura,MuraseandLu,2004).Afterricepaddiesareflooded,oxygenisrapidlyconsumedbyaerobicbacteriaandabioticchemicalreactions.Immediatelyafterflooding,ahighconcentrationoftheoxidizedformsofinorganicoxidantscanmaintainreductantssuchasdihydrogenandacetateattoolowaconcentrationformethanogenesistobethermodynamicallyfeasible(Conrad,2020b).Organicmatterissequentiallyoxidizedbyavailableelectronacceptorsbasedontheirredoxpotential:nitrate>manganeseoxide>ferriciron>sulfate>CO2.Differencesintheredoxpotentialofeachelectronacceptor,includingoxygenatoxicinterphases,giverisetomicroscalespatial-temporalchemicalgradientsofaerobicbacteria,nitratereducers,manganesereducers,ironreducers,sulfateredu-cers,andfermentingbacteriaandmethanogens.Nitrateisformedintheoxicpartsofthericepaddy,suchaswaterandthewater-soilinterphase,fromammoniumreleasedfromureawhichisaddedtothesoilasafertilizer.Nitratecanbereducedtodinitrogen,nitrite,nitrousoxide(anothergreenhousegaswhichhashigherGWPthanCO2)orammonia.Inricesoils,ironcontentisusuallyhighenoughtopreventtheaccumula-tionofhydrogensulfide(Liesack,SchnellandRevsbech,2000).Importantly,reducedinorganicionsarereoxidizedwhenthesoilisaerated,orifastronginorganicelectronacceptorisaddedtothesoil(forexample,addingnitratewillregenerateferricironand11Methaneemissionsinlivestockandricesystemssulfate;Conrad,2020b).Also,thereoxidationofreducedelectronacceptorsoccursatthewater-soilandthesoil-rhizosphereinterphases.Thedepthandtheconcentra-tionofoxygenintheseoxicinterphasesincreasesduringthedaywithphotosynthesis(Liesack,SchnellandRevsbech,2000).Oncealloftheinorganicelectronacceptorsarereducedtosuchanextentthateachprocessreachesathermodynamicequilibrium,anaerobicdegradationtoCO2andCH4proceedsthroughfermentation,andacetoclasticandhydrogenotrophicmethanogenesis(Figure2andFigure3).Ligninandxylanscanalsocontributemethanol,whichcanserveasaminorsubstrateformethanogenesis,particularlyconsideringthatligninanaerobicdegradationisslowandincomplete(Benner,MacCubbinandHodson,1984).Thedegradationofricestrawreleasesphenyla-cetateandphenylpropionateasminorproducts,whicharedegradedexclusivelyviahydrogenotrophicmethanogenesis,astheyaremetabolizedtobenzoate,CO2anddihydrogen,butnotacetate.Whilethedegradationofricestrawprogresses,theproportionofCH4producedviahydrogenotrophicmethanogenesisincreasesandacetoclasticmethanogenesisdecreases,asmorerecalcitrantOMisdegradedtoCO2anddihydrogenwithnoacetateisbeingproduced(Liesack,SchnellandRevsbech,2000;Conrad,1999,2020a,2020b).Asinotheranaerobicenvironments,dihydrogenturnoverisveryhigh.Alowcon-centrationofdihydrogenapproachingthethermodynamicthresholdofmethano-genesismakesdihydrogen-releasingreactionsthermodynamicallyfeasible(Conrad,1999).MicroorganismsinvolvedinthedifferentphasesoftheOMoxidationinanaer-obicdegradationmustaccommodatetheiractivitiestothethermodynamicfeasibil-ityofchemicalprocesses,buttheycontributesignificantlytoacceleratingthekinet-icsofthoseprocesses.Theoretically,theanaerobicdegradationofcellulosewouldresultinequimolaramountsofCH4andCO2aswellasmorethantwo-thirdsofCH4formedfromacetateandlessthanone-thirdfromhydrogenotrophicmethanogenesis.However,theproductsofdegradationcanbemodifiedbyacetateoxidationfollowedbyhydrogenotrophicmethanogenesisandacetateoxidationbysoilorganiccom-pounds,andbyreductiveacetogenesis.Otherorganicandinorganicelectrondonors,acceptorsandcarriersalsofurtherinfluencethestoichiometryofthefinalproductsofanaerobicdigestioninsoils(Conrad,1999,2020a,2020b).Temperatureinfluencesthepredominantsubstratesformethanogenesis,witharelativeincreaseinacetateandadecreaseindihydrogenatlowtemperatures(ChinandConrad,1995;Conrad,2020a).Furthermore,atlowtemperatures,acetateproductionincreasesrelativetoCO2and,asaconsequenceofreductiveacetogenesis,dihydrogenisfavouredoverhydrogenotrophicmethanogenesis.Hydrogenotrophicmethanogensbegintobeoutcompetedbyreductiveacetogensatlowtemperaturesbecausebacterialesterlipidsaremorefluidthanarchaealetherlipidsatlowtemperatures,andbecausedihydrogenproductionandacetateoxida-tionarethermodynamicallylessfavourable(Conrad,2020a).DecreasesinsoilpHalsonegativelyimpactmethanogenesis,althoughtheratioofhydrogenotrophictoacetoclasticmethanogenesisisnotaffected(Conrad,2020a).Soilmethanogenesisisstronglyinhibitedbyoxygen,andyetsoilmethanogenshaveevolvedtoadapttosucceedingeventsoffloodinganddesiccation,andcantole-ratethepresenceofoxygen,despitenotformingsporesorcysts.Methanogenshavegenerallybeenfoundtodecline,butnotdisappear,withsoildesiccation(Conrad,2020b).Methanogenesishasbeennotedtooccureveninanoxicmicronichesinoxic12Sourcesandsinksofmethaneemissionsfromfoodandagriculturesoils.MethanogensbelongingtotheordersMethanocellales,MethanomicrobialesandMethanosarcinaleshavebeenreportedtocarrygenesinvolvedinresistancetooxygen(Knief,2019).Inthisregard,soilmethanogensmaydifferfrommethanogensfoundinsuchenvironmentsastherumenoranaerobicdigesters,whichliveundermorestable,anoxicconditions.Naturalwetlands,landfillsandricepaddiesallcontributetoCH4emissionsintheatmosphere,butbacterialmethanotrophsinwell-aeratedsoilsoxidizeabout4per-centofatmosphericCH4.TheactivitiesofCH4-cyclingmicroorganismsdeterminethenetproductionorconsumptionofCH4insoils(Knief,2019).Inmostdrysoils,however,theatmosphericconcentrationofCH4istoolowtoinduceaerobicCH4oxidizingactivity(Conrad,2020b).Andyetaerobicmethanotrophicbacteriasituatedinoxic/anoxicinterphasescanoxidizeupto80percentofsoil-producedCH4beforeitisreleasedintotheatmosphere(Knief,2019).Thepresenceofoxygenintherhizo-sphereofriceorotheraquaticplantsenablesCH4oxidationtooccur,especiallyduringdaytimewhentheextensionoftheoxicinterphasesincreasesduetophotosynthesis.Notwithstandingthis,muchCH4escapesintotheatmosphereasbubblesandaboveallthroughtheplants’aerenchyma(Liesack,SchnellandRevsbech,2000).AnaerobicCH4oxidation,conductedbybothbacteriaandarchaea,canalsoelim-inatesubstantialamountsofCH4formedinsomesoilsbeforeitreachestheatmo-sphere.Methaneoxidationcoupledwithsulfatereductionisimportantinmarinesediments,butitmayalsobeimportantinterrestrialsoilssubjectedtocyclesofsulphurreductionandoxidation.Nitrateandnitrite,ferricironandmanganesecanalsoactaselectronacceptorsinCH4oxidation(Knief,2019).Figure3MethanedynamicsinfloodedricesoilCH4CCHH44FloodingwaterOxidationOxiclayersoilAnoxiclayersoilCH4CO2DiffusionEbullitionOxidationOrganiccarbonCH4CO2(e.g.plantresidue,riceroots)AerenchymaltransportCH4AnaerobicAcetatedecompositionMethano-CO2,H2genesisSource:Authors’ownelaboration.13Methaneemissionsinlivestockandricesystems1.3METHANEEMISSIONSDURINGTHESTORAGEOFMANUREMethaneemissionsfrommanuremanagementareanimportantcontributortotheGHGbudgetforthefarmsandtheagriculturalsector(Cluettetal.,2020).Manuremanagementfromlivestock(ruminantsandnon-ruminants)hasbeenestimatedgloballyat2.52GtCO2eq,themainsourceofemissionbeingmanurestorageandparticularlyliquidmanurestoragewhereanaerobicconditionsaremaintained.Ruminantmanurecontributed2.3GtCO2eqandswinemanure0.2GtCO2eq;thetotalCH4productionfromlivestockmanurewasestimatedat17.5milliontonnesperyear,incomparisonto85.6milliontonnesperyearofentericCH4(Steinfeldetal.,2006).IntheUnitedStates,theEnvironmentalProtectionAgency(EPA,2006)estimatedtheCH4emissionfrommanureatbetween470and523megatonnes(Mt)CO2eqperyear.FortheEuropeanUnion,manureCH4emissionsrepresent44MtCO2eq(Eurostat,2018).Methaneisproducedunderanaerobicconditionbyarchaeamainlyinstorageconditions,usingtheOMpresentinanimalexcreta.Hence,theproductionofCH4frommanuremainlyoccursinslurryandliquidmanure.Chianese,RotzandRichard(2009)indicateanaverageCH4emissionfromcoveredslurryof6.5kg/m3peryear,whichisreducedinuncoveredslurryto5.4kg/m3,whiletheemissionsfromstackedmanureareestimatedat2.3kg/m3andvarywithbothambienttem-peratureandtimeinstorage(Hristovetal.,2013b).ThemagnitudeofCH4emissionsgeneratedduringstorageofmanuremainlydependonthedurationofmanurestorage,thestoragesystemused,temperatureandmanurecomposition(Dennehyetal.,2017;PhilippeandNicks,2015).Forinstance,Petersenetal.(2013b)foundthatthecumulativeCH4emissionsfromstoredpigmanureinsummerwereover100timesgreaterthanthoseinwinter;however,thisdependsonthegeographicalcontext.EmissionsofN2Otypicallyrangefromlessthan1to4.3percentofthetotalnitrogeninstoredcattleandpigfarmyardmanureheaps,butemissionsashighas9.8percenthavebeenreported(Chadwicketal.,2011).1.4METHANEEMISSIONSFOLLOWINGTHEAPPLICATIONOFMANUREWhilethevastmajorityofmanure-derivedCH4emissionsemanatefromstoredmaterial,therehasbeensomeinterestinquantifyingemissionsfollowingapplica-tiontosoil.Bourdinetal.(2014)investigatedtheimpactofslurrydrymatter(DM)content,theapplicationtechniqueandthetimingofapplicationontheoverallGHGbalancefromcattleslurryappliedtograsslandsoils.Thetreatmentsonplotswereacontrol,calciumammoniumnitrateandcattleslurryderivedfromeithergrass-basedormaize-baseddiets,withvaryingDMcontents,spreadbymimickingtrailing-shoeandsplash-plateapplications.ThestudyvariedtheDMcontentsbymixingdifferentratiosoffaecesandurine.Althoughammonia(NH3)volitalizationlossesweresubstantiallyincreasedonplotsspreadwithslurry,thecumulativedirectN2Oemissionsandcorrespondingemissionfactorsweresignificantlyhigherwhencalciumammoniumnitratehadbeenapplied.IntermsofGHGfieldbalance,thepotentialdecreaseinindirectN2Oemissions,calculatedfromareductionofNH3volitalizationlossesusingtrailing-shoeasopposedtosplash-platemethods,couldbeeasilyoffsetbyanincreaseindirectN2Oemissionsandecosystemrespiration.SwitchingfromsummertospringapplicationwasmuchmoreefficientinmitigatingbothNH3andGHGemissions,duetofavourablesoilandclimaticfactors,which14Sourcesandsinksofmethaneemissionsfromfoodandagricultureenhancedcropgrowth.Theauthorsconcludedthatanypotentialtrade-offbetweenNH3andN2Oemissionswascancelled,resultinginanoverallpositiveeffectonreactivenitrogenlossesandinagronomicbenefitsforfarmers.However,arecentanalysisofalargedatasetofCH4fluxesfromagriculturalsitesacrossIrelandandtheUnitedKingdomofGreatBritainandNorthernIrelandindicatethatthesesoilsaresmallnetemittersofCH4ratherthansinks,withfluxesoccurringfollowinganimalmanureapplicationsespecially(Cowanetal.,2021).TherehavebeenmanyreportedstudiesofN2Oemissionsfollowingmanurespreading.Emissionfactors(i.e.cumulativeN2O-NlossasaproportionoftotalNappliedtomanure)canrangefromlessthan0.1to3percent.Higheremissions(of7.3to13.9percent)havebeenmeasuredduringlandapplicationofpigslurry(Velthof,KuikmanandOenema,2003).TherangeinN2Oemissionfactorsfollow-ingslurryandsolidmanureapplicationsreflectsdifferencesinsoiltype,soilcondi-tions(temperatureorwater-filledporespace),manurecomposition(NH4+-N,Ccontentandform)andthemeasurementperiod.OtherinvestigatorshaveexaminedthepotentialCH4oxidizingcapacityofsoils,particularlyasregardsremovingCH4emissionsfrommanureanddigestateslurry.Themagnitudeoftheeffectdependsonthesoil’schemicalandphysicalpropertiesthatframethelivingconditionsofthemethanotrophicbacteria,aswellasonthetimeandintensityofexpositiontoCH4(Oonketal.,2015).1.5TRADE-OFFBETWEENGHGANDOTHERGASEOUSEMISSIONSAccordingtoareviewofCH4emissionfactorsbyO’BrienandShalloo(2016),severalcountriesrecognizethatCH4emissionsfromcattleandlivestockmanurearelinkedtootherGHGemissionsfrommanure,suchasN2O.Somecountriesuseaprocess-basedmodel,onethatsimultaneouslyquantifiesGHGandNH3emis-sionsfromlivestock,forconsistency.ReducedNH3lossesfrommanurespreadingarelikelytoincreaseNavailabilityinagriculturalsoilsandthis,inturn,mayaffecttheproductionandreleaseofN2O(Brink,KroezeandKlimont,2001).AsN2OisamuchmorepotentGHG(IPCC,2007),thiscouldberegardedaspollutionswap-ping(StevensandQuinton,2009)sinceattemptstoabatethereleaseofoneecologi-callyharmfulgasresultinanincreaseintheemissionsofanother.AnintegratedassessmentoftheeffectsofmitigationmeasuresonNH3,CH4and(directandindirect)N2Oemissionsiswarrantedacrossthewholemanuremanage-mentchain.Intheirmeta-analysis,Hou,VelthofandOenema(2015)foundthatloweringthecrudeprotein(CP)contentoffeedandacidifyingslurryarestrategiesthatconsistentlyreduceNH3andGHGemissionsinthewholechain.1.6SPATIO-TEMPORALVARIATIONINMETHANEEMISSIONSReliable,high-resolutionspatio-temporalinventoriesofCH4emissionfromlive-stockproductionsystemsarerequiredtoensureanaccurateandequitablenationalinventorypreparation.Forexample,thebalancebetweenentericandmanureemis-sionswillbeinfluencedbyseason(particularlyforpastoral-basedproductionsys-tems),theprevalentlivestockspecies,andthevectoralzonationofproductionsys-tems(forbothpastoralandmixedsystems).Asaconsequenceofthetemporalandspatialvariabilityofemissionsfromlivestockhousingandmanuremanagement,measurementsandmonitoringthatcoverboththedailyandseasonalvariationsinemissionsneedtooccuroverlongerperiodsoftimeinordertoaccuratelyreflect15Methaneemissionsinlivestockandricesystemsannualemissions(NASEM,2018).Herreroetal.(2015)highlightedtheproblemsassociatedwithquantifyingemissionsfromlivestockproduction.Theauthorsemphasizethelargespatialvariationsofemissionsduetodifferencesinsoiltype,climaticparametersandwaterconditions,orindeedvariedsoilfertilizationaswellasmanuremanagementpracticesandcomposition.Adetaileddiscussiononquan-tificationmethodsispresentedinSection2.Inaddition,theconditionsformanurearefarlesswellcontrolledthaninthecaseofentericemissions,wherethephysi-ologicalregulatorymechanismsoftheruminantareinplace.1.7CONTRIBUTIONOFHUMANFOODANDANIMALFEEDWASTETOMETHANEEMISSIONSFoodwastageisaglobalissueintrinsicallylinkedwiththegrowingchallengesoffoodsecurity,resourceandenvironmentalsustainability,andclimatechange.Indevelopedeconomies,thelargestfoodwastagestreamoccursintheconsumptionstageattheendofthefoodchain.Du,AbdullahandGreetham(2018)pointoutthat,historically,livestockanimalsfunctionedasbioprocessors,convertingmateri-alsinediblebyhumansintonutritiousmeat,eggsandmilk.Theauthorsconsiderthatcontemporarytreatmenttechnologiescanassistinconvertingfoodintosafe,nutritiousandvalue-addedfeedproducts,insteadofwastingit,anddeemrecover-ingunconsumedfoodforanimalfeedingtobeaviablesolutionthatsimultaneouslyaddressesthereductionoffoodwaste,foodsecurity,resourceconservation,andpollutionandclimate-changemitigation.Thereductionandsustainablemanagementoffoodwastearefundamentaltenetsofthecircularbioeconomyconcept.Globally,around1.3×109tonnesoffoodwastearedisposedofinlandfills(Hao,KarthikeyanandHeimann,2015).About13.8per-centoffoodproducedin2016waslostfromfarmtofork,excludingtheretailandhouseholdstagesoftheglobalfoodsupplychain(FAO,2019).Inaddition,theglobalannualgenerationoffoodlossandwasteisestimatedat4.4GtCO2eq,orapproxi-mately8percentoftotalanthropogenicGHGemissions(Maketal.,2020).Therecy-clingoffoodwastethatcouldnotbereducedbyamoreefficientfoodsupplychainandinformedconsumerbehaviourprovidesemployment,reducesGHGemissions,decreasesdisposalcosts,mitigatesthenegativeenvironmentalimpactsandsupportssustainablewastemanagementpracticesthatfallunderthebiocirculareconomycon-cept.Comparedwithtraditionaldisposalmethods(i.e.landfilling,incinerationandcomposting),anaerobicdigestionfollowedbyCH4usageasbiogasisapromisingtechnologyforfoodwastemanagement,butithasnotyetbeenfullyappliedduetoanumberoftechnicalandsocialchallenges(Xuetal.,2018).Indeed,intheUnitedStates,lessthan2percentoffoodwasteisanaerobicallydigested.Themanagementoffoodwastethroughbiologicalprocessingisamoreenvironmentallysustainableapproachthanthermo-chemicalconversionorlandfilling.ThecompositionandCH4generatingthepotentialofsomecommonfoodwastestreamshasbeensummarizedbyXuetal.(2018).However,thecomposition,andthephysico-chemicalandbiologicalcharacteristicsoffoodwaste,canaffecttheoverallbiologicalprocesswhenitcomestoproductyieldanddegradationrate.Thepretreatment(i.e.grindingordrying)offoodwasteaheadofanaerobicdigestionhasbeenproposedinordertoovercomethismajorbottleneckinthesystem.Codigestingfoodwastewithmanure,sewagesludgeandlignocellulosicbiomasscouldbebeneficialduetothedilutionoftoxicchemicals,enhancedbalanceofnutrientsandsynergisticeffectsofmicroorganisms.16SourcesandsinksofmethaneemissionsfromfoodandagricultureTherearemajorvariationsinlegislationaddressingtheinclusionofunconsumedhumanfoodintothedietsoflivestockbetweendifferentcountriesandcontinents.Thesevariationsrangefromreducingthefoodwasteenteringlandfillstofearsoverthehealthimplicationsofitsinclusioninlivestockdiets.1.8ANAEROBICDIGESTIONMethanegashasbeenidentifiedasapromisingalternativeintheglobalefforttoreplacefossilfuelswithmoreenvironmentallysustainableandrenewableenergysources.Thishasledtoarapidincreaseintheconstructionofbiogasplantsworld-wide.Inaddition,thepotentialofanaerobicdigestion(AD)tomitigateGHGemis-sionshasgainedattention.UndertheEURenewableEnergyDirective,bioenergypathwaysmustreachminimumthresholdsofGHGemissionssavingstocounttowardsrenewabletargetsandbeeligibleforpublicsupport(Giuntolietal.,2017).Thefeedstockbeingusedhasimportantimplicationsfortheoverallsustainabil-ityoftheADsystem.TheproductionofCH4throughbiologicalprocesses(biogas)hastheadvantageofusinglignocellulosicagriculturalandlivestock-derivedby-productswhich,followingbiologicallybasedprocessing,areconvertedtoelec-trical,heatandpowerenergiesthrougharelativelyeasy-to-manageprocessinsmallindustrialandagriculturalunits(Antoni,ZverlovandSchwarz,2007).Digesterdesignsvarywidelyinsize,functionandoperationalparameters,andhavebeenreviewedinthecontextofdifferentproductionsystemsbyHristovetal.(2013b).WhilestronglyrecommendingtheuseofanaerobicmanuredigestersasaCH4-mitigationstrategyfortheagriculturesector,Gerberetal.(2013b)cautionthatcarefulmanagementisnecessary,sothattheydonotbecomeemittersofCH4intotheatmosphere.Theauthorssuggestthattheadoptionofthistypeoftechnologyonfarmsofallsizesmaynotbewidelyapplicableandwillheavilydependonfinancialandtechnicalcapacity,climaticconditionsandtheavailabilityofalternativesourcesofenergy.Whenlivestock(i.e.cattleandpig)derivedslurriesareusedinAD,thereisgener-allyanimprovedenvironmentalperformancecomparedtotraditionalmanureman-agement(Vadenbo,HellwegandGuillen-Gosalbez,2014).Thisislargelyduetotheemissionsthataremitigatedthroughtraditionalmanurestorageandapplication(Hamelinetal.,2014).Asaresult,theuseofanimalwastessuchasmanuresandslurryinADisencouraged,withsomestudiessuggestingthatpolicyshouldpri-oritizethedigestionofmanurestomaximizeGHGmitigation(Styles,DominguezandChadwick,2016).Whilemanureshavealowerbiomethanepotentialincom-parisonwithotherfeedstocks,ithasbeensuggestedthatfocusingonsmallerbiogasplantswithlowerenergyconversionefficiencymaybepreferableasastrategyforwastemanagementtorenewableenergygeneration,wheretherearemoreefficientalternativesintermsofcostandlandrequirements,suchaswindandsolarenergy(Styles,DominguezandChadwick,2016).ThemethodologyappliedbytheJointResearchCentre(JRC)oftheEuropeanCommissiontocalculatetheGHGemissionsassociatedwithbioenergypathwaysoutlinedintheRenewableEnergyDirectiveisasimplifiedattributionallifecycleassessment(LCA)forGiuntolietal.(2017).AccordingtotheJRC,biogasproducedfrommanurecanreceiveemissioncreditsforemissionsavoidedfromthetraditionalmanagementofmanure,includingCH4andN2O,providedthatmanureisnotstoredfortoolong.UsingmanureinADsystemsisconsideredanimprovedagricultural17Methaneemissionsinlivestockandricesystemsmanagementtechniqueandtheemissionsavoidedthroughthemanagementoftherawmanurearecreditedtothebioenergypathway.Thevalueofthecreditisequalto-45gCO2eqperMJofmanureused(Giuntolietal.,2017).However,theJRCrecog-nizesthatthecreditsarenotanintrinsicpropertyofthebiogaspathwaybuttheresultofacommon,althoughlessthanoptimal,agriculturalpractice(Giuntolietal.,2017).Furthermore,itisacknowledgedthatifgas-tightstorageofrawmanurebecomesastandardpracticeinagriculture,thecreditformanureinthebiogaspathwaywouldceasetoexist.Anaerobicdigestersutilizetheenergeticpotentialcontainedinthemanurefortheproductionofheatandelectricity,whichreducesN2Oemissionduringthetreatmentthrougharelativelyclosedsystem,andresultsinbiogasdigestatethatmakesforavaluablefertilizerstillcontainingmostofthenitrogen(Kreidenweisetal.,2021).However,comparisonsbetweenopenandcloseddigestionsystemsofammoniaemissionsarelackingintheliterature.Existingpublicationscomparedigestedtorawmanure,whichisnottherightcomparisonforevaluatingtheoveralleffectofadigester.Closeddigestersdonotaddresstheconcernsforammoniaemis-sionlevelsinneighbouringcommunitiesthattendtobenearlargeanimalfeedingoperationstoagreaterextentthanopensystems.1.8.1LeakageofmethanefromanaerobicdigestionfacilitiesMethanefromthewastesectoraccountsforaround3percentofglobalanthropogenicGHGemissions(Bogner,PipattiandHashimoto,2008),andforabout12percentoftotalglobalanthropogenicCH4emissionsforthe2008–2017period.Bakkalogluetal.(2021)suggestthatCH4emissionsresultingfrombiogasgenerationmaybebetween0.4and3.8percentofthetotalgasproduction,andcouldaccountfor1.9percentofthetotalCH4emissionsintheUnitedKingdom,excludingsewagesludgebiogasplants.ScheutzandFredenslund(2019)recentlymeasuredtotallossesofCH4from23biogasplantsbyapplyingatracergasdispersionmethodacrossplantsthatvariedinsize,substratesusedandbiogasutilization.Methaneemissionratesvariedbetween0.4and14.9percentofbiogasproductionwithanaveragelossof4.6percent.Methanelossesfromthelargerbiogasplantsweregenerallylowerthanthosefromthesmallerfacilities.Ingeneral,CH4losseswerehigherinwastewatertreatmentbiogasplants(7.5percentonaverage)comparedtoagriculturalbiogasplants(2.4percentonaver-age).TheauthorsconcludedthatfugitiveCH4lossmayconstitutethelargestnegativeenvironmentalimpactonthecarbonfootprintofbiogasproduction.182.MethanesinksGlobalCH4emissionsarelargelyoffsetbytheatmosphericandsoilCH4sinks.TheatmosphericsinkoccursthroughthechemicaldegradationofCH4byhydroxyl(OH)andchlorine(Cl)radicalsinthetroposphereandstratosphere(IPCC,2007)andisresponsiblefor90to96percentofglobalCH4sink(WuebblesandHayhoe,2002;Shukla,PandeyandMishra,2013;Saunoisetal.,2019),equivalentto550Tgperyear.Thesoilaccountsforabout4to10percentoftheCH4degraded(Born,DorrandLevin,1990;DuxburyandMosier,1993;Saunoisetal.,2019).TheoceanactsasasmallCH4sinkforatmosphericCH4ofabout4Tgperyear(Shukla,PandeyandMishra,2013).2.1SOILMETHANESINKThemostimportantsoilsinkforCH4isuplandsoil,accountingfor6percentofthetotalCH4consumption,equivalentto30Tgperyear(IPCC,2001;Knief,LipskiandDunfield,2003;Tianetal.,2016)withanuncertaintyof11Tgto49Tgperyear(Tianetal.,2016;Saunoisetal.,2019).ThebacterialgroupresponsiblefortheCH4sinkactivityinthesoilsarespecializedmembersofeubacteria,calledmetha-notrophsandammoniumoxidizingbacteria(Shukla,PandeyandMishra,2013).ThekineticsofthisprocessisanaerobicreactionwiththeenzymeCH4mono-oxygenase,inwhichCH4isoxidizedasanenergyandcarbonsource(BenderandConrad,1992;Roslev,IversenandHenriksen,1997).Amonguplandsoils,forestsoilsarethemostefficientCH4sinkinbothtem-perateandtropicalregions(Henckeletal.,2000;Steinkamp,Butterbach-BahlandPapen,2001;Singhetal.,1997),withaglobalannualaverageuptakerateof5.7kg,3.3kgand2.64kgCH4/hafortemperate,tropicalandborealforestbiomes,respectively(DutaurandVerchot,2007).Grasslands,shrublands,andsteppeandsavannabiomeshaveanaverageannualuptakeof2.32kg,2.25kgand1.49kgCH4/ha(DutaurandVerchot,2007).Croplandanddeserthavethelowestuptakeratewithanannualmeanrateof1.23kgand1.1kgCH4/ha,respectively(DutaurandVerchot,2007).Methanesinkestimatebybiomevariesconsiderablydependingontheestimationmodel(Saunoisetal.,2019;ItoandInatomi,2012)but,owingtothecombinationofareaandoxidationrate,forestsrepresentthelargestCH4soilsinkfollowedbygrazinglands(Murguia-Floresetal.,2018;Yuetal.,2017).Withingrazinglands,thedrygrazinglandsinbothtemperateandtropicalclimateshaveabout2to3timestheuptakerateperhectareofmoistgrazinglands(Yuetal.,2017).2.1.1FactorsaffectingthesoilmethanesinkcapacityTheCH4oxidationpotentialandmethanotrophiccommunitysizeandstructurecanbeaffectedbymanyenvironmentalandanthropogenicfactors(Boeckx,VanCleemputandVillaralvo,1997).EnvironmentalfactorsaffectingthesoilCH4sinkcanbedividedintotwotypes:thosethathavepurelyphysicaleffects(primarilyondiffusion),andthosethatinfluencemethanotrophpopulationsandactivity.Watercontenthasbothphysicalandmicrobiologicaleffects(Dunfield,2007)asdrysoilincreasesgasdiffusionandCH4consumptionbutinsufficientsoilmoisturereducesmethanotrophactivity.19MethaneemissionsinlivestockandricesystemsChangingclimateandclimaticfactors,particularlyseasonalprecipitationvariationsinsemi-aridregionsanddrylands,likewiseaffectthesoilCH4sinkcapacity,directlyandindirectly.SoilOMalsoincreasesCH4consumptionthroughbothpathways–porespaceandporesizeincreasewithincreasingsoilOM,whilecarbonandnutrientsinsoilOMincreasemethanotrophnumbers(Gaticaetal.,2020;Tangetal.,2019b).Physicalfactorsincludetemperature(weakbecauseofcompetingeffectsonmethano-trophactivity,soilwatercontentandgasdiffusionrates),texture(uptakeincreasesassandincreases)andbulkdensity(uptakeincreasesasbulkdensitydecreases)(Shukla,PandeyandMishra,2013).ThelanddegradationthatreducessoilOMandincreasessoilbulkdensityconsequentlyreducesthesoilsinkcapacity,whereasrestorationincreasesthesink;however,theincreaseinsinkcapacitywithrestorationisslowerthanthelossofsinkcapacitywithdegradation(Wuetal.,2020).TheadditionofinorganicnitrogendepressesuptakebecauseammoniacompetesforCH4monooxygenaseenzymeactivesitesandnitriteproducedduringnitrificationand/ordenitrificationistoxictometha-notrophs(Dunfield,2007).Whennitrogenismixedwithorganicamendmentssuchasmanure,nitrogenhasalessereffectonCH4uptake.Pesticidesandherbicides,metalpollutionandlandusepatternscanalsohaveasignificanteffectonCH4oxidationandmethanotrophiccommunity(Boeckx,VanCleemputandMeyer,1998;PrieméandEkelund,2001;Shukla,PandeyandMishra,2013).2.1.2LandmanagementeffectsonthesoilmethanesinkI.PastureAglobalmeta-analysisshowedthataddingnitrogentopasturesreducedthesoilmethanesinkcapacitybymorethan10percent,butthattheapplicationofphos-phoruswiththenitrogenroughlyhalvedthatreduction(Zhang,L.etal.,2020).Thelivestockstockingrate,hasanimportantbutyettobequantifiedeffectonCH4uptake.Comparedwithmoderateorlightgrazing,heavygrazingintensityreducesthesinkcapacityby12percentglobally,duetotheeffectofheavygrazingpressurewhichreducesplantproductivityandsoilOMwhileincreasingsoilbulkdensityfromhoofaction(Tangetal.,2019b).Forlowproductivitygrazinglandswithlowlivestockstockingrates,thesoilsinkcanbeanimportantpartofthegrazingsys-tem’sCH4budget.AnempiricalmodelforthesteppesinChinashowedthatthepastureCH4sinkwasequalto50percentofCH4fromentericfermentationandmanurefromgrazingsheepatastockingrateof1sheep/haperyearand20percentatastockingrateof4sheep/haperyear(Tangetal.,2019a).Thereismuchinterestinadaptivemulti-paddockgrazing,butcarefulanalysisoftheliteraturerevealsafailuretocontrolkeyfactors,particularlystockingrates.Furtherresearchisneededtoensurethatstudiescontrolstockingrates,repeatsoilcarbonandmethanefluxmeasurements,andcollectotherpertinentfielddata.II.ForestryTreespeciescompositioninthesystemisafactorthataffectsthesoilCH4sink(Dunfield,2007)becausesoilsunderdifferentforestcompositionssupportdifferentCH4uptakerates(Borken,XuandBeese,2003).Theeffectsoftreespeciesareprob-ablymediatedthroughsoilchemistry,moistureandmicrobiology,buttheprecisemechanismsarecomplex(Dunfield,2007).Uptakeratesarehigherinprimaryfor-estthansecondaryforestorplantations(Gaticaetal.,2020).20SourcesandsinksofmethaneemissionsfromfoodandagricultureIII.CroplandCroplandtypicallyhasnitrogenadditionandthatreducesitsCH4sinkcapacity.Otherwise,theCH4sinkonuplandcroplanddoesnotappeartobestronglyaffectedbymanagementasthereisnoconsistenteffectofthetillagesystem(Venterea,BurgerandSpokas,2005;JacintheandLal,2005;Kessavalouetal.,1998),biocharaddition(Cong,MengandYing,2018)orcovercrops(Singh,AbaganduraandKumar,2020)onCH4uptake.IV.AgroforestryBecausesoilundertreestypicallyhasagreaterCH4uptakerate,thetreedpor-tionofthelandhasahighersinkthantheuntreedcropland(Amadi,VanReesandFarrell,2016).InanexperimentcarriedoutinColombia,anintensivesilvopastoralsystemactedasaCH4sinkwithanaccumulatedflowof-1.01mg/m2perhourcomparedwithanimprovedpasturethathademissionsequivalentto46.7mg/m2perhourduringthesameperiod(Rivera,CharáandBarahona,2019).Inaddition,thecarbonsequesteredintheshrubsand/ortreesofsilvopastoralsystemspro-videsanopportunitytooffsetsome(Monjardino,RevellandPannell,2010)orall(Torresetal.,2017)oftheglobalwarmingeffectofalllivestockrelatedCH4emissions.21PART2Quantificationofmethaneemissions3.MeasurementFigure4depictstheflowchartindicatingthecategorizationofcurrenttechniquesusedtodeterminemethaneemissionsattheanimal,facilityandlarge-scalelevels.3.1ANIMAL-BASEDTECHNIQUESTherearemanydifferenttechniquesandmethodologiesusedtomeasureCH4emis-sionsfromruminants(Hammondetal.,2016),includinggasexchangemeasurements(e.g.respirationchambers,headorfacemasks,orspotsampling),tracergasandopen-pathlasertechnologies(Hilletal.,2016;Lassey,2007;Stormetal.,2012).Table1outlinesthecriticalaspectsofdifferenttechniques.Thesetechniqueshavespecificrequirements(i.e.methodologies)andassumptionsthatmaylimittheirapplicationoutsideoftheirintendedpurpose,andexacerbateCH4measurementsifthecondi-tionsarenotconsistentwiththeoriginalassumptions.Forinstance,sometechniquesaremoresuitableforgrazinganimals(e.g.sulphurhexafluoridetracergas,SF6),whileotherscanmainlybeusedforconfinedanimals(e.g.open-pathlaser).TracerreleaserateorairflowratearethemostcriticalmeasurementstoderiveaCH4emissionsrate.Figure4Aschematicflowchartofcurrenttechniquesusedtodeterminemethaneemissionsattheanimal,facilityandlarge-scalelevelsAnimal-basedtechniquesFacility-basedtechniquesManurestoragesDirectgasmeasurementtechniquesDirectmethodsRespirationchambersVentilationrateClosedcircuitCO2balanceOpencircuitNon-CO2externaltracergasPolytunnelsSensorsBiodomesMethaneconcentrationSpotsamplingOthermethodsHeadstallsInversemodellingInlinesniffersStaticchambersPortablechambersFacemasksLarge-scaletechniquesHand-heldlaserdetectorAircraftSatelliteanddroneimageryTracertechniquesSulphurhexafluorideOpen-pathlasertechniquesInvitrotechniquesSource:AdaptedfromTedeschi,L.O.,Abdalla,A.L.,Álvarez,C.,Anuga,S.W.,Arango,J.,Beauchemin,K.A.,Becquet,P.,Berndt,A.,Burns,R.,DeCamillis,C.,Chará,J.,Echazarreta,J.M.,Hassouna,M.,Kenny,D.,Mathot,M.,Mauricio,R.M.,McClelland,S.C.,Niu,M.,Onyango,A.A.,Parajuli,R.,Pereira,L.G.R.,delPrado,A.,Tieri,M.P.,Uwizeye,A.&Kebreab,E.2022.Quantificationofmethaneemittedbyruminants:Areviewofmethods.JournalofAnimalScience,100(7):1–22.https://doi.org/10.1093/jas/skac19725MethaneemissionsinlivestockandricesystemsTable1.CharacteristicsofdifferenttechniquesusedtomeasuremethaneTechniquesCostLevelEnvironmentApplicationsAdvantagesDisadvantagesAnimal/ResearchResultsaredifferentfromRespirationGenerallymanureGrazing/Highlyaccurate,free-rangeanimals;configurationsandhighpasture,ResearchcontrolledstillvaryfromoneresearchgroupaccumulationAnimalindoorsfreeResearchenvironment;toanother;ananimaladaptationchambersstallortieinformationaboutperiodisrequired;every2–3hAnimalstallResearchindividualanimals;accumulationchambersmustHoodand/ModeratetoAnimalincludeemissionsreleaseCO2thatbuildsup;needorheadboxhighInvitroResearchandfromhindgutcalibration.systemscommercialfermentationPen/bar/DonotmeasurehindgutTracersModeratebuildingResearchPortableandlessemissions;ananimaladaptationPaddock/Researchexpensivethanaperiodisrequired;somemaybeGassensorLowpasturechamber;requirelessdesignedforgrazingsituations;capsulesBasin/regionspacerecoverytestneeded.DiverseInvitroLowAccurate;fewReliesonSF6,whichisaPastureinterferencesbygreenhousegasitself;techniquesothergases;doesnotcompletelycapturealltheanimalcantracersand,therefore,reliesonOpen-pathHighfree-rangespotconcentrationmeasurements;laserhighcontactwithanimal,whichCompatiblewithcandisruptnormalbehaviour;newelectronichighlylaborious.technologies;reliesonsmall,Informationabouttherelationlow-costsensors;betweenconcentrationandcontinuousflux(emission);stillundermeasurementsdevelopment.HighreproducibilityOutcomescanbedifferentfrombutusedtoactualmeasurements;methodrankfeedsforreliesondonoranimalsforrumenmethanogenicenvironment;standardizationcanpotentialratherthanbedifficult.measurementsofflux;allowsdifferentRequireexpensiveandaccuraterumenmicrobialmeasurementapproaches;dataenvironmentstobeprocessingheavilyinfluencedbyevaluatedmicroclimaticconditions;lossofdatacanbehigh.Informationaboutmanyanimals;Highvariabilityanddifficultyofdataproducedinairflowmeasurement.anaturalgrazingenvironmentUnpersonResearchandEstimatetheOnlyCH4concentrationaerial/groundcommercialdistributionofmeasurements.vehiclesproduction;not(UAV/UGV,ResearchandlimitedtoanyTheycanbedifferentfromrealdrones)commercialconfigurationscenarios;stillrelyoninputdatamadefromrespirationSatelliteandaccumulationchambersmeasurementsaswellastracerComputerLowmethods.modelsLiDARModerateGrazingResearchAirborne;detectsCO2andCH4concurrentlySource:BasedonHill,J.,McSweeney,C.,Wright,A.-D.G.,Bishop-Hurley,G.&Kalantarzadeh,K.2016.Measuringmethaneproductionfromruminants.TrendsinBiotechnology,34(1):26–35.https://doi.org/10.1016/j.tibtech.2015.10.00426Quantificationofmethaneemissions3.1.1Gasexchangetechnique3.1.1.1RespirationchambersRespirationchambershavebeenthegold-standardtechniquetodeterminetheenergyexpenditureofindividualanimals.TheindirectcalorimetrymethodologyreliesonthegasexchangeofmainlyO2,CO2andCH4,eitherusingopen-circuitchambersthatanalysethecompositionofinflowandoutflowairorclosed-circuitchambersthatanalysethecompositionofairaccumulatedoversometime(JohnsonandJohnson,1995).Alimitationofrespirationchambersisthatanimalsmaynotexhibitnormalbehaviours–theymight,forinstance,decreasetheirfeedconsump-tion–thusresultinginanunderestimationofactualCH4emissionswhencomparedtofree-ranginganimalsunderfarmconditions(Huhtanen,RaminandHristov,2019).Anumberoffactorsareessentialwhenusingthistechnologyforcontrolledexperi-ments,suchasgasrecovery,routinemaintenance,chambertemperature(<27°C),relativehumidity(<90percent),CO2concentration(<0.5percent)andventila-tionrate(250–260L/min),assuggestedbyPinares-PatiñoandWaghorn(2014).Theutilityofrespirationchambersisalsolimitedtoquantifyinggaseousemissionsfromrelativelyfewanimals(fewerthan20).Furthermore,emissionsfrommanure,ifaccumulatedinthechamber,mustbeaccountedfor(Mathotetal.,2016).Respirationchambersarerelativelyexpensivetobuildandmaintain,butlow-costsystemsexist(Abdallaetal.,2012;CanulSolisetal.,2017;Hellwingetal.,2012).Thesesystemsusethesameprinciplesasforopen-circuitindirectcalorim-etrybuttheyemploylocallyavailablematerialsforconstructionandtheair-con-ditioningsystemsthataresimplerthanthosedescribedforotheropen-circuitsys-tems.Thesystemistypicallylocatedinthedailyenvironmentofcows(CanulSolisetal.,2017;Hellwingetal.,2012)orsheep(Abdallaetal.,2012).Itmayconsistoftransparentpolycarbonatechambers,thermicpanelswithacrylicwindowsorsheepmetabolismcagescoveredwith3-mmtransparentpolycarbonatewalls,withatotalvolumeof9.97m3and17m3forcowsand1.9m3forsheep.Flowandgasconcentra-tionsmaybemeasuredcontinuouslyintheoutletorinairsampledfromtheoutlet,usinganinfraredanalyserorgaschromatograph.Averagerecoveryratesrangefrom99±7to104±9percent.Anevensimplerversionoftherespiratorychamberisthepolytunnelthatcon-sistsofonelargeinflatableortenttypetunnelmadeofheavy-dutypolyethyleneorpolyvinylchloridefilm,inwhichindividualorgroupsofcattlecanbeconfinedforselectedperiodsoftimeduringwhichtheamountofCH4theyproduceiscol-lectedandthenmeasured(Goopy,ChangandTomkins,2016).Polytunnelscanbeplaceddirectlyonpasturessimulatingsemi-normalgrazingconditions(Murrayetal.,2001)orfixedclosetothepastureswherethedailyofferandintakeofforagescanbemeasured(Gaviria-Uribeetal.,2020;Molinaetal.,2016).3.1.1.2SpotsamplingHeadstalls,alsoreferredtoasAutomatedHead-ChamberSystems(AHCS)(Hristovetal.,2015b)(e.g.GreenFeedEmissionMonitoring™system),andsniffers(e.g.GASMET4030system)arebasedonspotsamplingofeructatedandexhaledgasesfromtheanimals’mouthandnostrils.Sniffersonlymeasureconcentration.Headstallsareusuallyprogrammedtodeliverasmallamountoffeedtolureanimalstoinserttheirheadinsideachamberthatwillcollectthegasesusinganactiveairflow.MethaneemissiondeterminedwithGreenFeedandempiricalregressionsdevelopedfrom27Methaneemissionsinlivestockandricesystemsrespirationchambershadahighcorrelation(r=0.958)andlowmeanbias(12.9per-centoftheobservedmean)fordairycows(Huhtanen,RaminandHristov,2019).Theadequacyofheadstalls(e.g.GreenFeed)inmeasuringCH4highlydependsonthedailyfrequencyofthevisitsbytheanimal(within-dayvariation),animalbehaviour(within-animalvariation),trialdesignandthenumberofdaysofdatacollection(Hammondetal.,2015;ThompsonandRowntree,2020).GunterandBradford(2017)recommendedatleast2.4visitsperdayfor6.3days.Hristovetal.(2015b)proposedsamplingeighttimesduringa24-hourfeedingcycle,staggeredintimeover3days.Arbreetal.(2016)measureddailyvaluesandobtainedarepeatabilityof70percentin17days;theycouldincreaseitto90percentin40days.Coppaetal.(2021)reportedarepeatabilityof60percentforaone-weekmeasurementofdailyCH4thatheincreasedto78percentforaneight-weekmeasurementperiod.TheGreenFeedEmissionMonitoringSystemcanbeusedforlarge-scalemea-surementsandcommercialproductionconditionsoflargeandsmallruminants(Zhaoetal.,2020),butdifferentunitsareneededdependingonanimalsize.Thesystemissuitableforpasture(i.e.grazingconditions)andindoorfreestallsortiestalls,butrequiresanimaltraining.Formaximumaccuracy,onemustper-formCO2andCH4calibrationsfivetimesatthebeginningandthreetimesattheendofeachgasmeasurementexperiment.ItisalsonecessarytoperformtheCO2recoverytestatleastonce(threereleasesisaboutonecylinderofCO2)beforeeachgasmeasurementexperiment.Forcontinuousapplications,onemustperformtherecoverytestoncepermonth(Hristovetal.,2015b).Benefitsincludeitslowercostcomparedtorespirationchambersbut,similartosniffersandgastracers,itdoesnotconsiderCH4emissionsfromhindgutfermentation.Sniffersareplacedneartheanimal’smuzzleatthefeedorwatertroughs,andexhaledairiscontinuouslysampled.Unfortunately,theprecisionofsniffers(Belletal.,2014)islowerthanthatofrespirationchambers(Yanetal.,2010),likelybecauseCH4concentrationdependsonthedistancebetweenthesnifferandtheanimal’smuzzle;ideally,itshouldbelessthan30cm(Huhtanenetal.,2015).Portableaccumulationchambershavebeenusedfrequentlytodetermineshort-termCH4emissionsingrazingsheep(Goopyetal.,2011).Madewithplexiglassonthesidesandtop,thesearebottomlessboxesthatarelowereddownonanimalsandsealed(ThompsonandRowntree,2020).Therearethreesamplingportsonthetopoftheseboxes,whichallowtofollowgasaccumulationintime.Comparisonswithrespirationchambermeasurementshaveindicatedmoderatecorrelations(upto0.6)foruptotwo-hoursamplingdurations(Goopyetal.,2011;Goopyetal.,2015).Onelimitationofportableaccumulationchambersisthatmeasurementsareshortterm,andrepresentonlyaportionofthe24-houremissioncycle.Chagunda(2013)evaluatedthehand-heldlasermethanedetector(LMD)on-farm.TheLMDisaninfraredabsorptionspectroscopythatusesanexcitationsourceandthesecondharmonicdetectionofwavelengthmodulationspectroscopy.Thisnon-invasiveandnon-contacttechniqueenablesthemeasurementofCH4emissionsfromthebreathofruminantanimals.Somerecoverytestsmaybeneeded.Methanehastwostronggroupsofabsorptionlines,centredat3.3micrometres(v3band)and7.6micrometres(v4band).Mostlaser-baseddevicesoperateatnear-infraredwave-lengths,limitedtobelow2.2micrometres.ThemostrobustabsorptionbandofCH4islocatedat1.64to1.70micrometres(2v3band).Thiscorrespondstothesingle-mode,single-frequencyemissionwavelengthofindiumgalliumarsenide-distributed28Quantificationofmethaneemissionsfeedback(DFBlaserdiode).Hand-heldCH4detectionsystemsareusedinotherindustriesandhavebeendescribedbyvanWelletal.(2005).BecausetheinstrumentmeasuresCH4atarangeofseveralmetres,itdoesnotdisturbanimalbehaviour.TheinstrumentaccountsforthethicknessofanyCH4plumes,andtheresultisexpressedintermsofCH4concentration.TheLMDthusenablesreal-timemeasurementwithafastresponse.Furthermore,theLMDcansegregatetheCH4concentrationfromdairycowsperformingdifferentphysiologicalactivities,suchasruminating,feed-ingorsleeping.AstheLMDonlyspotsamplesananimal’sbreath,attemptshavebeenmadetousethesemeasurementstocalculatetotalemissions(gperday).OnestudyreportedthattheCH4measurementwiththeLMDhadastrongagreementwithmeasurementsinrespirationchambers(r=0.8)(ChagundaandYan,2011).OneofthechallengesofusingtheLMDisrelatedtotheabsenceofgassampling.Itisnecessarytoseparatetheeructationepisodesfromthenormalbreathcycleoftheanimals(exhalation-inhalationcycles).Torespondtothischallenge,athresholdvalueforseparatingthetwoeventsisbeingtested.Applyingthisapproachtograzingani-malsisalsochallengingbecausewindspeedanddirection,relativeairhumidityandatmosphericpressurecanhaveasignificanteffectontheresultantconcentrationofCH4.WindspeedisnegativelycorrelatedwithCH4concentration(r=-0.41).Anadditionallimitationisthatthedeviceshouldbeattherightdistancefromtheanimal(Sorg,2022)toavoidcontaminationfromaneighbouringanimal.TheLMDinstru-mentisrelativelynovelwhenitcomestoruminantanimals,andextensivestudieswillberequiredtodeterminethemeasurements’repeatability(Chagunda,2013),whichcouldbeusedtodevelopstandardprotocolsfordatameasurementandanalysis(Sorg,2022).Thatsaid,suchtechniquesmayhelpimprovetheaccuracyofthecurrentCH4inventoriesandmonitortheefficacyofmitigationoptions(Chagunda,2013).3.1.2TracertechniqueMethaneemissionscanalsobedeterminedbyusingaknownquantityoftracergas(e.g.SF6)releasedintherumen.TheCH4emissionrateisthencomputedbytheknownreleaserateofthetracergasandtheratioofCH4andtracergasconcentra-tions(Johnsonetal.,1994).Unfortunately,thedifferenceinmeasurementbetweentheSF6methodandrespirationchamberscanbegreaterthan10percent(Stormetal.,2012;Ramírez-Restrepoetal.,2020),likelyonaccountoftheinconsistentreleaseofSF6fromthepermeationtubesdepositedwithintherumen,variationsintheanimal’sbreathcollectionefficiency,interruptionofnormalbehaviourduetothesamplingequipmentharness,andinabilitytocollectCH4emissionsproducedinthehindgut(Lassey,2007).ToimprovethepredictabilityoftheSF6method,somemodificationshavebeenproposed,suchasthecontinuouscollectionataconstantratefor24hoursandtheincorporationoforificeplatesratherthancapillarytubestorestricttherateofsamplecollection(Deightonetal.,2014).Arbreetal.(2016)suggestedthatathree-daymeasurementperiodwasneededtoachievearepeatabilityof70percentforCH4emissionsperunitoffeedintake(i.e.CH4yield),withoutanyfurtherincreaseinrepeatabilityformoreextendedmeasurementperiods.TheSF6tracergastechniqueissuitableforlargeandsmallruminants,anditcanpotentiallybeusedinoutdoor(Ramírez-Restrepoetal.,2010)orindoor(Ramírez-Restrepo,ClarkandMuetzel,2016)systems.However,Hristovetal.(2016)notedthatSF6isbettersuitedforopenspacesorwell-ventilatedbuildingsbecause,inpoorlyventilatedbuildings,back-groundCH4couldaffecttheinterpretationofresults.Thetechniquecannotbeused29MethaneemissionsinlivestockandricesystemsclosetootherCH4sources(e.g.slurry,manure,otheranimalsandwetareas)andSF6sources(e.g.electricitytransformersandindustrialsites)(JonkerandWaghorn,eds.,2020).TheSF6techniqueisrelativelyinexpensive,butonlyoneanimalperunitcanbemeasured.Anadequatecalibrationofthereleaserateofthetracergasfromthepermeationtubeshouldbeconductedinadvanceofplacementintherumen,andtheexperimentshouldbecarriedoutsoonafterthiscalibrationsincethereisadecreaseinthepermeationrateofthetube.Adjustmentsforthechangingpermeationrateshouldbeperformedinlong-termtrials(JonkerandWaghorn,eds.,2020).Madsenetal.(2010)proposedpredictingCH4fromCO2calculatedbasedonbodyweight,energy-correctedmilkyieldandthedaysofpregnancy,providedthattheefficientenergyuseformaintenanceandproductionisconstantfordairycows.IndividualCH4concentrationwasrecordedforthreedaysinanautomaticmilkingsystemwithaportableairsamplerandanalyserunit,basedonFouriertransforminfra-reddetectionandusingCO2asatracergas(Lassen,LøvendahlandMadsen,2012).Airwasanalysedevery20secondswhentheanimalsweremilked,andtheratiobetweenCH4andCO2wasusedtomeasureCH4emissions.Therepeatabilityofthemeasure-ment(CH4:CO2ratio)was0.39and0.34forHolsteinandJerseycows,respectively(Lassen,LøvendahlandMadsen,2012).TheseresultssuggestedthattheCH4:CO2ratiocouldbeusedforgeneticevaluationsofdairycows(Lassen,LøvendahlandMadsen,2012).Unfortunately,efficientcows(i.e.thoseyieldingmoremilkperfeedconsumed)producelessheatandconsequentlylessCO2perunitofmetabolicbodyweightandenergy-correctedmilk;thus,thereisariskofoverestimatingtheirCH4production.Hence,thegeneticselectionforlowCH4emittersusingCO2productionrateasarefer-encewillfavourinefficientdairycows(Huhtanenetal.,2020).ThesemethodologicalissuesoftheCH4:CO2ratiotechniqueshouldbetakenintoaccount.3.1.3Open-pathlasertechniqueTheopen-pathlasertechniquequantifiesthedispersionofaspecificgasfromthesourceandusesthedownwindconcentrationofthegastoestablishtheemissionratebyadoptingan“inversedispersion”approach(McGinnetal.,2006).Thetech-niquehasbeenusedforCH4(McGinnetal.,2006)andNH3(McGinnetal.,2007)emissions.Validationassayshaveshownitslimitationswithregardstothetimeofdatacollection(McGinnetal.,2006,2008).Theopen-pathlasertechniquehasbeenupdatedbyhavingdifferentanalysersandatmosphericparametersintegratedintoaflyingplatform,thusshowingmorereliableandpromisingresults(Hackeretal.,2016).Theseauthorsindicatedthatwiththerevisedapproach,CH4andNH3couldbedetectedwithinadistanceofatleast25and7km,respectively,fromthesource.Tomkinsetal.(2011)comparedtheopen-pathlasertechniquewithanatmosphericdispersionmodelforgrazinganimalstoanimalswithrespirationchambersconsum-ingfreshlycutChlorisgayana.Dailyestimateswereof136gand114gCH4,respec-tively,andtheauthorsconsideredthatfurthercomparisonsusingdifferentforagesandherdswereneeded.Subsequently,TomkinsandCharmley(2015)testedtheopen-pathlasertechniquerelyingontheexpectedbehaviourofherdinganimalsaroundwaterpointsduringtheday.Themeasurementwasconductedover4to16daysfor78hours,withdatacollectedevery10minutes.Historicalmeteorologicaldataforwinddirectiondeterminedthephysicalarrangementofequipmentateachtestedsite.Thedataneededtobefilteredbasedonenvironmentalconditions,includinglightlevel,surfaceroughness,atmosphericstabilityandvariationofwinddirection,30Quantificationofmethaneemissionscomparedtohistoricaldata.Basedontheirresults,theauthorsconcludedthattheopen-pathlasertechniqueworkswellwhenusedonaggregationsofgrazingcattlefor7to8hoursperdayover7to14days,andthatitisalsoanoptionfordirectlymeasur-ingCH4emissionsfromcattleattheherd-scaleinextensivegrazingconditions.3.1.4InvitrotechniquesTheinvitrofermentationtechniqueshavebeenusedforseveralyearstoevaluateruminalfermentationoffeedstuffsand,morerecently,toassesstheeffectofdiffer-entnutritionalstrategiesinmitigatingCH4productionunderstandardizedcondi-tions(Yáñez-Ruizetal.,2016).DuetothecomplexityandcostofmethodologiesforevaluatingentericCH4emissionsdirectlyfromanimals,thepossibilityofobtainingresultsthroughinvitrosystemswouldbeanalternative,mainlyinprovidinganini-tialscreeningofalargernumberofsampleswithdifferentmethanogenesis-reducingoptions,suchastannins,plantsecondarymetabolitesandessentialoils(Tedeschietal.,2021).Availableinvitrotechniquesvaryfrombatchculturesystems(Mauricioetal.,1999;PellandSchofield,1993;Theodorouetal.,1994)tosemi-continuousfer-menterssuchasRUSITEC(CzerkawskiandBreckenridge,1977)orthedual-flowcontinuousculturesystem(HooverandStokes,1991).MostinvitrotechniquesarederivedfromTilleyandTerry’s(1963)two-stagemethod,whichconsistsofsimulatingrumenconditions(temperature,pH,anaerobiosis)usingarumeninoculum(strainedrumenfluid),abuffertoavoidsignificantpHvariation,andmediatoprovideneces-sarynutrientstorumenmicrobiota.TheCH4productionisusuallyexpressedperincubatedunitoronadigestedDMorOMbasis.3.2FACILITY-BASEDTECHNIQUES3.2.1ManurestorageThreedifferentapproachestothequantificationofmanureCH4emissionsfromanimalhousingfacilitiesarecommonlyused:directmeasurementmethods,inversemodelling(manureandanimals)andthechambertechnique(manureemissions)(HassounaandEglin,2016).Atthebarnlevel,theremovalofcattletoestimateemis-sionsfrommanurehasbeenperformed(Edouardetal.,2019;Mathotetal.,2012,2016).Themeasurementmethodsthatexisttodayweredevelopedforscientificpur-poses,whichiswhysomeofthesemethodscanbeimplementedformeasuringemis-sionsfrombarnandmanurestorageatanexperimentalscale(Mathotetal.,2016).Theirimplementationincommercialfarmsistooexpensiveandtime-consuming.Sincethereisnointernationalstandardizationofthesemethodstodate,itisyettobeclearlydemonstratedthatthemeasurementofventilationratecanhaveanimpactontheresultobtained(Quetal.,2021).Moreover,oneofthecurrentchallengesisthedevelopmentofnewmethods,whichwouldbeeasiertoimplement,lessexpensive(Robinetal.,2010;Hassounaetal.,2010)andadaptabletodifferentcontexts,soastomeetobjectivessuchasthecertificationofemissionreductionsinrealconditionsorthequantificationofemissionfactorstakingintoaccountintracategoryvariability.3.2.1.1DirectmethodsDirectmethodsarethemostwidelyused.Anemissionrateiscalculatedastheproductofthehousingventilationrateandthein-houseCH4concentration,minusthebackgroundconcentration(Hassounaetal.,2021).Amethodologytoquan-tifytheuncertaintyofaerialemissionsforthedirectmethodshasbeenoutlinedby31MethaneemissionsinlivestockandricesystemsGatesetal.(2009),onewhichcombinesthestatisticaluncertaintyoftheemissionsconcentrationmeasurementandtheventilationratemeasurement.Measurementsassociatedwiththeventilationratehavebeendemonstratedtobethemajorcon-tributortotheemissionsrateuncertaintywhenrelyingonthedirectmethods.3.2.1.1.1VentilationrateFortheventilationratequantification,threemethodshavebeenimplementedmainlyinstudiesandcomparedinliterature:internalgasandexternaltracergas(indirectmethods),andtheuseofsensors(directmethod).i.CarbondioxidebalanceForthismethod(Barreto-Mendesetal.,2014;Liu,PowersandHarmon,2016),whichusesCO2asthetracergas,themainhypothesisisthattheVRdeterminestherelationshipbetweenCO2productioninthebarnandthedifferenceinCO2concen-trationsbetweentheinsideandoutsideofthebarn(ΔCO2).Inthebarn,CO2pro-ductioncomesfromanimals,deeplitter,andgasorfuelheatingsystemsifapplicableinthebarn.Pedersenetal.(2008)donotrecommendusingthismethodtocalculatetheventilationrateinananimalhousewithdeeplitterbecauseofitshighandvari-ableCO2production.AnimalCO2productioncanbeestimatedfromanimalheatproduction,theCO2productionperheatunitandanimalactivity.Inmanystudies,theseparametersarecalculatedwithmodelsgivenbytheInternationalCommissionofAgriculturalEngineering(CIGR,2002).AccordingtoZhang,PedersenandKai(2010),associatederrorsrangingfrom10to20percentandmorerecentmodelsthattakeintoaccounttheprogressofanimalgeneticsshouldbetakenintoconsiderationtoimprovetheaccuracyoftheVRestimations.ConcerningtheaccuracyofVR,Calvetetal.(2011)demonstratedthatthedailyvariationofCO2productionthatdependsonanimalactivityshouldbeconsideredtohaveanaccurateestimationofthedailyvariationoftheventilationflow.ThisCO2balancemethodalsorequiresΔCO2.VanOuverkerkandPedersen(1994)suggestedthatΔCO2valuesshouldnotbelowerthan200ppminorderforthemethodtoyieldreliableresults.ii.ExternaltracergasThetracergasmethodformeasurementoftheemissionsinlivestockbuildingsreferstoatechniquethatreliesonthereleaseofatracergasthatisnotproducedinthebarn.Thismethodisoftenusedinnaturallyventilatedbuildings(Oginketal.,2013).ThemostwidelyusedgasisSF6becauseitiseasytodetect,chemicallyinertandnotproducedinthebuilding.Thebarnventilationrateiscalculatedusingthetracergasinjectionrateandthetracerconcentrationgradient,assumingtheperfectmixingoftheairinsidethebarn,aswellassteady-stateconditions.BecauseofthehighGWPofSF6,lowconcentrationsofSF6shouldbeinjected,andtheconcentrationmeasure-mentshavetobedoneusingasensorwithalowdetectionlimit.Inlivestockbuildings,thismethodcouldbeimplementedbymeansoftwodifferentapproaches:aconstantdosingofthetracergasorwithspotinjections(concentrationdecaymethod).Fortheconstantinjectionmethod,thetracergasisdosedintothebarnor,moregenerally,closetoanemittingareaorpointsource.ThistracergasmimicsthedynamicflowanddilutionofCH4orothertargetgasessuchasN2OorNH3(Schradeetal.,2012).Forthetracerdecaymethod,adoseoftracergasisinjectedandmixedintothehous-ingunituntilthedesiredthresholdisachievedandauniformdistributionofthetracer32Quantificationofmethaneemissionsgasisreached.Thentheinjectionisstopped,andthedecreaseoftracergasconcentra-tionismonitoredforagivenperiodtocalculatethebarn’sVR(Mohnetal.,2018).Thismethodrequiresasensorordevicetomeasuretracerconcentrationwitharea-sonablyfastanalysisfrequencyinhighlyventilatedbarnssuchasopenbarns,andisnotsuitableforlong-termairflowmeasurements(Oginketal.,2013).ManystudieshavecomparedthismethodwiththeCO2methodindifferenttypesoflivestockbuildings.Edouardetal.(2016)foundthatbothmethodsyieldedsimilarresults,withtheCO2massbalancemethodbeingquantitatively10to12percentlowerintheestimateofemissionscomparedtotheSF6tracermethods.iii.SensorsInmechanicallyventilatedhouses,continuousmonitoringofthestaticpressuredif-ferentialandtheoperatingstatus(on-off)ofeachfancanbeusedtoestimatethefan’sVR,basedonitstheoreticalormeasuredperformancecharacteristics.Ideally,theinsituperformanceofeachfanisdeterminedfirst,andtheVRofthehousecanbeesti-matedbysummingupalloperatingfanflowrates.Gatesetal.(2004,2005)developedandimprovedafanassessmentnumerationsystem(FANS)tomeasuretheinsituper-formancecurveofventilationfansoperatinginanegativepressure,mechanicallyven-tilatedanimalhouse.Thisapproachcanprovideventilationestimateswithuncertain-tiesoflessthan10percentinlowairflowconditionsandlessthan25percentinhigherairflowconditionswhenregularinsitucalibrationisconducted(Gatesetal.,2009).Fornaturallyventilatedhouses,Jooetal.(2014)proposedamethodthatreliesontheimplementationofahighnumberofultrasonicanemometersattheopeningsofthebarn.Inthemethodstheydeveloped,anypositivevelocitiesindicatedairoutflows,whilenegativevelocitiesdenotedairflowingintothebarns.Thetotalairinflowratewasassumedtobethesumofairinflowsattheinlets,whilethetotalairoutflowratewasthesumofairoutflowratesattheoutlets.3.2.1.1.2MethaneconcentrationmeasurementsForthequantificationoftheemissionrate,CH4concentrationshavetobemeasuredinsideandoutsideofthebarn.Mostofthetime,thesamedeviceisimplementedforbothmeasurements,whichimpliesthatthedevicehastohavetheadapteddetec-tionrange.PowersandCapelari(2016)listedmanytechniquesthatarecommonlyimplementedforCH4concentrationmeasurements,includinggaschromatography,infraredspectroscopy,Fouriertransforminfraredspectroscopytechnologies,pho-toacousticspectroscopy,massspectroscopy,tunablediodelaserabsorptionspec-troscopytechnology,andsolid-stateelectrochemicaltechnology.Thesetechniquesaremainlyspectroscopicandportable,buttechniqueswithaveryselectivedetec-tionsystem(suchaslasers)arepreferredforcontinuousmeasurements.Hassounaetal.(2013)highlightedinterferenceproblemswithnon-selectivemethodssuchasthe(commonlyused)photoacousticinfraredspectroscopythatcanleadtoover-estimationCH4emissions.Gaschromatographycanalsobeimplemented,butthecontinuousmeasurementismorecomplicatedoncommercialfarmsbecauseregu-larcalibrationisrequired.Nevertheless,notallsensorsandgasanalysersonthemarketaresuitablefordetectingCH4inbarnsduetotheadverseconditionsfoundthere(dust,moisture,NH3,animals).Thereliabilityofmeasurementsovertimeisnotalwaysguaranteed.Testingnewequipmentcanrequirealongperiodoftime.Moreover,theavailablesensorsanddevicesaretypicallycostly.33Methaneemissionsinlivestockandricesystems3.2.2SoilfluxesForthecollectionofsoilCH4fluxesinsitu,thetwopossibleapproachesdrawonchamberandmicrometeorologicalmethodsthatcomeinmanifolddesignsandhavevaryinglevelsofcomplexity.ThesuitabilityofagiventechniquefordeterminingCH4fluxratesdependsonmultiplefactors,including,butnotlimitedtothepur-poseoftheexperimentalstudy,thegeographicscale,measurementfrequency,rep-licabilityaswellasavailablefundsandlabour.ThesetechniquesalsorelyonthedeploymentofdifferentgasanalyserstoquantifyCH4fluxeswithdifferentlevelsofprecisionandtemporalresolution.3.2.2.1ChambertechniquesBothclosedandopenchamberscanbeusedforthecollectionofCH4fluxesfromricepaddysoilsandfromvariousmanurehandlingsystems,includingliquidandsolidstoragesystems(Husted,1993;KreuzerandHindrichsen,2006).Theprin-ciplesforcollectionandmeasurementviachambersapplytobothsoilsandmanurestoragesystems.Asolidorclearopen-bottomedchamberofaknownvolumeisfittedontoapermanentlyinstalledringorcollartoencloseagivenheadspace.Forclosedorstaticchambers,theconcentrationofCH4buildsupintheheadspaceofthechamberovertimeandairsamplesfrominsidethechamberareextractedatgiventimeintervals(e.g.at0,10,20,30andupto45minutes),dependingontherateofCH4evolution,thekindsofmanualchambersemployedorthetypeofequationsusedtoderivetheemissionrates(Tiwarietal.,2015).Fornon-CO2tracegaseslikeN2OandCH4,longertimeintervalsareoftenrequiredduetothelow,negligibleornegativefluxesofthesegases(Collieretal.,2014).Methanemeasurementinricefields,however,havetoenclosetheplantsinthechamberastheplant’saerenchymaisaconduitofCH4.TheenclosureintervalitselfisclearlylimitedduringdaytimetoprotecttheplantsfromthestresscausedbyincreasingtemperatureandCO2deple-tion.Alternatively,tolimitthese,closedchambersmayalsobeexposedatnight-time(Wassmann,2019).Althoughemissionratesareloweratnight,thediurnalpat-ternsmaybetakenintoaccountforintercomparisonsofvarietiesandtreatments.Oneortwosmallfansaretypicallyinstalledinsidethechambertothoroughlymixtheatmosphericgases(Tiwarietal.,2015).Gassamplescanbecollectedviasyringeandtransferredintovialsforoffsiteanalysis(Sassetal.,1990,1991)orinsituanalysisifusingadynamicsystemwithautomatedsamplingdevices(Wassmann,PapenandRennenberg,1993;Wassmannetal.2000;Hall,WintersandRogers,2014).Theobviousadvantagesofdynamicsystemsarethehightemporalresolu-tionsandseamlessobservationperiods,asinwhentheemissionmeasurementsencompasstwo-hourintervalsoverentire24-cyclesandstretchovertheentirecroppingseason(Wassmann,NeueandLantin,2000).Thesedirectmeasurementsystemscanbevaluableincombinationwithmodellingapproaches,namelythevalidationofTier2regionalequationscreatedusingsuchmeasurementsorsimula-tionmodelsasDayCentorLandscapeDNDCappliedtothespecificconditionsofricefields(Welleretal.,2016;Krausetal.,2016;Janzetal.,2019).ItisvitaltonotethatsometimessimulationmodelsrequirealotofinputdatawhichisnotavailableforsmallholderfarmsinAsiaandAfrica.Intermsofapplicability,theclosedchambersystemswithmanualsamplingpro-ceduresrepresentbyfarthemostcommonapproachusedforricefieldsandarenowoperatedbymanyresearchgroups.AliteraturesearchinGoogleScholarfor34Quantificationofmethaneemissionstheterms“rice”and“closedchamber”hasyielded23hitsfortheyear1991,101for2001,241for2011and632for2021,illustratingthegrowingnumberofthesemeasurements.Atthispoint,closedchambermeasurementsinricefieldshavebeenconductedinalmostallrice-producingcountriesoftheworld–inmanycasesaspartofTier2approachesofGHGinventoriesundertheNationalCommunicationstotheUnitedNationsFrameworkConventiononClimateChange(UNFCCC).Thecaveatofthesewide-rangingapplicationsisthatthemeasurementresultsoftenremainas“greyliterature”withnopeer-reviewedpublicationsandarenotalwaysavailabletoaninternationalaudience;forexample,theIPCCemissionfactordata-baseshowsonly24emissionfactorsbasedonmeasurementsforCH4inricepro-duction(asofJanuary2022).Whiletherearealotmorepeer-reviewedpublica-tionsthanincludedinthedatabase,theprocessofenteringemissionfactorsintotheIPCCdatabaseisnotstraightforwardandhencethereareveryfewentriesinthisdatabase.Openchambers,i.e.dynamicorsteady-statechambers,replaceairinsidethehead-spacewithambientairthroughaninletport,andtheCH4fluxisestimatedasthedifferencebetweenthegasconcentrationsattheinletandoutletports(Pumpanenetal.,2004).Aswithclosedchambers,gasanalysiscanoccurinsituorthroughcol-lectioninglassvialsforoffsiteanalysis.Althoughthesesystemscaninprinciplebeusedforemissionmeasurementsforallkindsofgases,theirrealadvantagescomeintoplayforhighlyreactivegasessuchastheNO-NO2-O3triad(Breuningeretal.,2012).Giventhecomplexityofthegassamplingpatterns,however,dynamicchambersystemsarerarelyusedfornon-reactivegaseslikeCH4,i.e.thecurrentspikeinavailableemissionmeasurementsinricefieldsisexclusivelybasedonclosedcham-bersystems.GaschromatographyistheconventionalmethodusedtoanalyseCH4concen-trationsingassamplesfromsoilsandmanurehandlingsystems.AsforCH4analy-sis,theflameionizationdetector(FID)(Weiss,1981)isthegaschromatographydetectorofchoice,whereasotherdetectorsmaybedeployedforspecificpurposes,suchasmassspectrometry(Ekebergetal.,2004)todetermineisotopiccompositionofoneormultiplegasanalysissystemsfortheparallelassessmentofseveralGHGs(Hedley,SaggarandTate,2006;Sitaula,LuoandBakken,1992).Lasertechnolo-gies,Fourier-transforminfraredandotheropticaltechniquescontinuetogrowinpopularityforanalysingCH4concentrationsbecauseoftheirlowdetectionlimits,higherdegreeofprecision,andabilitytomeasuremultipleGHGssimultaneouslyatthesamplinglocation(Brannonetal.,2016;Harveyetal.,2020).Theavailableoptionsincludethequantumcascadelaser(Cowanetal.,2014;Nelsonetal.,2002)andotherspectroscopictechniqueswithquantumcascadelasersuchascavityring-downspectroscopy(Brannonetal.,2016;Christiansenetal.,2015)aswellasoff-axisintegratedcavityoutput(Waldoetal.,2019;Brannonetal.,2016)(Harveyetal.,2020).Infraredabsorptionmeasurementdetectorsarewellsuitedforsitua-tionsthatrequirefrequent,highprecisionmeasurements,e.g.tocapturedielvaria-tionandforshort-termresponsestoexperimentaltreatments(Ruanetal.,2014).Otherauxiliarymeasurementslikesoilandwatertemperature,airtemperatureinsideandoutsidethechamber,andsoilmoistureshouldbecollectedatthetimeofcollection(Pavelkaetal.,2018)foruseinseasonalandannualCH4fluxcalculations.Regardlessofchambertype,careshouldbetakentoensurethatthecollectionofgassamplesdoesnotintroduceartificialenvironmentsorconditionsthatalterthe35MethaneemissionsinlivestockandricesystemsCH4flux.Collectionsringsorcollarsshouldbeinstalledwellinadvanceofsamplecollection,i.e.morethan24hours,toallowthediffusionofgasfromthesoilorlit-terlayertotheatmospheresufficienttimetoequilibratefollowingthedisturbanceevent.Moredetailsaboutrobusttracegasestimationwithclosedandopencham-berscanbefoundinPavelkaetal.(2018),Collieretal.(2014),andRochetteandHutchinson(2005).Bothopenandclosedchambersarewidelyacceptedintheliterature,butselectingbetweenchambertypesdependsoncostconsideration,labouravailability,experimentaldesignandsamplingconditions(e.g.siteaccessibility,climate,soiltype).Closedchamberswithmanualsamplingareadvantageousbecausetheyrequireonlylowinvestmentandaresimpletodeploy,buttheyinvolvegreatermanuallabourcosts(Savage,PhillipsandDavidson,2014).Bothnon-flow-throughandflow-throughchamberscanalterthetemperature,moistureandgasdiffusiondynamicsduringsamplecollection(Husted,1993)leadingtoerrorsinfluxestimation(Pihlatieetal.,2013;Ueyamaetal.,2015).Errorsinfluxestimationwithclosedchamberscanbesignificantlyreducedbyincreasingchambersize,i.e.height,areaandvolume(Pihlatieetal.,2013).Thelongtimeneededformeasurementwithclosedchamberscanalsoalterthediffusiongradients(Davidsonetal.,2002;Savage,PhillipsandDavidson,2014).Openchambers,particularlyflow-throughsystems,allowformorefrequent,andlesstime-andlabour-intensivemeasurements(Ueyamaetal.,2015;Savage,PhillipsandDavidson,2014).Furthermore,openchambersmaybemoreappropriateformanurehandlingsystemsgiventhedifferencesinthegasdiffusiondynamicsrela-tivetosoils(Husted,1993).However,thesechambersrequiregreatercapitalinvest-mentsandmaintenance,andmaynotbesuitableinlowinfrastructurecontexts(Collieretal.,2014).Detailedinstructionsonhowtocustomizemanualchamberdesignfortropicalorsemi-aridregionsoftheworldandotherlowinfrastructurecontextsareprovidedbyTiwarietal.(2015).3.2.2.2MicrometeorologicaltechniquesThemainmicrometeorologicaltechniqueformeasuringCH4fluxesfromsoilsisbyeddycovariance.Eddycovariancereliesoninstantaneouscovariancemeasurementsofupanddowndraftsofair,i.e.“eddies”,andtheconcentrationofCH4orotherGHGswithintheatmosphericboundarylayer(Baldocchi,2014,2003;Baldochi,HinksandMeyers,1988).Samplesaretakenrapidly(morethan10timespersecond)forlongdurations(ofmorethan30minutes)tocalculateGHGfluxdensitybetweenthesoiland/orvegetationandtheatmosphere,thusprovidingrelevantspatio-temporalfluxestimatesforwholeecosystems(Baldocchi,2014).Oneofthemainadvantagesofmicrometeorologicaltechniquesisthattheyallowforcontinuousgassampling,andthattheycancapturetemporalvariabilityinGHGfluxes,whichisamajorchallengewithchambertechniques.Theyalsoofferlow-tono-disturbanceandnon-destructiveecologicalsampling(EugsterandMerbold,2015;Baldocchi,HinksandMeyers,1988).However,eddycovarianceislesswellsuitedforsmall-scalemanipulationexperi-ments,andexhibitssomebiaswithrespecttospatiallyheterogeneousgaseslikeCH4andN2O(Baldocchietal.,2012).EddycovariancemaythereforebemoreappropriateforecosystemmonitoringofCH4fluxesand,whenappliedinexperimentalcontexts,itshouldbecombinedwithchamber-basedmethodsratherthancompletelysubsti-tutedforthem(EugsterandMerbold,2015).Anotheraspecttoconsideristhelarge36Quantificationofmethaneemissionsarea(“fetch”)requiredforeddycovariancemeasurementsthatconstitutesamajorimpedimentforintercomparisonsofdifferentagronomictreatments.Whilethemini-mumfetchforeddycovariancemeasurementsdependsontheheightwherethesensorsareplaced,thetypicalsetupofatwo-metre-highmastinaricefieldtranslatesintoa100-metreradiusandacoherentexperimentalfieldof4ha(Albertoetal.,2009).Giventhatthesemeasurementsystemsarerelativelyexpensive,apracticalsolutioncanbea“rovingtower”thatisroutinelyshiftedfromoneexperimentalfieldtoanother(Albertoetal.,2012).Whilethefluxrecordsthatintegrateoverlargerareasdonotpresenttheartificialpatchinessofchambermeasurements,theneedforasteadyhorizontalairflowwithinthefetchputsadditionalconstraintsoneddycovariancemeasurements.Thiscrucialrequirementoftenleadstogreaterdatagapsduringnight-timeandeffec-tivelyprecludeseddycovariancemeasurementsduringperiodsofhighturbulence,whichareoftenthecaseintropicalregionsduringtherainyseason.EddycovariancemeasurementstodetermineCH4emissionsfromricefieldshavebeenappliedinsev-eralcountries,e.g.theUnitedStates(Rebaetal.,2020),China(Geetal.,2018),India(Swainetal.,2018)andthePhilippines(Albertoetal.,2014).Additionalresearchisneededtounderstanddifferencesinseasonalfluxestimatesforricepaddiesmeasuredwithchambersversuseddycovariance(Rebaetal.,2020).3.3LARGE-SCALETECHNIQUES3.3.1AircraftsAirborneCH4measurementsofdairyfarmscanbeconductedusingaseriesofcon-centric,closedflightpaths,theemissionratesbeingestimatedbyapplyingGauss’stheorem(Conleyetal.,2017).Atthebarnlevel,theCH4mixingratio,pressure,temperatureandhorizontalwindaremeasuredwhileanaircraftisflyingaseriesofconcentricclosepathsaroundthefarmfacilitiestocalculatetheCH4emissionsforthewholefacility.InCaliforniandairies,aircraftmeasurementswerecomparedtoopen-pathmeasurementsusinginversedispersionmodellingandvehiclemeasure-mentsmadewiththetracerfluxratiomethod.TheestimatedCH4emissionrateswerecomparedonawhole-farmlevelandforprimarysourceswithinafarm,suchasanimalhousingandliquidmanurelagoons(Arndtetal.,2018;Daubeetal.,2019).3.3.2SatelliteanddroneimageryPrecisionimagery,suchasdroneorsatelliteimagery,canbeutilizedtodetermineandmonitorsoilandcrophealth,andtoestimatetheyieldofcropsgiventhegoodcorrelationbetweentheleafareaindexandthenormalizeddifferencevege-tationindex(Lambetal.,2011;Nagy,FehérandTamás,2018;Wahab,HallandJirström,2018).Dronesandsatelliteshavealsobeenusedtotrackandcountanimals(Laradjietal.,2020)andtodetectCH4leaksfromoilandgasfacilitiesinnaturalgaspipelines(Barchyn,HugenholtzandFox,2019;Lauvauxetal.,2022;Tannantetal.,2018;Varonetal.,2018).Thereisapotentialforadaptingthesetechnologiestoassessandbenchmarklivestock-relatedCH4emissionsonfarms.Anewgenerationofremotesensingandsatellite-basedmonitoringsystemscon-tinuetosupportthequantificationandmonitoringofCH4fluxesfromricepro-duction.SatellitemeasurementsofCH4emissionsprovidebetterspatio-temporalcoverageofemissionsandemissionshotspotsthanmoretraditionalinsitumeasure-menttechniques.EarlysatellitemeasurementsofglobalCH4emissionsweremadewithSCIAMACHY(Frankenburgetal.,2006),andlaterwithGOSAT(Kuze37Methaneemissionsinlivestockandricesystemsetal.,2016;Houwelingetal.,2014).ThenumberofdedicatedCH4-focusedmissionshaveincreasedoverthepastseveralyearsandincludeGHGSat(Varonetal.,2018),GOSAT-2(Glumb,DavisandLietzke,2014),geoCARB(Polonskyetal.,2014),andMethaneSAT(Staebelletal.,2021).Eventhoughsomemissionsquali-fiedashyperspectralimagersorknownasimagingspectrometersarenotoptimizedforCH4mapping,theysamplethestrongCH4absorptionat2300nmwithtensofspectralchannels,whichcanbeexploitedforCH4retrieval(Varonetal.,2021;Guanteretal.,2021).Satellite-basedmeasurementsrelyoninversemodellingtounderstandandquantifyCH4emissionsatregionalandglobalscales(UNEPandCCAC,2021).Underinversemodelling,theatmosphericmeasurementsmadewithsatellitesareusedtoback-calculateboththelocationofanemissionssourceandtherateofemission(Houwelingetal.,2014;UNEPandCCAC,2021).ZhangG.etal.(2020)usedSCIAMACHYandGOSATatmosphericCH4-concentrationmeasurementscombinedwithMODIStimeseriesimageryofricepaddyproductiontobetterunderstandspatio-temporaldynamicsofriceCH4emissionsincontinentalmonsoon-proneAsia.TheyfoundastrongcorrelationbetweenareaswherericeisproducedatthecontinentalscaleandatmosphericCH4concentration,andconsistenciesinseasonalricegrowthandatmosphericCH4concentrations.Thecombinationofgeographicinformationandsatellitemeasure-mentscouldhelpreducethespatialuncertaintiesassociatedwithriceCH4estimatesinempiricalandprocess-basedmodels(ZhangG.etal.,2020).However,Zengetal.(2021)reanalysedthesameatmosphericCH4concentrationdatawithCH4simula-tionsfromachemicaltransportmodel,andfoundinsufficientevidencetosupporttheclaimthatspatialareasofriceproductionandatmosphericCH4concentrationsarecorrelated.Theseauthorscautionagainsttheuseofcorrelation-basedinferencetoestimateCH4emissionsfromriceproductionatregionalandcontinentalscales,andpointoutthatmoreworkcombiningsatelliteobservationsandmodelsimula-tionsisneededtoparseoutdifferentCH4emissionssources(Zengetal.,2021).Airborneandground-basedinsitumeasurementscontinuetobethemainmethodsformeasuringCH4concentrationsdespitetheirlimitations.Workpre-viouslycarriedoutinCaliforniaonrice(Peischletal.,2012)anddairy(Arndtetal.,2018)productionsystemsshowshowremotesensingtechniquescancaptureseasonalCH4emissionsdynamicsforthoseregionalproductionsystemsthatarenotaccountedforintraditionalbottom-upapproaches.Thesemeasurementtech-niquesarealsosensitivetocapturingCH4emissionsdynamicsunderdifferenttypesofmanagementsystems,i.e.residueburningvsresiduesoilincorporation(Peischletal.,2012),liquidslurryvsdrymanurestorage(Arndtetal.,2018),withimplica-tionsforGHGinventoriesandclimateactions.3.4UNCERTAINTIESMeasurementerrorassociatedwiththequantificationofaerialpollutants,suchasCH4,comprisesbothsystematicandrandomcomponents.Uncertaintyrep-resentsthequantificationoftherandomcomponent.Becauseuncertaintyestab-lishestherangeofvaluesthatthetruevalueofthemeasurementwillbewithin,theuncertaintyofemissionsmeasurementsmustbeknownwhenusingthemeasure-mentstodevelopemissioninventories,identifyemissionfactorsorcertifyemis-sionmitigation.Gatesetal.(2009)demonstratedhowcomponenterroranalysiscouldbeusedtoquantifyuncertaintiessuchastheairflowassociatedwithdirect38QuantificationofmethaneemissionsmeasurementofaerialpollutantemissionssuchasCH4.Hristovetal.(2018)examinedtherootsofuncertaintiesinpredictingCH4forinventorypurposes,andreportedthatanimalinventory,dry-matterfeedintake,thechemicalcompositionofthediets,CH4emissionfactorsandpredictionsofentericCH4emissionsarethemainculprit.Unfortunately,untilnow,uncertaintyhasnotbeenevaluatedforallpublishedemissionsvalues,whichmakescomparingresultsbetweenthedifferentpapers,evaluatingthequalityoftheresultsandcertifyingtheemissionreductionsdifficult.Onefuturechallengewillbeprovidingastandardmethodologyforuncer-taintyassessmentassociatedwithemissionmeasurements.Hristovetal.(2018)concludedthatquantitativeattributionofchangesinatmosphericCH4concentra-tionstoCH4sourcesbasedonδ13CH4data(stableisotopesignature,specifically13C/12Cusedintop-downmethodology)isatleastquestionable.394.Estimation4.1BOTTOM-UPAPPROACHESTheso-called“bottom-up”approachessumuptheestimatesofallidentifiedsourcecomponentsofagivenregionorboundarytoachieveanestimateoftheglobalsourceofCH4emitters,includingenteric,manureandsoil/cropemissions.AccordingtoLassey(2008),manyofthesecomponentsareill-quantifiedandthereisalackofagreementbetweendistinctestimates.The“bottom-up”approachesseemtofol-lowamoremechanistic,conceptual,build-upratherthanareconciliatoryapproach(e.g.“top-down”)thatmaybeunsuitableiftheactualsourcesarenotknown,andleadtoincorrectlyassigningestimatesharestoknownsources.Vibartetal.(2021)providedanextensivediscussionofmathematicalmodelsthatcanpredicton-farmCH4andN2Oemissions.4.1.1ModellingtoestimateentericmethaneTherearemanydifferenttypesofmathematicalmodellingmethodsinagriculture;themostcommononescanbeclassifiedaseitherempiricalormechanistic,stochas-ticordeterministicandstaticordynamic(FranceandKebreab,2008;ThornleyandFrance,2007).Forpredictabilitypurposes,somemathematicalmodelsofnutritionmayincorporatedifferent(andsometimescomplementary)methods,oftencalledlevelsortiersofsolutions(TedeschiandFox,2020a).Thesimplicityofempiricalmodelsiscommonlythedominantfactorinthedecision-makingprocesswhenselectingmodelstopredictCH4emissions.Inpart,themodels’simplicityisafunctionoftheinputsrequiredfortheexecutionofthemodel(essentiallyderivedfromstatisticalregressionmodelsandmethods),whichfavourstheselectionofempiricalmodelsovermorecomplex(andsometimesmorecomplete)typesofmodellingsuchasmechanisticorevenagent-basedmodels.Empiricalmodelsdonottakeintoaccounttheunderlyingbiologicalmechanismsbehindanaturalphe-nomenon,buttheyservetheirintendedpurposeofmakingdeterministicpredic-tions(TedeschiandFox,2020a)ifallinputs(e.g.variables)areavailableandwithintherangeoftheoriginaldatasetusedtodevelopthestatisticalregression.Anotherfactorwhichisrarelyconsideredisthatthenewinputsmusthavesimilarcorrela-tionsamongthemselvesastheinputsoftheoriginaldataset;otherwise,thevari-able’scoefficientsmightbeincorrect,andthepredictionwillbebiased.Cautionarynotesshouldthereforeaccompanymodelpredictionsbecausetheirlimitationsandintendedusemaynotbetheappropriatemathematicalmodelforalltypesofpro-ductionscenariosandspecificconditions.Ideally,differentalternativesformodelpredictabilityusingcontrastingmodellingmethodsshouldbeavailableandcon-sidered.Forinstance,theBeefCattleNutrientRequirementsModel(BCNRM)bytheNASEM(2016)providedempiricalandmechanisticoptionstopredictCH4productioninbeefcattle.TheBCNRM’sempiricaloptionwasdevelopedbasedonselectedempiricalequationsfortypicalbeefcattleproductionscenariosinNorthAmerica(Escobar-Bahamondesetal.,2017),whereasitsmechanisticoptionwasdevelopedbasedonmechanisticandempiricalapproachestomodeltherumenfunctions(Foxetal.,2004;NRC,2000),oftencalledfunctionalmodelsbecause40Quantificationofmethaneemissionstheysimultaneouslyhaveempiricalandmechanisticelementstosupportaspe-cificpredictivegoal(TedeschiandFox,2020a).Unfortunately,fewmathemati-calnutritionmodelshaveexplicitlymodelledCH4emissionfromthehindgutofruminants,inpartbecausetherumenrepresentscloseto90percentoftheCH4emission(Murrayetal.,1976;TedeschiandFox,2020a),andalsobecausethereisalackofinterestinpredictingthefermentationdynamicsinthehindgutbecausetheycontributelittle,ifatall,toruminantanimalperformanceandproduction.4.1.1.1EmpiricalmodelsBottom-upmodelsforpredictingemissionshavebeenusedinlieuofactualmea-surement.Thesemodelsdrawonregionalactivitydatatoestimateemissions.TheIPCC(2019)developedstandardpredictivebottom-upmodels.Thesemodelsaregenerallystratifiedintotiersdependingonthelevelofcomplexity.Tier1usesdefaultemissionfactorsbasedongeneralliteratureduetothepaucityofdatainaregion.Itdoesnotconsiderthecharacterizationoflivestocksystemsprevalentinaregion,suchasbreedtypes,ageofanimals,physiologicalstates,levelofproduc-tivity(exceptforcattleandbuffaloinTier1a),anddiet(intakeandcomposition).Tier2isbasedonemissionfactorsrefinedtoconsiderfeedandanimalcharacteriza-tion.Theemissionfactorsforeachlivestockcategoryareestimatedbasedonthegrossenergyintake(GEI)andCH4conversionfactor(Ym,expressedinpercentofGEIconvertedtoCH4).Tier3isbasedonyearsofextensiveresearchintheregion.TheIPCCmodelshavebeencriticizedbecausetheyassumeadlibitumfeedintakeandthatuncertaintiesaccompanyingthederivedemissionfactorsareill-defined,whichisoftenthecasewhenprevailingconditionsinaregionarenotconsidered(Goopyetal.,2018).Thereareseveralempiricalpredictionmodelsthathavebeendevelopedinthelastdecade(e.g.Benaoudaetal.,2019;Moraesetal.,2014;Niuetal.,2018;vanLingenetal.,2019).Thesemodelsarebasedondietaryintake,proportionsandcompositions,andanimalcharacteristics.ThereisageneralagreementwithinthescientificcommunitythatDMIiscrucialinpredictingCH4production.Forinstance,Benaoudaetal.(2019)reviewed36empiricalmodelsinvolving16dietaryandanimalvariables,andfoundthat56percentofthemodelsusedDMIasthebestpredictorofentericCH4productionwhile28percentofthemodelsselectedGEIasthemainpredictorofCH4production.Niuetal.(2018)developed42empiricalmodelsandsuggestedthatincreasedcomplexityimprovedprediction.TheyalsoreportedthatmodelswithDMIonlyhadagoodaccuracyofpredictionwhileotherdietaryvariablesfurtherimprovedthepredictionofthemodels.ThesefindingsareconsistentwiththosediscussedbyAppuhamy,FranceandKebreab(2016),whoreviewed40modelswith20variablesandfoundthat43percentofthemodelsusedDMItopredictCH4production.DeterminingDMIforstall-fedandconfinedanimalsisstraightforward,butmanylivestocksystemsinvolveruminantsgrazingonnativepastures,theirdietsupplementedwithcropresiduesandcultivatedfodder/forageinmixedcrop-livestocksystems.Workingoutthedietaryamountsandcompositioninthesesys-temsiscomplicated.Inpart,voluntaryfeedintakedependsonthedigestibilityofthediet(orthedigestibleenergy),which,inturn,dependsonthelevelofintake(Tedeschietal.,2019).Thisproblematicnaturebecomesmoreinvolvedstillowingtoalackofpropercharacterizationoftheprevailinglivestocksystems(i.e.numbers,41Methaneemissionsinlivestockandricesystemsbreeds,herdstructures,bodyweight,physiologicalstatesandlevelofproductivity)aswellasoftheterrestrialcharacterizationofthepastureland.TypicalmethodsforestimatingDMIincludeemployingempiricalmodelssuchasthosebasedonthenetenergysystem(NASEM,2016;NRC,2001;NRC,2007)orthosefactoringinani-malcharacteristics,pastureconditionsandsupplementation(CSIRO,2007),theuseofinternalandexternalmarkers,andherbagedisappearance(Macoonetal.,2003;Undietal.,2008).Thesemethods,beingestimates,haveinherentuncertaintiesthatcompoundandincreaseuncertaintiesinCH4predictivemodels.Insuchcases,itwouldbeadvisabletoadaptDMIestimatestolocalconditionsasmuchaspossible.Onesuchadaptationistherecoursetoa“feedbasket”,atermreferringtopropor-tionsoffeedsonofferinagivenseasonandinagivenregion,makinguptheseasonaldietoflivestockinthatlocality(Goopyetal.,2018;Marquardtetal.,2020).Anypredictivemodelisasgoodastheaccompanyinglevelofuncertainty.Itispossiblethatthemoreregion-specificthedataandmodel,thelowertheaccompa-nyinguncertainty.Predictivemodelsareusedtodevelopnationalemissioninven-toriesformonitoring,reportingandverifyingnationallydeterminedcontributionstowardsthemitigationofemissions(Bodanskyetal.,2016).Additional,targetedinputsmightfurtherimprovetheadequacyandpredictabil-ityofempiricalmodels.Anexampleisthemid-infraredspectrumofmilkcom-ponentsasaproxyforestimatingindividualCH4emissionswithchemometricmodels.Indeed,commonmetabolicprocesseswillaffectboththeamountoferuc-tatedCH4andthelevelofmilkcomponents(e.g.fattyacids).Milkmid-infraredspectrarepresentthechemicalbondsfromthecomponentspresentinthemilk.Moreover,milkmilk-infraredspectracanbeobtainedroutinelyatareasonablecost(alreadycollectedformilkpayment,milkrecordingorboth).Thisproxypresentsasignificantinterestforlarge-scalestudies(thatcompareanimals,herds,periods,geographicalregions,geneticstudies)(Vanlierdeetal.,2020),buttheinformationaboutthelimitationandapplicabilityofmilkmid-infraredspectraofmilkcompo-nentsisstilllacking.4.1.1.2MechanisticmodelsMechanisticmodelsrepresenttheunderlyingprocessesthatcontrolemissionsandtheirinteractions.ThereareveryfewmechanisticmodelsdevelopedtopredictCH4emissions.Adynamicmechanisticmodeldesignedtosimulatedigestion,absorp-tionandoutflowofnutrientsintherumenwasdevelopedbyDijkstraetal.(1992).Themodelcontains19statevariablesrepresentingN,carbohydrate,lipidandvol-atilefattyacid(VFA)pools.EntericCH4productionisestimatedbasedonVFAstoichiometrydevelopedbyBanninketal.(2006),whichrelatestheVFAproducedtothetypeofsubstratefermentedintherumen.Theassumptionisthatthehydro-genproducedintherumenfromthefermentationofcarbohydrateandproteinisused:i.)tosupportrumenmicrobialgrowth,ii.)forbiohydrogenationofunsatu-ratedfattyacidsandiii.)forproductionofglucogenicVFA(propionateandvaler-ate).TheremaininghydrogenisusedforthereductionofCO2toCH4.Thepre-dictionfromrumenmethanogenesisandhindgutfermentationisdescribedbyMillsetal.(2001).ThemodelhasbeenusedtoestimateentericCH4emissions,mostlyfromdairycattle(Alemu,OminskiandKebreab,2011;Kebreabetal.,2008;Morvayetal.,2011).AversionwithanupdatedVFAstoichiometrythatincludestheeffectofrumenpHonthestoichiometryofVFAformeduponthefermentationofsoluble42Quantificationofmethaneemissionssugarsandstarch(Bannink,ReijsandDijkstra,2008)isusedasaTier3methodforCH4inventoryintheKingdomoftheNetherlands(Bannink,vanSchijndelandDijkstra,2011).Ellisetal.(2010)introducedmodificationstothemodelinordertobeabletohandlepredictionsforbeefcattlebetter.MOLLYisanotherdynamicmechanisticmodelthatsimulatesrumendigestionandwhole-bodymetabolisminlactatingdairycows(Baldwin,FranceandGill,1987;Baldwin,FranceandGill,1987;Baldwin,ThornleyandBeaver,1987;Baldwin,1995).Themodelwasconstructedinasimilarwayasdescribedabove,buttheVFAstoichiometryisbasedontheequa-tionsdevelopedbyMurphy,BaldwinandKoong(1982),andlaterupdatedbyArgyleandBaldwin(1988),whichrelatetheamountofVFAproducedtothetypeofsub-stratefermentedintherumen.Inadditiontothestoichiometricdifferencesdescribedabove,thetwomechanisticmodelsdifferinthenumberofmicrobialpools;MOLLYusesonemicrobialpool,whereasthemodelproposedbyDijkstraetal.(1992)usesthreepools(amylolytic,fibrolyticandprotozoal).Severalstudieshaveevaluatedthepredictivepotentialofempiricalandmechanis-ticmodelsforentericCH4productionfromcattleusingindependentdatasources(Alemu,OminskiandKebreab,2011;Benchaaretal.,1998;Kebreabetal.,2006,2008).Benchaaretal.(1998)comparedthepredictivecapacityoftwomechanis-ticandtwolinearmodelswithadatabaseconstructedfromexistingliterature.Predictionsfromlinearequationswerepoor;themodelsexplainedbetween42and57percentofthevariation.Themechanisticmodels,ontheotherhand,explainedmorethan70percentofthevariation.Alemu,OminskiandKebreab(2011)com-paredempiricalmodelsandtheVFAstoichiometryusedinmechanisticmodelstoestimateandassesstrendsinentericCH4emissionsfromwesternCanadianbeefcattle.Theauthorsconcludedthatamorerobustapproachmightbetousemecha-nisticmodelstoestimateregionalYmvalues,whichwouldthenserveasinputforIPCCmodelsforinventorypurposes.AnothermathematicalmodelforpredictingVFAandruminalpHthatcanbeusedtoforecastCH4emissionwasdevelopedbyPittetal.(1996)andPittandPell(1997)withintheCornellNetCarbohydrateandProteinSystemframework.Basedonthemassbalanceapproach,theassumptionsindevelopingthemodelincluded:i.)theruminaldegradationoftrueproteinyieldsnegligibleamountsofVFAandCH4;ii.)CH4isthemainsinkofH2;iii.)ruminalNbalanceispositive;andiv.)theendproductsofruminalfermentationareessentiallycomputedasoneminusbacterialyield,multipliedbytheamountofruminallydegradedcar-bohydratecorrectedforbacterialash,CPderivedfromNH3-Nandthecarbonskeletonsofnon-carbohydratesources(TedeschiandFox,2020a,2020b).FurtheradditionstoPitt’smodelwerediscussedbyTedeschiandFox(2020a,2020b)andincorporatedintotheNASEM(2016),includingtheimpactofpectinonruminalpH,adjustmentsforbacterialnitrogen,andoptimizationforruminalpHgiventheratesofdegradationandescapeofcarbohydrates,VFAandlactate,andbuf-feringcapacityfromsalivaproductionandfeedcomposition.DespitethelimitedevaluationoftheVFA-pH-CH4modelconductedbyPittetal.(1996),theCH4emissionhasnotbeenfullyvetted.ThemodeldevelopedbytheFrenchInstituteforAgriculturalResearch(INRA,2018)servesasthebaseofaTier3methodtoestimateCH4emissionsofindoorandgrazingproductionsystems,givenavailableinformationonthetypeofanimal,productionlevel,anddietcharacteristicsandconsumption(Eugèneetal.,2019).43Methaneemissionsinlivestockandricesystems4.1.2Modellingtoestimatemanuremethane4.1.2.1EmpiricalmodelsSimilartoentericCH4,IPCC’sguidelinesfornationalgreenhousegasinventories(2019)indicatethreetiersofcomplexityinestimatingCH4producedduringthestorageandtreatmentofmanureandfrommanuredepositedonpasture.TheTier1approachisbasedondefaultemissionfactorspervolatilesolid(VS)unitbyanimalcategoryandmanurestoragesystem.Tier2isbasedoncountry-specificestimatesofVSandtheimpactofinteractionsbetweenmanuremanagementsystemsandanimalcategoriesontotalCH4emissionsduringexcretionandstorage,includingmanuretreatmentssuchasbiogasproduction.RecentemissionfactordatabasesmayhelptorefinetheTier2approachinlinewiththedistributionofclimateregionswithinacountry(Beltranetal.,2021;Viganetal.,2019;vanderWeerdenetal.,2020).Finally,Tier3requiresspecificmodellingapproachestailoredtocountry-specificmethodo-logiesormeasurement-basedapproachestoquantifyingemissionfactors.Likewise,severalmodelshavebeenusedtoestimatetheCH4emissionsfrommanurestoragesystems,buttheypossessahigherdegreeofuncertainty.Forexample,usingtheIPCCTier2methodforthemanagementofliquidmanureinanaerobiclagoonsandslurrystoragesystems,thereportedCH4emissionswereintherangeof368±193and101±47kgCH4/headperyear,respectively(OwenandSilver,2015).4.1.2.2MechanisticmodelsMechanisticmodellingofCH4emissionsischallengingbecauseofthecomplexdatarequirementandmodelparameterization(Lietal.,2012),whichlimitstheiruseforregionalorcountryestimates.Furthermore,theuseofmechanisticmodelsinLCAanalysisremainselusive.Asinthecaseofentericemissions,mechanisticmodelsofmanureemissionsarescarce.Onesuchmodel,Manure-DNDC(Lietal.,2012)isanextendedversionoftheDeNitrification-DeComposition(DNDC)model(Li,Frolking,S.andFrolking,T.,1992).Manure-DNDCwasdevelopedtosimulatebiogeochemicalcyclesofC,Nandphosphorus(P)inlivestockfarmsandcanbeappliedtosimulateGHG,ammoniaandnitricoxideemissionsfrommajorcompo-nentsoflivestockproductionfacilities.Themodelcontainsfundamentalprocessesdescribingtheturnoverofmanure’sOM.Arelativelycompletesuiteofbiogeochem-icalprocesses,includingdecomposition,ureahydrolysis,ammoniavolatilization,fermentation,methanogenesis,nitrificationanddenitrification,havebeenembed-dedinManure-DNDC,whichallowsthemodeltocomputethecomplextransferandtransformationsofC,NandPinlivestockproductionsystems.ThemodelhasbeenextensivelycalibratedforCaliforniancroppingsystemsandusedfordevelopingCalifornianCH4emissionsinventoryfromricepaddiesandN2Oemissionsinven-toryfromsyntheticfertilizersandcropresidue(Dengetal.,2018a,2018b).4.1.3Soil/cropmodelling4.1.3.1Empiricalmodels/IPCCmethodologyTheIPCCmethodologyforestimatingCH4emissionsfromricecultivationwasapprovedinternationallyasapartoftherevisedIPCCguidelinesfornationalgreenhousegasinventoriesin1996(IPCC,1996).Therespectiveguidelineswereupdatedin2006(IPCC,2006),followedbythe2019refinement(IPCC,2019).Theguidelinesforricecultivationcompriseafairlysimpleempiricalmodelbasedonemissionandscalingfactorsincombinationwithactivitydataoncropstatisticsand44Quantificationofmethaneemissionsmanagementinformation.ItshouldbenotedthattheseguidelinesweredevelopedforestimatingemissionsatthenationalscaleasrequiredintheGHGinventoriesunderthenationalcommunicationstobesubmittedtotheUNFCCC.Inthemean-time,however,themethodologyhasbeenappliedinavarietyofcontexts,rangingfromthelocaltotheglobalscale,andthusdevelopedintoastandardapproachforcalculatingCH4emissionsfromriceproduction.4=∑(,,×,,×,,×10−6),,Where:CH4Rice=annualmethaneemissionsfromricecultivation,GgCH4yr-1EFijk=adailyemissionfactorforconditionsi,j,andk,kgCH4ha-1day-1tijk=cultivationperiodofriceforconditionsi,j,andk,dayAijk=annualharvestedareaofriceforconditionsi,j,andk,hayr-1i,j,andkrepresentdifferentecosystems,waterregimes,typeandamountoforganicamendments,andotherconditionsunderwhichmethaneemissionsfromricemayvaryThedifferentconditionstobeconsideredinclude:i.)riceecosystemtype(irri-gated,rainfed,deepwater,anduplandriceproduction);ii.)floodingpatternbeforeandduringricecultivationperiod;andiii.)typeandamountoforganicamend-ments.OtherconditionssuchassoiltypeandricecultivarcanbeconsideredforthedetailedestimationifthespecificinformationabouttherelationshipbetweentheseconditionsandCH4emissionsisavailable.Threetierscanbeuseddependingondataavailability.Tier1appliestocoun-trieswhereeitherCH4emissionsfromriceproductionarenotakeycategoryorwherecountry-specificemissionfactorsdonotexist.InTier1,CH4emissionsareestimatedbasedontheavailabledataregardingtheannualharvestareaofriceafterthedisaggregationoftheareaaccordingtoitswaterregime:irrigated,rainfedandupland.Thecalculationsaredoneseparatelyforeachwaterregimeandorganicamendment.Tier2appliesthesamemethodologyasTier1,butcountry-specificemissionfactorsand/orscalingfactorsshouldbeused.Tier3comprisestheappli-cationofsimulationmodelsthatmustbevalidatedbyindependentobservationsfromcountryorregion-specificstudies(IPCC,2006).Irrespectiveofthetier,IPCCrecommendsusingactivitydatathatisdisaggregatedatthesubnationalleveluptothebest-possibleresolutionavailableforarespectivecountry.Ideally,theactivitydatawillroutinelybeupdatedthroughmonitoringnetworkstailoredtoaddressthenationalcircumstancesofricecultivation.4.1.3.1.1DailyemissionfactorandscalingfactorsAglobalCH4baselineemissionfactorproposedinthe2019refinementis1.19kgCH4ha-1d-1,withaconfidenceintervalof0.80to1.76.RegionalCH4baselineemis-sionfactors,rangingfrom0.65to1.32kgCH4ha-1d-1(IPCC,2019),arealsopro-posedtoenablethecollectionofmoredisaggregatedactivitydata.Theemissionfactorisadjustedtodifferentscalingfactorsinordertoaccountforthedifferenceinwaterregimeduringandbeforethecultivationperiod,andthetypeandamountoforganicamendmentapplied(IPCC,2019).InTier2,thescalingfactorsforsoiltypeandricecultivarcanbeincluded.45MethaneemissionsinlivestockandricesystemsInthecaseofcontinuouslyfloodedfields,thescalingfactorforwaterregimesdur-ingthecultivationperiodrangesfrom0.06fordeep-waterriceto0.71forafieldwithasingledrainageperiod(IPCC,2019).Thescalingfactorforuplandricecultivationiszero.Forwaterregimesbeforethericecultivationperiod,itrangesfrom0.59incaseoffieldswithoutafloodedpreseasonoveroneyearto2.41forthosewithafloodedpreseasonlongerthan30days.Thescalingfactorfororganicamendmentsisdeterminedasafunctionofboththeapplicationrateandthetypeoforganicamendments.Thelattercomprisescon-versionfactorsrangingfrom1forfreshricestrawto0.17,thelowestvalue,forcompost(IPCC,2019).4.1.3.1.2ActivitydataEstimationofCH4emissionsfromricecultivationbyempiricalmodelsisprimar-ilybasedonharvestedareastatistics,whichshouldbeavailablefromanationalstatisticsagency.Inmanyricegrowingcountries,thedurationofthecultivationperiodcanalsobeobtainedfromstatisticsbecausethisfactoriscloselyrelatedtothericevariety.Intherefinementof2019,thedefaultcultivationperiodofriceisestimatedonaglobalscale(113dayswithanerrorrangeof74to152days)aswellasonasubcontinentalscale(102to139days)(IPCC,2019).Acorrelationoflocallyverifiedcultivationareaswithavailabledatafortheemissionfactorswouldbeinvaluable.Internationaldatasourcesarealsoavailablefortheannualharvestedareaofrice,althoughthosedonotdistinguishbetweenriceecosys-tems(irrigatedvsrainfedrice),whichisanimportantfeatureofthemethodologyusedtoestimatemethaneemissions.DatarelatingtothericeareaharvestedcanbeobtainedfromFAOSTATontheFAOwebsite(www.fao.org/faostat).TheRicepediaonlinesourceprovidedbytheInternationalRiceResearchInstitute(IRRI,https://ricepedia.org/rice-around-the-world)featuresharvestedareasofricebyecosystemtypeformajorrice-producingcountries,alongsideotherusefulinformationsuchasaricecropcalendarforeachcountry.4.1.3.2MechanisticmodelsAmongthesoilbiogeochemicalprocess-basedmodels,theDeNitrification-DeComposition(DNDC)modelisprobablythemostwidelyusedtoevaluateGHGemissionsfromriceproduction(Gilhespyetal.,2014).However,othersoilbiogeochemicalmodelsliketheDailyCentury(DayCent;Partonetal.,1998;DelGrossoetal.,2001)andCH4MOD(Huangetal.,2004)havealsobeenusedforreportingnationalGHGemissionsattheTier3levelintheUnitedStates,andJapanandChina,respectively(IPCC,2019).TheDayCentmodelhasalsobeenparameterizedandvalidatedforChinesericeproductionsystems(Chengetal.,2013,2014).Bymeansofaspecificmethanogenesissubmodel,DayCentintegratessoilredoxpotential,soiltemperatureandCsubstratesupplydynamics–viathesoilorganicmatter(SOM)andplantproductionsubmodels–tosimulateCH4produc-tion(Chengetal.,2013).TheDNDCmodelisalsowellparameterizedforestimatingCH4emissionsfrommajorriceproductionregions(Giltrap,LiandSaggar,2010),anditisusedasaTier3methodinJapanforitsnationalGHGinventory(IPCC,2019;Katayanagietal.,2017).Themodelwasexplicitlydevelopedtorepresentcarbonsequestrationandtracegasemissionsinagriculturalproductionsystemsbymodellingmicrobial46Quantificationofmethaneemissionsactivitiesinresponsetoaerobicandanaerobicconditions,thelatterbeingcriticaltotheformationofCH4insoils(Li,2007).Forexample,theuseoftheDNDCsimu-lationsofCH4emissionswasabletobetterrepresentinbothJapan(Katayanagietal.,2017)andIndia(Pathak,LiandWassmann,2005)themanagementfactorsthatinfluenceCH4productioninricesystems–thatis,organicmatterinputs,totalproductionarea,drainageclasstypesandwatermanagement.InJapan,theDNDCmodelsimulationswereusedtogeneraterevisedemissionfactors(EF),whichresultedinhighernationalCH4emissionsthanpreviouslycalculatedbutreduceduncertaintyrelativetoTier1estimates(Katayanagietal.,2017).Whilemostsoilbiogeochemicalprocess-basedmodelssimulateabove-andbelow-groundplantCandNinputs,thesemodelswerenotdevelopedwiththeintentofrigorouslymodellingtheimpactsofvaryingcultivartypesandcertainenvironmentalconditionssuchaspestoutbreaksoncropyields,andtheresultingvariationinplantCandNinputstosoils.AsimportantdriversofsoilCseques-trationratesandtracegasemissions,theoverorunderproductionofcropCandNinputsdirectlyinfluencestheGHGbalanceofthecropproductionsystem(Katayanagietal.,2017).Inordertoovercomethischallenge,Tianetal.(2021)combinedthedecisionsupportsystemforagrotechnologytransfer(DSSAT)(Jonesetal.,2003;Sarkar,2012;Tianetal.,2014)cropgrowthmodel,whichincorpo-ratesricegeneticparameters,withDNDCtobetterrepresentcropyield,GHGemissionsandwateruse,andtoidentifybestmanagementpracticesforminimiz-ingthefood-water-GHGemissionstrade-offsinChina.FutureeffortstocombinecropgrowthandproductionmodelswithsoilbiogeochemicalmodelscouldhelpimproveGHGemissionestimatesfromricepaddysystemsbutalsoidentifyco-benefitsandtrade-offsassociatedwithmanagementdecisions.Unlikeinotherscientificfields,theuseofensemblemodellingisstillnotcom-moninsoilscience.AnensemblemodellingapproachcombinesmultiplemodelsormodelversionstosimulateGHGemissions.Thisapproachhelpsaddressuncer-taintyinrepresentingGHGemissionsdynamics,whichgenerallystemfromdif-ferencesinmodelstructureandrepresentationofdifferentbiogeochemicalpro-cesses(Parker2013),butalsotheuseofdifferentmodelinputdatasets(Tianetal.,2019).Whilesomeresearchinthisareahasaddressedcropproductionandyields(Assengetal.,2013),thereislimitedworkonapplyingensemblemodelsimulationstosoilN2O(Ehrhardtetal.,2018;Tianetal.,2019)andsoilCdynamics(Sándoretal.,2020).ThesubjectofensemblemodelsimulationsofCH4emissionsfromriceproductionremainsamajorgapintheliteratureandakeyareaforfutureresearch.4.2TOP-DOWNAPPROACHESTop-downapproachesmayprovidethemostaccurateestimatesofglobalCH4aftermassbalanceisappliedtoglobalsourcesandsinks(Lassey,2008).MeasurementsofCH4emissionsaremadealongaspectrumofspatialandtemporalscales,rang-ingfrominstantaneousforindividualsourcestoglobalassessmentsofannualCH4emissions.Bottom-upapproachestypicallyinvolvemeasuringatthescaleofindividualCH4emitters,suchaslivestockormanurestoragefacilities.Theseapproachesuseemissionsfactorsdevelopedbasedondatacollectedfromindivid-ual,activityandsometimesmechanisticmodels.Top-downapproaches,incontrast,estimateemissionbyusingobservationsofatmosphericCH4concentrationsandmodelsthataccountforatmospherictransportfromanemittertoanobservation47Methaneemissionsinlivestockandricesystemslocation(NASEM,2018).TheisotopiccharacterizationofCH4emissionsmaypro-videpowerfuldiscriminationbetweensources(Nisbetetal.,2020).Theproportionofbiogenicemissions(fromwetlands,ruminantsorwastes)resultsinashifttonegativevaluesofδ13CCH4(atmosphericCH4changingthecarbonisotoperatio)(Nisbetetal.,2019).Varioustop-downtechniquesareusedformeasuringCH4emissions,includingremoteobservation(e.g.atmosphericCH4byinfraredspec-trometry),towers,aircraftandsatellites.Manymodellingapproachesaresuitableforspatialscalesof10to100m(Lassey,2007).However,suchestimatesstillhaveahighuncertaintyandalsomaybedisputed,asinthecaseofHristovetal.(2013a).4.2.1Comparisonbetweenbottom-upandtop-downapproachesComparingestimatesproducedfrombottom-upandtop-downtechniqueshashelpedidentifyinformationgapsandresearchneeds.Insomecases,top-downesti-matesofemissionsandbottom-upinventorieshavesignificantlydiffered,leadingtoare-examinationofestimatesfrombothapproaches(NASEM,2018).Thechal-lengefortop-downapproachesisthatestimatesincludeemissionsfromallsourcesandmayhavedifficultyinattributingemissionstospecificsources.Bottom-upapproaches,ontheotherhand,provideestimatesfromspecificsources.Milleretal.(2013)usedatmosphericCH4observations,spatialdatasetsandahigh-resolu-tionatmospherictransportmodeltoestimateCH4sourcesintheUnitedStates.Theauthorsconcludedthatemissionsduetoruminantsandmanureareuptotwicethemagnitudeofthebottom-upapproachesusedbytheUSEnvironmentalProtectionAgency(EPA).Hristov,JohnsonandKebreab(2014)challengedtop-downestimatesmadebyMilleretal.(2013),andshowedthattheEPAestimatesagreewellwithothermorerefinedmodelsusedtoquantifyemissionsattheindi-vidualscale.AccordingtoNASEM(2018),uncertaintiesintop-downCH4emis-sionestimatesariseduetouncertaintiesintheatmospherictransportmodels.Furthermore,NASEM(2018)reportsthatcurrentglobalandregionalatmospherictransportmodelsareunlikelytobeabletoaccuratelyrepresentsmall-scalepro-cesses,makingitdifficultforthemtoaccuratelysimulateobservedCH4atcon-tinentalsites.Contemporaneoustop-downandbottom-upmeasurementswereconductedbyArndtetal.(2018).Theauthorsshowedthatwhole-facilityCH4emissionestimateswerecomparableinopen-path,vehicleandaircraftmeasure-ments.EmissionsfromanimalhousingweresimilartoEPAestimates,butCH4emissionsfromliquidmanurestoragewere3to6timesgreaterduringthesummerthanduringthewintermeasurementperiods.Short-termmeasurementsshouldthereforenotreplacelong-termmeasurements.Top-downandbottom-upmethodscouldbecomplementaryinidentifyinggapsandmayleadtoabettercharacterizationofCH4emissions.48PART3Mitigationofmethaneemissions5.MitigationstrategiesformethaneemissionsInthissectionweprovideabriefdescriptionofstrategieswithapotentialtodecreaseentericCH4emissionsfromruminantproductionsystems.Theapproacheshavebeenbroadlyclassifiedas:i.)animalbreedingandmanagement;ii.)feedmanagement,dietformulationandprecisionfeeding;iii.)forages;andiv.)rumenmanipulation.Someofthestrategiesarewellresearchedandavailableforimmediateadoptionwhileothersareconsideredexperimental.Inallcases,theadoptionpotentialofagivenstrat-egydependsontheproductionsystemandtheregionalorlocalconditions;hencetheneedfornumerousapproaches.Strategiesthatdifferinmodeofactionmayhavepotentiallyadditiveeffectswhencombined;however,thereisstillaneedforresearchontheefficacyofcombinedmitigationapproaches.Extensiveproductionsystemswithgrazingruminantsrepresentauniquechallengeformitigationbecausemanyofthedietaryandrumenmanipulationstrategies(e.g.feedadditivesupplementation)maynotalwaysbeapplicableinthosesystems.Forthosesystems,itwillbenecessarytoevaluatethemitigationoptionsandanypossiblelimitations.SeveralmetricsmustbeconsideredwhenaddressingtheefficacyofaparticularentericCH4mitigationstrategy.Somestrategiesdecreaseabsoluteemissions(gramsofCH4peranimalperday),somedecreaseemissionsyield(gramsofCH4perkilogramofDMI),andothersdecreaseemissionsintensity(gramsofCH4perkilogramofmeatormilkproduced).MethanemitigationcanalsobeevaluatedintermsofCH4energylossasaproportionofingestedgrossenergy(GE,Ym),andasCH4producedperkilogramofdigestedOM.Methaneyield,CH4producedperkilogramofdigestedOM,andYmareimportantvariablesforhelpingtounderstandhowemissionsaremitigatedbyacertainstrategyandthepotentialconsequencesitmayhaveontheanimal’senergyutilizationefficiency.ByadjustingforDMI,CH4yieldassesseshowefficaciousamitigationstrategymaybeindependentlyofpossiblechangesaffect-ingfeedintake,giventhatfeedintakeisthemainfactoraffectingCH4production.MethaneproductionperkilogramofdigestedOMfurtheradjustsfortheproportionofingestedfeedthatisactuallydigested.AsaproxyofthefeedfermentedintherumenavailabletoproduceCH4,itcanreflectchangesintherumenfermentationprofile.Inturn,YmprovidesametricofhowmuchextraingestedenergyispotentiallyavailableforanincreaseinanimalproductionwhenCH4formationintherumenisdecreased.Inthisdocument,wehavesubjectivelydefinedlowefficacyasdecreasesinCH4emissions(anymetric)lowerthan15percent,moderateefficacyasdecreasesbetween15and25percent,andhighefficacyasdecreaseshigherthan25percent.ItisimportanttoconsiderthatmitigationofentericCH4emissionsfromafarm,aregion,asectororacountry,orglobally,doesnotdependsolelyontheeffectsofamitigationstrategyonabsoluteCH4emissionsoronCH4emissionintensity.Mostrumenmanipulationstrategiestargetruminalmethanogenesisandthusdecreaseabsoluteemissions,withoutaffectinganimalperformance.StrategiesthatincreaseanimalperformanceandefficiencyofproductiontendtodecreaseCH4intensitybecausetheydilutethefeedenergyassociatedwithanimalorherdmaintenance.51MethaneemissionsinlivestockandricesystemsWhiledecreasedCH4emissionintensityrepresentsadesirableimprovementinGHGefficiency,absoluteCH4emissionscanactuallyincreaseiffeedconsumptionandproductionincreaseproportionallymorethanthedecreaseinCH4emissionintensity.However,thisisnotcommonlyobserved.RespiratoryCO2andCO2ofrumenoriginexpelledbyanimalsdonothavegreenhouseeffectsbecausetheyresultfromtheoxidationoforganiccarboncom-poundsingestedbytheanimals,whichareinturntheresultofplantbiomassaccre-tionfromatmosphericCO2byphotosynthesis;thus,CO2expelledbyanimalsisagrossbutnotanetsourceofCO2intheatmosphere.5.1ANIMALBREEDINGANDMANAGEMENT:INCREASEDANIMALPRODUCTION5.1.1DescriptionIncreasingbeefandmilkproductionthroughimprovementsinmanagement,nutri-tion,diseasepreventionandtreatment,andselectivebreedingorgeneticimprove-mentreducesCH4emissionintensitybutinmostcaseswillincreaseabsoluteemis-sionsonadailybasis.Variouspracticesandtechnologiesinanimalfeedingandhusbandrycanbeusedtoincreaseanimalproduction,suchasimproveddietfor-mulation,reducedenvironmentalstress,diseasepreventionandselectivebreedingforgreaterweightgainormilkyield(Knappetal.,2014;Beaucheminetal.,2020).5.1.2ModeofactionIncreasedanimalproductionreducesCH4emissionintensitybythedilutioneffectofmaintenance(CapperandBauman,2013),astheproportionofingestedfeedthatsupportsanimalmaintenancefunctionsisdecreased,whileincreasingthepropor-tionoffeedthatsupportsmeatand/ormilkproduction.However,increasedanimalproductionisgenerallyassociatedwithincreasedintakeandabsoluteemissions,unlessfeedconversionefficiencyisalsoimprovedsothattheincreaseinproductionisobtainedwithoutanincreaseinfeedconsumption.5.1.3EfficacyThemagnitudeofCH4intensitymitigationisvariable,rangingfromhightolow.Mitigationpotentialislargerinlow-producingthaninhigh-producinganimalsystems(Gerberetal.,2013a).Themitigationpotentialisgreatestforsmallholdersinlow-incomecountriesthattypicallyrelyonlargenumbersoflow-producinganimalstomeetthedemandforfoodproduction(Tricarico,KebreabandWattiaux,2020).Forexample,thereductionsarelargestindairysystemsthatproducelessthan2000kgoffat-andprotein-correctedmilkpercowannually,withreductionsinCH4intensitybecomingsmallerasproductionincreases(Gerberetal.,2011).Inallcases,thereductioninCH4emissionintensitymustbeaccompaniedbyareductioninanimalnumberstodecreaseabsolute(daily)CH4emissions.Thisisbecausehigherproducinganimalsconsumemorefeedtomeetnutrientrequirementsforgreaterproduction,therebyproducingmoreentericCH4andmanuredaily.Therefore,theincreaseinindividualdailyCH4emissionsmustbecompensatedbyaproportionallygreaterreductioninthenumberofanimalstodecreasethetotalemissionsofthecountryorregion.Replacingspecializedbeefherdsorsomeportionthereofwithdairyherdspro-ducingbeefdeservesconsideration.Bymakingmaintainingorevenincreasingbeefproductionpossiblewithfeweranimals,thiscoulddecreaseabsoluteemissionsand52Mitigationofmethaneemissionsemissionintensity.Butsuchasolutionmaynotbeapplicabletoallsituations,asinmanycountriesorregionsbeefcalvesareraisedonpastureswithlowerqualitysoilsthatcanonlymeettheenergyrequirementsofbeefcowsforgestationandlactation,ratherthanforfatteninganimals.Thus,semi-intensiveorintensivedairyproductionmaynotbepossibleunderthoseconditions.5.1.4PotentialtocombinewithothermitigationstrategiesIncreasedanimalproductioncanbeachievedusingacombinationofvariousprac-ticesandtechnologiesinanimalfeeding,breedingandhusbandry(CapperandBauman,2013).ThepotentialforcombiningthesepracticesandtechnologieswithmorefocusedCH4mitigationstrategies,suchastheuseoffeedadditivesormanurehandlingtechnology,isveryhigh(Knappetal.,2014).5.1.5EffectsonotheremissionsIncreasinganimalproductionmayincreaseCH4andN2Oemissionsfrommanurestorageandlandapplication,owingtotheincreaseinfeedintake(Gerberetal.,2013b).Inaddition,upstreamCO2emissionsmayalsoriseasaresultofgreaterenergyuseforcropcultivationandanimalmanagementassociatedwithincreasedanimalproduction.Ifgrazinglandsareabandonedasaconsequenceofincreasedanimalproduction,wildherbivorepopulationsmayreoccupytheecologicalnichesoflivestock,causinganetincreaseinCH4emissions(ManzanoandWhite,2019).5.1.6Productivityandthequalityofmeat,milk,manure,crop,andairAnimalproductionisincreasedalongwithmanureproductionandcropcultivationduetotheincreasedfeedintakebyindividualanimals.However,resourceuseeffi-ciencyandemissionsincreaseperunitofproductdecrease.ThiscanincreasefarmprofitabilitywhilereducingCH4emissionintensity(Knappetal.,2014).Increasinganimalproductioncanminimizethetrade-offsbetweenCH4mitigation,foodsecu-rityandproducerwelfare,particularlyinlow-producingsystems.5.1.7SafetyandhealthaspectsMostanimalfeedingandhusbandrypracticesleadingtogreateranimalproductionaresafefortheanimalsasarethefoodproductsderivedfromthem(FAOandIDF,2011).5.1.8AdoptionpotentialTheadoptionpotentialforpracticesandtechnologiesthatincreaseanimalproduc-tionishighinallanimalproductionsystems,butespeciallythosecharacterizedbylowproductivity.Educationandknowledgetransfer,availabilityofnaturalandtech-nologicalresources,andapositivereturnoninvestmentforproducersareneededtoimplementthesestrategies.Furthermore,successfuladoptionrequirestheidentifica-tionandbreakdownofbarriersfordifferentlivestocksystemsandregions,asdemon-stratedbyfailuresandsuccessesinadoptingrecognizedbestpracticesforincreasinganimalproductioninlow-incomecountries(Owen,SmithandMakkar,2012).5.1.9ResearchrequiredStudiesquantifyingtheeffectsofimprovednutrition,health,reproductionandgene-ticstoincreaseanimalproductionanddecreaseCH4emissionintensityarerequiredonaregionalbasissothatthesemeasuresarerelevantandcanbeimplemented.53MethaneemissionsinlivestockandricesystemsThisinformationisneededtohelpfarmersmakemanagementdecisionsbasedoneconomicandenvironmentaloutcomes.Akeyresearchquestioncentresonthepoliciesimplementedtoachievelowerglobalemissionsfromlivestockproduction.Iffeedconversionefficiencyisnotimprovedorifanimalnumbersarenotcapped,thengreaterproductivityincreasesCH4emissions.Reducingemissionintensitybecomesmoreimportantwhenexpandingruminantproductiontomeetthedemandforfoodofagrowingpopulation.5.2ANIMALBREEDINGANDMANAGEMENT:SELECTIONFORLOWMETHANE-PRODUCINGANIMALS5.2.1DescriptionAnimalbreedingthatexploitsnaturalanimalvariationinCH4emissionsisaninex-pensive,permanentandcumulativemitigationstrategy(Hayes,LewinandGoddard,2013).AtpresentthereareonlyafewinstanceswhereCH4istakenintoconside-rationinbreedingprogramsaroundtheworld,includingalarge-scalecommercialtrialwithsheepfarmerscurrentlyunderwayinNewZealandandaprogramintheKingdomoftheNetherlandswhichintegratesCH4emissionsintothebreedingdairyvalues(Roweetal.,2019;deHaasetal.,2021).5.2.2ModeofactionAnimalbreedingexploitsnaturalbetween-animalvariationinCH4emissions(deHaasetal.,2017).Variouspossiblemodesofactionhavebeenidentified:lowerfeedrequirement,increasedfeedefficiency,increasedfeeddigestibility,decreasedrumensize,increasedrateofpassage,improvedhealthandadifferentrumenfermentationprofile,hydrogendynamicsandmethanogenactivity.5.2.3EfficacyThemagnitudeofpossibleCH4mitigationisnotfullyunderstood.Earlierstudieshavebeenrelativelysmall-scale(Chagunda,RossandRoberts,2009;Garnsworthyetal.,2012;LassenandLøvendahl,2016),andlarger-scalestudiesareneededtodrawdefini-tiveconclusionsonthepotentialforincludingCH4inbreedingprograms(deHaasetal.,2017).IthasbeenestimatedthatdecreasesinCH4intensityindairyproductionrangingfrom13to24percentarepossiblebetween2018and2050,theirmagnitudedependingontheeconomicweightofCH4production(deHaasetal.,2021).5.2.4PotentialtocombinewithothermitigationstrategiesGiventhatgeneticselectionisbothcomplementaryandadditionaltoothermitiga-tionstrategies,geneticselectionforCH4canbecombinedwithothermitigationstrategies.AchallengingaspectisthatselectionforaCH4traittakesselectionpres-surefromothereconomicallyimportanttraitsofinterest.5.2.5EffectsonotheremissionsSelectionfordecreasedCH4mayalterOMdigestibility.5.2.6Productivityandthequalityofmeat,milk,manure,crop,andairSelectingsolelyforalowtotalCH4productionmaysimplyselectforlowerDMIandcanresultinlowerproduction(LassenandLøvendahl,2016;deHaasetal.,2017;Breider,WallandGarnsworthy,2019).Also,lowCH4-producinganimals54Mitigationofmethaneemissionsshouldintheoryhaveabetterconversionofdigestibletometabolizableenergy;however,theirlowerrumenretentiontimesmayresultinlowerdigestibility(McDonelletal.,2016;Løvendahletal.,2018).ToincludeatargetedselectionforCH4productionwithinabreedingprogram,thelinkbetweenCH4,animalproduc-tivityandeconomicsneedstobeconsidered.5.2.7SafetyandhealthaspectsNoadverseissuesrelatedtobreedinghavebeenreportedintheliterature.5.2.8AdoptionpotentialTheadoptionpotentialishighbutrequiresconsiderableinvestmentbyindustrytomeasureandidentifylowCH4phenotypes.Assessingananimal’sCH4phenotypeisdifficultbecauseCH4mustbemeasuredoveranextendedperiodoftime(weeks),andmeasurementsofthousandsofindividualsarerequiredtoincorporatethistraitintogeneticselectionprograms.ProxiesorindicatorsofCH4productionarebeingexploredasanalternativemeansofphenotypinglow-CH4animals.Oncethetraitisintegratedintothebreedingprogram,thereshouldbelittleimpedimentforadoption.Aconside-rabledifferenceintheadoptionpotentialinlow-incomeandhigh-incomecountriesistobeexpected.Aninvestigationofgenotypebyenvironmentinteractionwoulddeter-minewhetheroptimumgeneticsidentifiedinonecountryissuitableforanothercoun-tryorregion.Interactionswithdiettypesneedtobeexplored.5.2.9ResearchrequiredInformationisneededonlow-CH4animalphenotypes,whichwillinvolvemeasuringCH4productiononalargecohortofanimals(morethan2000)(deHaasetal.,2017).Substantialanalysisisrequiredtodeterminethemostappropriatetraitsforinclusioninaselectionindex;forexample,CH4emission(g/day),CH4intensity(g/kgproduct),CH4yield(g/kgDMI)orother.Eachtraitwillneedtobeevaluatedtoensurethattherearenonegativeconsequences.GeneticbreedingvalueswillhavetobedevelopedandestimatedagainsttheCH4traitofrelevance.Thefinalstepistoincludethetraitofinterestintheselectionindex.ThiscallsforalinkagebetweentheCH4traitofinterestandeconomics,whichcouldbedonebyplacingapriceonCH4emissions.5.3ANIMALBREEDINGANDMANAGEMENT:IMPROVEDFEEDEFFICIENCY5.3.1DescriptionImprovingfeedefficiency,definedastheratioofanimalproducttofeedintake(i.e.kgofmeatormilkperkgDMI),reducesCH4emissionintensity.Feedefficiencymaybeimprovedbyincreasingthenutrientdensityorfeeddigestibility,alteringtherumenmicrobialcomposition,enhancingfeedmanagementpractices(Knappetal.,2014),andselectivelybreedingforanimalswithnegativeresidualfeedintake2(Løvendahletal.,2018;Beaucheminetal.,2020)andsmallermetabolicbodyweight(VandeHaaretal.,2016),oracombinationoftheabove.2Residualfeedintakeisdefinedasthedifferencebetweenananimal’sactualfeedintakeanditsexpectedfeedintakebasedonitssizeandgrowth.55Methaneemissionsinlivestockandricesystems5.3.2ModeofactionImprovedfeedefficiencyreducestheamountoffeedanimalsconsumetomeetnutrientrequirementsinordertoproduceaunitofproduct(Løvendahletal.,2018).5.3.3EfficacyThepotentialforCH4mitigationthroughimprovedfeedefficiencyislowtomod-estindairycows(Knappetal.,2014),butmaybelargerinbeefcattleduetogreatergeneticvariability(Hristovetal.,2013a).5.3.4PotentialtocombinewithothermitigationstrategiesImprovingfeedefficiencycanpotentiallybecombinedwithothermitigationstrategies.5.3.5EffectsonotheremissionsImprovingfeedefficiencywillreduceabsoluteCH4emissions,CH4intensityandupstreamemissionsassociatedwithfeedproductionbecauselessfeedisrequiredtoproduceagivenquantityofanimalproduct.Inaddition,CH4andN2Oemissionsfrommanurestorageandlandapplicationarealsoreducedbecauselessmanureisproduced.Aswitchfromfibre-richforagetostarch-andprotein-richcultivatedfodderswillresultinincreasedfossilCO2emissions.DependingonthemagnitudeofthenaturalCH4emissionbaseline(ManzanoandWhite,2019)thisswitchmaynotresultinanetreducedwarmingeffect.5.3.6Productivityandthequalityofmeat,milk,manure,crop,andairImprovingfeedefficiencyincreasesanimalproductivityperunitoffeedandmayincreasefarmprofitabilitydependingonthecostoffeedwithrespecttothereve-nuesfrommeatandmilk.5.3.7SafetyandhealthaspectsCautionshouldbeexercisedinimplementingcertainanimalnutritionpracticesthatimprovefeedefficiencywhileincreasingtheriskofdigestiveupset,suchasagreaterinclusionofstarchorfatinruminantdiets(Knappetal.,2014).Cautionshouldalsobeexercisedwhenusinganunbalancedselectionfornegativeresidualfeedintakeasitcouldleadtoundesirableeffectsduetonegativelycorrelatedtraits(Løvendahletal.,2018).5.3.8AdoptionpotentialTheadoptionpotentialforimprovingfeedefficiencyrestsontheabilitytosafelyincreasethenutrientdensityordigestibilityoffeed,andthedevelopmentandincor-porationofacomplexfeedefficiencytraitinbalancedselectionindexes.Currently,genotypingananimalforfeedefficiencyiscostly.Howanimprovedfeedefficiencywillimpactonprofitabilitywillalsoneedtobeclearlydefined.5.3.9ResearchrequiredStudiesarerequiredtounderstandtheinteractionsbetweenfeedefficiencyandentericCH4emissions,astherehavebeenreportsofnegativecorrelationsbetweenthesevariables(FreetlyandBrown-Brandl,2013;Flayetal.,2019;Renandetal.,2019).UnderstandinghowthebiologicalfactorsthatinfluencefeedefficiencyandentericCH4emissionsinteract(Cantalapiedra-Hijaretal.,2018;Løvendahl56Mitigationofmethaneemissionsetal.,2018)requiresfurtherresearch.ResearchisalsoneededtostudytheeffectsonentericCH4emissions(bothintensityandabsoluteemissions)ofimprovingfeedefficiencyundervariousgenotypebyenvironmentbydietaryconditions.Thepotentialforcumulativeorsynergisticeffectsofimprovedfeedefficiencyandstra-tegicdietarymanagementaswellassupplementationshouldbeexamined.Aholis-ticbioeconomicevaluationofimprovingherdfeedefficiencyovertimeiscalledfor.Geneticselectionforfeedefficiencyisnotyetabreedingobjectiveinmostsystemsduetothelackofgenomictoolsdesignedtopredictfeedefficiency.5.4ANIMALBREEDINGANDMANAGEMENT:IMPROVEDANIMALHEALTH5.4.1DescriptionAnimalhealthimprovedthroughbreeding,diseasepreventionandtreatment,enhancednutritionorhusbandryisboundtoreduceCH4emissionintensity.5.4.2ModeofactionImprovedanimalhealthtypicallyincreasesanimalproduction(Dürretal.,2008;Hand,GodkinandKelton,2012)andimprovesfeedefficiency(Potter,ArndtandHristov,2018).Itdecreasesthefeedenergyandnutrientsusedbytheimmunesys-teminresponsetodiseaseandformaintainingtheanimal.Forexample,whenmas-titisoccurs,animmuneresponseiselicitedand,dependingonthepathogen,aseriesoflocalandsystemiceffectsmayoccur,includingadeclineinDMI(Ballou,2012)increasingemissionintensity.Ratherthanmobilizingtissuereservestocompensateforthislossofdietaryenergy,nutrientpartitioningchangesandanimalproductiondeclines(Ballou,2012).5.4.3EfficacyEfficacydependsonwhetherthediseaseitselfnegativelyaffectsfeedintake,digest-ibilityand/oranimalproductivity.ImprovedhealthislikelytoincreaseabsoluteentericCH4emissionsbuttodecreaseCH4emissionintensity(Potter,ArndtandHristov,2018).Areviewthatmodelledtheincreasedintakeandproductionaswellasanimallongevityresultingfromimprovedanimalhealthshowedareducedemis-sionintensity(vonSoostenetal.,2020).OtherstudieshavesuggestedthattherewouldbenoeffectorreductionofdailyentericCH4emissionsandlowtohighreductionsinemissionintensity(Hristovetal.,2015a;ÖzkanGülzari,VosoughAhmadiandStott,2018;Potter,ArndtandHristov,2018;vonSoostenetal.,2020).TheoveralleffectofimprovinganimalhealthonCH4emissionswillthusdependonwhetheranimalperformanceisnegativelyaffectedbydisease,andwhetherimprovedhealthincreasesproductivity.5.4.4PotentialtocombinewithothermitigationstrategiesMitigationeffectsofimprovedhealthareassumedtobecumulativewithotherCH4mitigationstrategies.5.4.5EffectsonotheremissionsImprovedanimalhealthislikelytoincreaseupstreamemissionsassociatedwithcropproductionifthefeedintakeandanimalperformanceincrease.Nitrousoxideemissionsfrommanuremightdecreaseiftheanimalsproducemore,asmoredietary57Methaneemissionsinlivestockandricesystemsnitrogenwouldberetainedinmeatandmilk(Arndtetal.,2015a).However,ifthereisanincreaseinfeedintake,theremayalsobeincreasedN2Ofrommanureasaresultofincreasednitrogenexcretion.5.4.6Productivityandthequalityofmeat,milk,manure,crop,andairAnimalproductionlossesandcostsofimprovinganimalhealthcanvarydependingonmanyfactors,suchasanimalageandpreviousinfections.Forexample,lossesfrommastitisvarydependingonthestageoflactationatthetimeofinfection,pre-viousinfections(Chaetal.,2013),parity(Bartlettetal.,1991)andthecausativepathogen(Chaetal.,2011).Milkproductionlosseshavebeenshowntorangefromaslittleas0.35kg/day(Halasaetal.,2009)toasmuchas4.18kg/day(Wilsonetal.,2004).Chaetal.(2011)reportedthat,onaverage,asinglecaseofmastitiscostfarmersbetweenUSD95.31andUSD211.03forthetreatment,thediscardedmilk,labourandculturingtests.Similarly,gastrointestinalparasitismineweshasbeenshowntoincreaseentericandmanureCH4intensity,andmanureN2Ointensity,by11,32and30percent,respectively(Houdijketal.,2017).Ingeneral,decreasingthemortalityofyounganimalswilllessenGHGemissions,asfewernon-productiveanimalswillhavetobemaintainedintheherd.Improvedanimalhealthalsodimin-ishesadultanimalcullingandtheneedforgrowingreplacements(Hristovetal.,2013b).5.4.7SafetyandhealthaspectsNoadverseeffecthasbeenreportedintheliterature.5.4.8AdoptionpotentialTheadoptionpotentialforexistingstrategiestoimproveanimalhealthisgreaterinhighincomecountries.However,inlow-andmiddle-incomecountries,itislowtomediumbecauseofthecostsoftreatmentsandpreventivecare,andofaccesstotreatments.5.4.9ResearchrequiredMostoftheresearchontheeffectofanimalhealthonCH4productionisbasedonmodelling(ÖzkanGülzari,VosoughAhmadiandStott,2018;vonSoostenetal.,2020)andonlyafewstudieshavemeasureddirectlytheeffectofhealthonentericCH4emissions(Arndtetal.,2015a;Houdijketal.,2018).Ingeneral,itispossibletocalculatetheimpactofthedecreasedmortalityofyoungandadultanimalsonthenumberofreplacementsandherdemissionsofentericCH4.MoreresearchisneededtobetterunderstandhowimprovementsinhealthimpactonentericCH4emissionofindividualanimalsbyaffectingDMI,includingitsmetabolismanddigestiveaspects.5.5ANIMALBREEDINGANDMANAGEMENT:IMPROVEDANIMALREPRODUCTION5.5.1DescriptionIncreasingthereproductiveperformanceofsucklerruminantsthroughmanage-ment,nutritionandbreedingresultsintheneedforfewernon-productivereplace-mentanimalswithinaherd.Indairyproduction,improvedreproductionincreasestheproportionoflactatinganimals.Improvedreproductiveperformancecanoccur58Mitigationofmethaneemissionsduetoreproductivemanagementandgeneticselectionforherdfertility.Theseapproachesshortenthecalvingintervalandageatfirstcalving,andincreasethelongevityofanimalsinaherd.5.5.2ModeofactionIncreasedfertilityreducesCH4emissionintensityofmeatandmilkproductionbyreducingthenumberofreplacementanimalsintheherd.However,theageprofileoftheherdincreases,andthisincreasesthetotaldailyemissionsfromtheherd.5.5.3EfficacyThemagnitudeofCH4mitigationdependsonthereproductivestatusoftheherd.Researchconductedgenerallyinvolvesmodellingatherdlevelratherthanthefarmsystemlevelwhichtakesintoaccountthegrowingandnon-productiveanimalsrequiredforeachproductiveanimalintheherd(Lovettetal.,2006a,2008;O’Brienetal.,2010;Lahartetal.,2021).5.5.4PotentialtocombinewithothermitigationstrategiesThepotentialtocombineincreasedreproductiveperformancewithothermitiga-tionstrategiesisveryhigh(Knappetal.,2014).5.5.5EffectsonotheremissionsImprovedreproductiveperformanceallowstoproducethesameamountofmilkorbeefwithfeweranimals.FeweranimalsdecreasethemanureoutputandassociatedemissionsofCH4andN2O.Inpasture-basedsystems,wherethelengthofthegraz-ingseasonislinkedtothecalvingdate,N2Oemissionsmaychangesubstantiallywithchangesinthelengthofthegrazingseasonandtherequiredfeedproduction.5.5.6Productivityandthequalityofmeat,milk,manure,crop,andairIncreasingreproductiveperformancemayincreaseanimalproductionifthepro-portionofmultiparouscowsintheherdincreases,becausetheyhavegreatermilkyieldsthanprimiparouscows(Hutchinson,ShallooandButler,2013).Increasinganimalfertilityshouldincreasefarmprofitability(Shalloo,CromieandMcHugh,2014)giventhatfewerreplacementanimalswouldberequiredtomaintaintheherd.Ifexcessreplacementheifersareusedtoproducebeef,thetotalanimalnumberswillincreasealongwithassociatedemissions.However,greaterbeefproductionfromexcessdairycalvescouldpotentiallyoffsetbeefproductionandemissionselsewhere.5.5.7SafetyandhealthaspectsNoadverseeffecthasbeenreportedintheliterature.5.5.8AdoptionpotentialAbalancedapproachisneededforgeneticselectionprogramstoincorporaterepro-ductivetraitsinadditiontoothereconomicallyimportanttraits.Selectingsolelyonthebasisofimprovedanimalproductionhasbeenassociatedwithreductionsinherdfertility.Theadoptionpotentialforpracticesandtechnologiesthatincreasefertilityishigh.However,therequirementsforsuccessfuladoptionareeducation,knowledgetransfer,availabilityofandaccesstoresources,andapositivereturnon59Methaneemissionsinlivestockandricesystemsinvestment.Implementationwillalsodependontheavailabilityofgeneticselectionprogramsthatincludefertilityinthebreedingobjectives.Inlow-incomecountries,theremaybelimitationsduetomanyofthesecomponents.5.5.9ResearchrequiredStudiesquantifyingtheeffectsofimprovedreproductiononCH4emissionsareneeded.Thisvaluableinformationwillenablefarmerstomakemanagementdeci-sionsbasedonbotheconomicandenvironmentaloutcomes.TheimpactonCH4emissionsofusingsexedsemenandembryotransfertoincreasethebeefmeritofanimalsfromthedairyherdshouldbequantified.Theuseofsexedsemenalongwithgoodherdfertilitycouldallowtargetedbreedingtomaximizegeneticgainwhileatthesametimemaximizingbeefmerit,therebylimitingherdexpansion.5.6FEEDMANAGEMENT,DIETFORMULATIONANDPRECISIONFEEDING:INCREASEDFEEDINGLEVEL5.6.1DescriptionInthissection,wediscusstheisolatedeffectsofincreasingthefeedintakeofanimals(i.e.feedinglevel)withoutalteringdietcomposition.Thatsaid,inpractice,theremaybefewproductionsituationsinwhichanimalscanbefedextrafeedwithoutalteringdietcomposition.Forexample,supplementinggrazinganimalswithcon-centratewilldecreasetheforagetoconcentrateratio.Atallerpasturewithgreatergrassavailabilitywilllikelybelessdigestible.Alteringfeedintakeanddietcompo-sitionaffectsanimalproduction.5.6.2ModeofactionIncreasingthefeedintakeofruminantsdecreasestheretentiontimeoffeedintherumenduetohigherpassagerates.ShorterretentiontimelimitsmicrobialaccesstoOM,thusreducingtheextentofruminalfermentation(GalyeanandOwens,1991)andleadingtoadeclineinCH4lossesperunitofDMIorasapercentageofgrossenergyintake(GEI).Inaddition,arapidpassagerateincreasesthegrowthrateofmethanogensandH2concentration,inhibitingacetate,H2andCH4productionandfavouringpropionateproduction,whichisacompetitivepathwayfortheuseofH2(Janssen,2010).Importantly,increasedfeedintakedecreasestheproportionofingestedandabsorbednutrientsandofenergyassociatedwithanimalmaintenance.Asaresult,increasedfeedintakedilutesCH4productionduetomaintenance,andagreaterproportionofCH4emittedisassociatedwithanimalproduction(Capper,CadyandBauman,2009).TheconsequenceofthisisthatthetotalCH4productionincreasesbecausethereismorefeedtoferment,butCH4asaproportionofDMIorGEIandCH4perunitofanimalproductusuallydecreaseathigherintakes.5.6.3EfficacyIncreasingfeedintakeincreasestotalCH4emissionsbutreducesCH4emissionrate(percentofGEIorYm)andyield(CH4/kgDMI)(BlaxterandClapperton,1965;Yanetal.,2010).Forexample,BeaucheminandMcGinn(2006)reportedthatYmdeclinedby0.77percentageunitsperunitincreaseinthelevelofintakeabovemain-tenance,whileHammondetal.(2013)observedadeclineintheCH4yieldofupto11percentperunitofDMIwithatwofoldincreaseinDMI.JohnsonandJohnson(1995)reportedanaverage1.6percentageunitdecreaseinYmperincreasedlevel60Mitigationofmethaneemissionsoffeedintakeabovemaintenance.Moreover,CH4intensity(perunitofproduct)decreaseswithincreasingintake,asincreasedintakeispositivelyrelatedtoincreasedproductivity.Knappetal.(2014)reporteda2to6percentdecreaseofCH4perenergy-correctedmilkforeachkilogramincreaseinDMI.EmpiricalpredictionmodelsforCH4productionshowgreatestaccuracywhenDMIisincludedasavariable(Appuhamy,FranceandKebreab,2016;Hristovetal.,2017;Niuetal.,2018),demonstratingthehighimpactofDMIonCH4pro-duction.Inthesemodels,thepositivelinearrelationshipbetweenDMIandpre-dictedCH4yieldshowedvariabilityacrossmodels(11.3to15.3gCH4/kgDMI),andwasmainlyattributedtodifferentchemicalcompositionanddigestibilityofdietswithinthedatasetsusedtodevelopthedifferentmodels(Niuetal.,2018),althoughthemeasurementtechniqueemployedcouldalsohaveaffectedtheestima-tions(Hristovetal.,2018).5.6.4PotentialtocombinewithothermitigationstrategiesWhileeasytocombine,inpracticetheeffectoffeedintakecaninteractwithotherstrategies(e.g.dietqualityandcomposition).Moreover,otherCH4mitigationstrategies,suchastheinclusionoftannins(Jayanegara,LeiberandKreuzer,2012)orcoconutoil(HollmannandBeede,2012)andotherlipids,maydepressfeedintake.5.6.5EffectsonotheremissionsAsisthecaseforincreasedabsoluteCH4emissionswithincreasedfeedintake,thetotalCO2andN2Oemissionsmayalsoincreaseduetotheadditionalfeedrequired,althoughCO2eqemissionsperunitofproductdecrease(Capper,CadyandBauman,2009).5.6.6Productivityandthequalityofmeat,milk,manure,crop,andairIncreasingthefeedinglevelcanincreaseproductivitydependingontheanimalcate-gory.Forexample,sucklerbeefcowsandsheepinthefirsttwo-thirdsofgestationmaynotbenefitfromincreasedoradlibitumfeedintakeduetotheirrelativelylowenergyrequirements.Inaddition,greaterintakeincreasestheexcretionoffecesandurine,potentiallyaffectingmanurecompositionandemissions(Hristovetal.,2013b),althoughperhapsnotperunitofanimalproduct.5.6.7SafetyandhealthaspectsThisisasafemitigationstrategyfortheanimal,theenvironmentandconsumers,onewhichhasbeenimplementedbyproducersandwhichdoesnotcallforgovern-mentregulations.However,anincreasedintakeofhighgraindietscanraisetheriskofrumenandsystemicacidosisandshouldbecarefullymanagedandmonitoredundersuchfeedingconditions.5.6.8AdoptionpotentialThismitigationstrategyiseasilyadoptableinproductionsystemswhereitispos-sibletoofferincreasingfeed.However,inextensivegrazingproductionsystemsthepossibilitiesofincreasingfeedintakecanbelimitedormayrequireconsider-ableadditionalexpense.Inallcases,decisionsaboutsupplementingextrafeedwilldependontheeconomicreturn.61Methaneemissionsinlivestockandricesystems5.6.9ResearchrequiredThegeneralprinciplesthatdeterminetheeffectsofincreasingfeedintakeondiges-tion,fermentationandCH4productionarewellestablished.However,itisimpor-tanttorefineexistingpredictionmodelsthatestimateCH4productioninresponsetoDMI,andtodevelopnewmodelsforparticularregionsordiets.Itisrecom-mendedthatthismitigationstrategybeaccompaniedbyabroaderevaluationofdietcharacteristicsthatcouldimpactefficacy.StudiesrelatedtoincreasingfeedintakeshouldalsoconsideritseffectontheemissionsofotherGHGs.5.7FEEDMANAGEMENT,DIETFORMULATIONANDPRECISIONFEEDING:DECREASEDFORAGETOCONCENTRATERATIO5.7.1DescriptionDecreasingtheforagetoconcentrateratioofthedietinordertoincreasetheenergydensityofthediet.5.7.2ModeofactionForagesarecomposedofmainlystructuralcarbohydrateswhileconcentratesarehighinsugars,starchandhighlyfermentablefibre.ThecompositionofthecarbohydratesconsumedaffectstheVFAprofileandCH4production(JohnsonandJohnson,1995).Aceticacidproductionispromotedinhigh-foragediets,resultingingreaterCH4productionperunitoffeed(HegartyandGerdes,1998;Janssen,2010).Ahigherpro-portionofconcentratesinthedietdecreasesthestructuralcarbohydrateproportionandincreasestherumenoutflowrate.HighergrowthratesofmethanogenscauseH2toaccumulate,whichinhibitsacetateandCH4productionandfavourspropionicproductionasanalternativesinkofmetabolichydrogen(HegartyandGerdes,1998;Benchaar,PomarandChiquette,2001;Janssen,2010).Moreover,therapidfermenta-tionrateofgrainslowersruminalpH,whichinhibitsthegrowthofmethanogensandprotozoa(vanKesselandRussell,1996;Hegarty,1999;Janssen,2010),therebydecreasingCH4productionperunitoffeedfermented.5.7.3EfficacyThereisgeneralagreementthatfeedingconcentratestoruminantsreducesCH4emissions,expressedrelativetoGEI,DMIandproduct,althoughtheadducedmagnitudevaries.JohnsonandJohnson(1995)reporteda2to3percentageunitdecreaseinGElostasCH4infeedlotsusinghighconcentratediets(i.e.morethan90percentconcentrate).McAllisteretal.(1996)reporteduptoa3.9percentageunitreductioninthepercentageofGEIlostasCH4withincreasingconcentrateintakebetween40and68gDM/kg0.75perday.BeaucheminandMcGinn(2005)reported1.5percentageunitlessCH4(4.5percentvs6.0percentofGEI)frombeefcattlefedprimarilygrainasopposedtoforagediets.Knappetal.(2014)reporteda2per-centdecreaseintheCH4toenergy-correctedmilkratioforeach1percentincreaseofnon-fibrecarbohydratesinthediet,uptoamaximumof15percentdecrease.SauvantandNozière(2016)quantifiedtheeffectsofconcentratepercentageonCH4/OMDfromtheresultsofcalorimetricmeasurementsgatheredinthe“Rumener”database,concludingthatenergylostasCH4isminimizedwithahighpercentageofconcentratefedathighintakelevels.ThedifferenceinmagnitudeoftheeffectofconcentratesonCH4,especiallyinmixeddiets,dependsonthe62Mitigationofmethaneemissionsproportionofconcentrateinthediet,thetypeofconcentrateandthefermentationcharacteristics(Moss,GivensandGarnsworthy,1994).SomeexperimentsevaluatingconcentratesupplementationofgrazinganimalshaveshownadecreaseofCH4perDMIandenergy-correctedmilk(Jiaoetal.,2014),whileothersreportednochange(Muñozetal.,2015;Lovettetal.,2005;YoungandFerris,2011).Thediscrepanciesforpasturestudiesmaybeattributedtothesubstitutionrate(concentratevspasture),pasturecharacteristicsordifferencesinmethodologyusedtoestimateDMI.AlthoughincreasingconcentratesupplementationdecreasesCH4productionperkilogramofDMI,OMDandanimalproduct,itcanleadtoanincreaseinabsoluteemissionsofCH4.ThisisbecauseconcentratesupplementationcanincreaseDMIanddigestibility(especiallyinlowqualityforagesystems),resultinginmoreOMfermentedintherumen.5.7.4PotentialtocombinewithothermitigationstrategiesThisstrategycanbeeasilycombinedwithothermitigationstrategies.SeveralstudieshaveshownadditiveeffectsofconcentrateandoilinclusiononmitigatingtotalCH4emissionsandemissionintensity(Lovettetal.,2003;Bayatetal.,2017).Methanogenesisinhibitorssuchas3-nitrooxypropanol(3-NOP)showsynergywithconcentrates,wherebythemitigationpotentialofinhibitorsinhighconcen-tratedietsisincreased(Schildeetal.,2021).Yeastshowedanadditiverelationshipwithincreasedconcentrateproportioninaninvitrostudy(Phesatchaetal.,2020);thoseresultswouldneedtobeconfirmedinvivo.5.7.5EffectsonotheremissionsIncreaseduseofgraintodecreaseCH4outputperproductwill,however,beaccompaniedbyincreasedemissionsofCO2andN2Ofromthefossilfuelsandnitrogenfertilizerusedtoproducethegrain(Boadietal.,2004;Beauchemin,McAllisterandMcGinn,2009).Conversionofpasturelandtocroplandresultsinthelossofsoilcarbon.SomestudieshaveshownareductionoftotalCO2eqperunitofproductwithincreasedconcentrate(Johnson,PhetteplaceandSeidl,2002;Lovettetal.,2006a).ThisemphasizestheneedtoevaluatetotalCO2eqemissionsusinganLCAforindividualfarmsandgeographicalregions(Beaucheminetal.,2008).ChangesinsoilcarbonneedtobeincorporatedintotheLCA.5.7.6Productivityandthequalityofmeat,milk,manure,crop,andairConcentratesarehighlydigestibleandthusfeedingconcentratesingeneralallowsforhigherlevelsofanimalproductivity.Milkandmeatfromanimalsfedconcen-trateshasmoresaturatedfatandfewerpolyunsaturated,rumenicandvaccenicacidscomparedtoanimalsfedconservedforages,especiallygrazinganimals.Ifincreasingconcentratepercentageinthedietincreasesintake,theamountofmanuremayalsobeincreaseddependingondigestibility.5.7.7SafetyandhealthaspectsIncreasingthepercentageofconcentratesinthedietisconsideredsafe,anditdoesnotrequireregulatoryapproval.However,increasingconcentratepercentageinruminantdietscancauseclinicalandsubclinicalacidosisandshouldthereforebeimplementedandmonitoredcarefully.63Methaneemissionsinlivestockandricesystems5.7.8AdoptionpotentialCerealgrainscanbeconsumedbyhumansandnon-ruminantanimals,whereasrumi-nantscanconvertfibrousfeedsthatareunsuitableforhumanconsumptiontohigh-qualityproteinsources(i.e.milkandmeat).Inthisregard,feedingruminantsconcen-tratesthatareediblebyhumansimpliesafeedvsfoodcompetition,andisregardedasundesirable.Inadditiontoforages,non-humanediblecropco-productsareconsumedinabundancebyruminantlivestock.ThisnicheroleofruminantsshouldthereforebebalancedagainstthedecreaseinCH4emissionsyieldandintensity(Boadietal.,2004),consideringalsothatanincreaseinabsoluteCH4emissionsmightoccur.Thisstrategyiseasilyadoptableinproductionsystemsinwhichintensificationispossible.Substantialincreasesincerealgrainusewouldbedifficultorevenimpossibletoimplementinmanyareasoftheworld,wherecerealcropscannotbegrownoraretooexpensive(Beauchemin,McAllisterandMcGinn,2009).However,ruminantsconsumeconsider-ablequantitiesoffoodwasteandco-products,convertingtheselow-valuematerialsintohigh-qualityproducts.Thereisanopportunitytoincreasetheuseofthesemateri-alsfromgrainsandoilseedsnotsuitableforhumanconsumption(e.g.frozen,off-grade,distillersgrainsandsoon)(Ominskietal.,2021).Adoptionwilldependonavailabilityandthecost-benefitratioofconcentratesupplementation.Itshouldalsobeconsideredthatsomeconsumerspreferanimalproductsfromgrazinganimals.5.7.9ResearchrequiredGiventhatthegeneralscientificconceptsarewellestablished,furtherresearchshouldbefocusedontheadoptionpotentialataregionalscale,usingtheLCAapproach.AquantificationofnaturalbaselineCH4emissionsinnaturalorrewildedgrazingecosystemsisneededtoassesshoweffectiveanincreasedratioofconcen-trateisinmitigatingglobalwarming.5.8FEEDMANAGEMENT,DIETFORMULATIONANDPRECISIONFEEDING:STARCHCONCENTRATESOURCESANDPROCESSING5.8.1DescriptionProcessingofgrainsandfeedingspecificsourcesofconcentratestopromotestarchfermentationintherumenand/orshiftthesiteofstarchdigestionfromtherumentotheintestines.5.8.2ModeofactionPromotingstarchfermentationintherumenincreasespropionateproduction,whichservesasanalternativesinkofmetabolichydrogentomethanogenesis(McAllisterandNewbold,2008;Ungerfeld,2015).Moreover,increasingstarchfermentationdecreasesruminalpHandinhibitstheproliferationofmethanogenicarchaea(vanKesselandRussell,1996)whiledecreasingtheabundanceofrumenprotozoa(FranzolinandDehority,2010).Theinhibitoryeffectonprotozoalimitstheirsymbioticroleinpro-tectingmethanogensfromoxygentoxicityandreducesthegenerationofH2assub-strateformethanogensinCH4formation(Newboldetal.,2015).Inaddition,theprocessingmethodandsourceofgraincanaffectDMandstarchdegradabilityintherumen.SlowerratesofruminalOMdegradabilitywillallowagreaterproportionofOMdigestiontooccurintheintestines,decreasingtheavailabilityofsubstrateforCH4productionintherumen.64Mitigationofmethaneemissions5.8.3EfficacyTheanti-methanogeniceffectofgrain-baseddietsdependsonthetypeofgrainandtheprocessingmethod(JohnsonandJohnson,1995).ThemagnitudeofCH4abatementfromgrainsourcesappearstofollowtheorder:wheat>corn>barley(BeaucheminandMcGinn,2005;Moateetal.,2017,2019).Feedingdairycowsawheat-baseddietreducesCH4emissions,yieldandintensitybyanaverageof30,48and41percent,respectively,comparedtocorn-basedandbarley-baseddiets.Similarly,Raminetal.(2021)reportedthatanoat-baseddietdecreasedCH4emissionsindairycowsby5percentcomparedtoabarley-baseddiet.Ithasalsobeenshowninfinishingfeedlotcattlethatfeedingacorn-baseddietreducedCH4yieldby30percentincomparisonwithabarley-baseddiet(BeaucheminandMcGinn,2005),possiblyduetodecreasedruminalstarchdigestibility(Yangetal.,1997).Furthermore,grainprocessingme-thods(theapplicationofvariouscombinationsofheat,moisture,timeandmechanicalactions)canmodifytheruminaldigestionofstarch(Theurer,1986),whichcouldinflu-encetheamountofCH4produced.Comparedtoadry-rolledcorn-baseddiet,feedingasteam-flakedcorn-baseddiettosteersreducedCH4yieldby17percent(Hales,ColeandMacDonald,2012).However,theanti-methanogeniceffectofgrainprocessingisvariableacrossstudies,andisgreatestforanimalsfedhigh-concentratediets.Methaneemissiondidnotdifferbetweensingle-rolledordouble-rolledbarley-baseddietsfedtodairycows(Moateetal.,2017),norforground-versuspressure-cookedcorn-baseddietsfedtocalves(Pattanaiketal.,2003).5.8.4PotentialtocombinewithothermitigationstrategiesThereislimitedinformationonthesynergisticeffectofcombiningthismitigationstrategywithothers.However,itappearsfeasibletocombinethiswithotherCH4mitigationstrategies,particularlytheuseofmethanogenesisinhibitors.InvitroexperimentshaveshownthattheCH4mitigatingeffectofwheatwasgreaterwhencombinedwithmethanogenesisinhibitors(nitrate,fator3-NOP),comparedtotheindividualeffectofwheat(Alvarez-Hessetal.,2019).5.8.5EffectsonotheremissionsFeedinggrain-baseddietsmayincreasetheGHGemissionsassociatedwithfeedproduction,especiallyifthegrainprocessingmethodinvolvestheuseoffossilfuelforthermaltreatment.Digestibilityofnutrientsmightdifferdependingonthegrainsourceandprocessingmethod,whichmightincreasetheexcretionofnutrientssuchasfermentableOMandnitrogen(BeaucheminandMcGinn,2005;Hales,ColeandMacDonald,2012),andtheamountofCH4,ammoniaandN2Oemissionsfrommanure(Gerberetal.,2013b).5.8.6Productivityandthequalityofmeat,milk,manure,crop,andairThisstrategyisexpectedtomaintainorimproveanimalperformance(milkyieldorweightgain)iftherationformulationiswellbalancedtosupplythenutrientrequire-mentsoftheanimals.However,milkproteinandfatconcentrationsmightdecreasewhenfeedingwheat-oroat-baseddiets,comparedtocorn-orbarley-baseddiets(Moateetal.,2019,2017;Ramin,FantandHuhtanen,2021)ifrumenpHdeclines.Thedecreaseinmilkcomponentscouldreducetheprofitabilityofdairyproducers.655.8.7SafetyandhealthaspectsGrainshavebeenroutinelyfedtohigh-producingruminantsanddonotposesafetyissues.However,feedinghigh-concentratedietscontaininggrainssuchaswheatandbarleycanlowerrumenpHandincreasetheriskofsubacuteacidosisandothermetabolicincidences,suchaslaminitisandliverabscesses,whichcouldimpairani-malhealth.5.8.8AdoptionpotentialThisCH4mitigationstrategyisreadilyavailableandcanbeeasilyimplementedinintensiveorconfinedfeedingsystemsbuthaslimitedpotentialforapplicationingrazingsystems.Processingandfeedingvariousgrainsourcesiseasilycarriedoutbyfarmersanddoesnotrequiregovernmentapproval.Formulatingdietswithgrainsourcescallsforsometechnicalexpertisetoensurethatthenutrientrequire-mentsoftheanimalsaremet.Thesuccessofthisstrategywilldependonthetypeofgrainavailable,thepricevolatilityofgrainsandthecostofprocessinggrains.Thecombinationofthesefactorscouldincreasefeedcostsandlimitthepotentialforadoptionofthismitigationstrategy.Moreover,aswellasmodifyingtheforagetoconcentrateratio(seeSection5.9),thisstrategycanincreasefood-feedcompeti-tionandmaycontrastwiththepositiveimageofruminantsasutilizinghuman-inediblefeedresources.5.8.9ResearchrequiredWhileaconsiderableamountofresearchondairycowshasbeenconducted,moreresearchisrequiredtocharacterizehowthegrainsourceandprocessingmethodcouldinfluenceentericCH4emissioninbeefcattleandsmallruminants.Theeffectofthegrainprocessingmethodanddegreeofprocessingontherateandextentofstarchdigestionneedstobeclarifiedwithregardtotheimpactonmetabolicdisor-derssuchasacidosis.AlthoughthemagnitudeofCH4abatementofwheat-baseddietsisattractivecomparedwithothergrains,thewideadoptionofthisfeedingstrategymightbelimitedduetothenegativeeffectonmilkfatproductionandprofitability.Thus,furtherresearchisrequiredtoidentifytheappropriaterationformulationbalancewithwheat-baseddietsthatwouldcounteractthenegativeeffectonmilkfatwhileretainingitsCH4mitigationpotential.Finally,theimpactofthisCH4mitigationstrategyonfeedemissionsandnutrientexcretionshouldbeconsideredwhenaccountingforthenetreductioneffectontheemissionintensityofmeatormilk.5.9FEEDMANAGEMENT,DIETFORMULATIONANDPRECISIONFEEDING:SUPPLEMENTATIONOFLIPIDS5.9.1DescriptionDietarysupplementationoflipids.5.9.2ModeofactionDietarylipidsproducetheirCH4mitigatingeffectthroughvariousmechanismsthatmodifytherumenecosystemandfermentation.Thesemechanismsincludetoxic-ityagainstmethanogensandprotozoa;biohydrogenationofunsaturatedfattyacidsservingasaminoralternativeH2sink;shiftingtheruminalfermentationprocesstopromotetheproductionofpropionateresultinginlowerCH4production;and66Mitigationofmethaneemissionsdecreasingfeedfermentabilityintherumen(Newboldetal.,2015;Honanetal.,2021).Lipidscanencapsulatefeedparticles,whichreducesrumenfermentation,leavingdigestiontooccurinthesmallintestine.Inaddition,aslipidsarelargelyunferment-able(exceptfortheglycerolmoiety),thereplacementofcarbohydrateswithlipidsreducesfermentableOM,thuscontributingtoadecreaseinentericCH4emissions.5.9.3EfficacySupplementationofdietarylipidsisaneffectiveCH4mitigationstrategy,althoughefficacydependsontheform(refinedoilvsoilseeds),sourceandamountofsupple-mentalfat,degreeofsaturationandnumberofcarbonsofthefattyacidsinthesupple-mentalfat,andnutrientandfattyacidcompositionofthebasaldiet(GraingerandBeauchemin,2011;Patra,2013).Variousmeta-analysisstudieshavebeenconductedtoelucidatetheCH4mitigatingeffectofdietarylipidsinruminants(Beaucheminetal.,2008;Eugèneetal.,2008;GraingerandBeauchemin,2011;Patra,2013,2014;Arndtetal.,2021).Thesestudiesshowthattheanti-methanogeniceffectsofdietarylipidsvaryconsiderablyoverabroadrangeofconditions.Beaucheminetal.(2008)reportedthataddingfattothedietsofsheep,beefanddairycattlereducedCH4yield(g/kgDMI)by5.6percentper10g/kgDMinclusionofsupplementalfat.Inothermeta-analysisstudies,CH4yielddecreasedby3.77percentincattle(Patra,2013)and4.30percentinsheep(Patra,2014)forevery10gfat/kgDMaddedtothediet.Patra(2014)indicatedthattheanti-methanogeniceffectofdietarylipidsisgreaterinsheepthanincattleduetothecomparativelylowerdepressionofDMdigestionandconsequentlowerdecreaseofCH4production.Medium-chainfattyacids(MCFA;lauric,myris-tic,andcapricandcaprylicacids)andpolyunsaturatedfattyacids(PUFA)arethemosteffectivefattyacidsforreducingCH4emissions.FeedingrefinedoilsrichinMCFA(e.g.coconutoilandpalmkerneloil)orpurifiedformsofMCFAsuchasmyristicacid(Machmüller,2006;Odongoetal.,2007;Hollmannetal.,2012)havebeenshowntoreduceCH4emissions.Similarly,feedingoilsoroilseedsrichinPUFAsources(e.g.fishoil,sunflower,canola,linseed,cottonseed,camelina,soybean,rapeseed)haveprovedeffectiveinreducingCH4emissions(Fievezetal.,2003;Jordanetal.,2006a;Martinetal.,2008;Graingeretal.,2010;Bayatetal.,2015;Ramin,etal.,2021).Mostoilseedsneedtobeprocessedpriortofeedingtoensureavailabilityofthelipidsintherumen.Oilsaretypicallymoreeffectivethancrushedoilseeds(Beaucheminetal.,2008),althoughthisdependsontheextenttowhichtheoilseedshavebeenprocessed.Inameta-analysis,Arndtetal.(2021)showedthatfeedingoils/fatsandoilseedshadcomparablemitigationeffectsondailyCH4production(-19percentand-20percent),CH4yield(-15percentand-14percent)andCH4intensityformilk(-12percentand-12percent).However,feedingoilseedshadnoeffectonCH4intensityforweightgain,whereassupplementaloilsandfatsreducedCH4intensityofweightgainby22percent(Arndtetal.,2021).Fewstudieshaveexaminedthelong-termeffectsofdietarylipidsonCH4emission;whilesomeresultsindicatethatlipidsupplementationhaspersistentanti-methanogeniceffects(Jordanetal.,2006b;Graingeretal.,2010),arecentstudyundergrazingconditionsshowedotherwise(Muñozetal.,2021).ExtrusionoflinseedbutnotofrapeseediseffectivefordecreasingCH4yieldandintensityindairy(Martinetal.,2011).TheinhibitoryeffectofdietarylipidsonCH4emissionisgreaterincon-centrate-basedasopposedtoforage-baseddiets(Patra,2013),possiblyduetolowerrumenpHassociatedwithconcentrate-baseddiets,whichenhancestheinhibitionoffattyacidsonmethanogens(Zhouetal.,2015).67Methaneemissionsinlivestockandricesystems5.9.4PotentialtocombinewithothermitigationstrategiesThesynergisticeffectofcombiningdietarylipidswithothermitigationstrategieshasbeeninvestigatedinonlyafewstudies.AnadditiveeffectofdietarylipidsonCH4abatementwasconfirmedwhencanolaoilwascombinedwith3-NOP(Zhangetal.,2021)andwhenlinseedoilwascombinedwithnitrate(Guyaderetal.,2015).However,therewasnoadditiveeffectwhensoybeanoilwascombinedwithanextractrichintannins(Limaetal.,2019)orsaponins(Maoetal.,2010).5.9.5EffectsonotheremissionsFeedingfatscancreateemissiontrade-offsfromfeedandmanure.Supplementingfatscanleadtoanincreaseinfeedemissionsassociatedwiththecultivation,pro-cessingandtransportationofrefinedoilsorprocessedoilseeds.TheeffectonfeedemissionscanbegreaterinthecaseofsoybeanandpalmkerneloilsourcedfromsomepartsofLatinAmericaandAsiaduetothehigherglobalwarmingpoten-tialassociatedwithsubstantialland-usechanges.Feedingahighconcentrationoffatscandecreasefeeddigestibility(Patra,2013,2014),whichmightincreasetheexcretionofOMandCH4lossesfrommanure(Mølleretal.,2014;HassanatandBenchaar,2019).However,feedingsupplementalfatsatlevelsthatdonotaffectfeeddigestibilitymightnotaffectemissionsfrommanure(Hristovetal.,2009).5.9.6Productivityandthequalityofmeat,milk,manure,crop,andairSupplementingfatsforupto4to6percentofthedietaryDM(totaldietaryfatof6to8percentmaximum)canimprovemilkproductionwhilereducingCH4emissions(-15percent)incattle(Patra,2013).However,feedinghigherconcen-trationsoffatscanhavedetrimentaleffectsonrumenfermentation,feeddiges-tionandanimalperformance(GraingerandBeauchemin,2011;Patra,2013,2014).Themeta-analysisconductedbyArndtetal.(2021)quantitativelyshowedthatfeedingoilsandfatsdecreasedDMI(-6percent)anddigestibility(-4percent)buthadnoeffectonmilkproductionorweightgain.FeedingoilseedsdidnotaffectDMIbutdecreaseddigestibility(-8percent)andweightgain(-13percent),withnoeffectonmilkproduction(Arndtetal.,2021).Supplementingdietarylipidsrichinlong-chainunsaturatedfattyacidscanimprovethenutritionalqualityofmeatormilkbyincreasingthecontentofhealthfulfattyacids,includingPUFA,conjugatedlinoleicacidsandvaccenicacid(Flowers,IbrahimandAbuGhazaleh,2008;Bayatetal.,2015).5.9.7SafetyandhealthaspectsThisstrategyisnotknowntoposearisktothesafetyofanimalsandhumansandisnotsubjecttoregulatoryapprovalprocesses.5.9.8AdoptionpotentialThisCH4mitigationstrategyisreadilyavailableandcanbeeasilyimplementedinintensiveandconfinedfeedingsystems.Rationformulationrequiressometechni-calexpertiseconsideringthatsupplementalfatsalsosupplydigestibleenergy,andcaremustbetakentoensurethatdietaryfatlevelsdonotexceedthethresholdof6to8percentofDMinthediet.Feedingrefinedoilscanbecostly,withlimitedpotentialforcommercialapplication.Asanalternative,processedoilseedscanbelessexpensiveandmightstimulatetheadoptionofsupplementingdietarylipids.68MitigationofmethaneemissionsAlthoughlimitedoptionsexisttoimplementthisstrategyingrazingsystems,pro-misingeffortshavebeenmadetobreedgrasseswithhighlevelsoffatsrichinPUFA(Winichayakuletal.,2008)orprovidingsupplementalfatthroughdrinkingwater(Osborneetal.,2008).5.9.9ResearchrequiredTostimulateuptake,furtherresearchisrequiredthatwouldidentifycost-effectivefatsourcesandtheirrespectivesupplementalleveltoreduceCH4emissionswithoutimpairingfeeddigestibilityandanimalproduction.Theinteractionoffatsandfattyacidswithotherdietaryfactors(suchasNDFandnon-fibrecarbohydrate)shouldbebetterunderstood,particularlyasregardstheCH4inhibitoryeffectofdietarylipids.ItmustbeensuredthatCH4inhibitionduetolipidsupplementationofdietsisnottheresultofadecreaseinfibredigestibility.Studiesarealsoneededtoascer-tainthelong-termeffectofsupplementalfatsinsuppressingCH4emissions.Givenitspotentialimpactonfeedemissionsandnutrientexcretion,theeffectivenessofthismitigationstrategyshouldbeaddressedusingLCA.5.10FORAGES:FORAGESTORAGEANDPROCESSING5.10.1DescriptionForagemanagementatorafterharvesting,suchastheformofpreservationorthealterationofparticlesize,tomodifyitsphysicochemicalcharacteristics.5.10.2ModeofactionMorethanonemodeofactionmaybeinvolved.Comparedtoitspreservationashay,ensilingforagecandecreaseCH4productionbecausethesolublecarbohy-dratefractionsfermentduringsilagemaking,therebyreducingrumenfermentation(McDonald,HendersonandHeron,1991).Processingstrategiessuchaspelletingincreasetherumenoutflowrate.ThegreaterpassageratedecreasesOMdegradationintherumen(Thomson,1972;HuhtanenandJaakkola,1993;Hironakaetal.,1996;LeLibouxandPeyraud,1999),whichresultsinlowerCH4production.Moreover,anincreasedpassagerateincreasesthegrowthrateofmethanogens,andconse-quentlytheH2concentrationincreasesaccordingtotheMonodfunction.AgreaterH2concentrationthermodynamicallyinhibitsH2production,withtheresultthatacetateproduction,whichreleasesH2,isalsoinhibited.LessH2beingproducedmeanslessH2beingincorporatedintotheCH4production.Fermentationisshiftedtowardspropionateproduction(Janssen,2010).5.10.3EfficacyJohnson,WardandRamsey(1996)reportedadecreaseofCH4yieldbetween20to40percentwhenforagewasgroundorpelleted,comparedwithfeedingani-malslongforage.Benchaar,PomarandChiquette(2001)reportedsimilarfindingsinasimulationstudy,withapproximatelya20percentreductionofCH4produc-tion(g/dayandpercentageofGEI)forpelletedincontrasttolongalfalfahay.TheefficacyofforageprocessingindecreasingCH4productionisgreatestwhenanimalsarefedadlibitumratherthanrestrictively(JohnsonandJohnson,1995;LeLibouxandPeyraud,1999).PelletingalsopromotesincreasedDMIwhenintakeislimitedbyrumenfill(Vermorel,BouvierandDemarquilly,1974),withtheeffi-cacyofpelletingbeingmorepronouncedforlow-qualityforages(Hironakaetal.,69Methaneemissionsinlivestockandricesystems1996).RelativelyfewstudieshaveexaminedtheeffectoftheforagepreservationmethodonCH4production(Knappetal.,2014).Benchaar,PomarandChiquette(2001)simulated33percentlessCH4(g/day,percentofGEI)foralfalfasilagecom-paredwithalfalfahay,duetoalowerruminaldegradationofOMascarbohydratesarepartlyfermentedinsilagemaking(McDonald,HendersonandHeron,1991).However,adecreaseinCH4productionduetothereducedruminaldigestionofOMwhenensilingorpelletingfeedmaynotdecreaseCH4perunitofmeatandmilkproduced,unlessDMIandanimalproductionincrease.Theeffectsofthepre-servationmethodwilldependonforagespeciesandthestageofmaturityoftheharvestedforage(Evans,2018).5.10.4PotentialtocombinewithothermitigationstrategiesForageprocessingandstoragemethodsareeasilycombinedwithotherCH4mitiga-tionstrategies,butwhethertheinteractionsarepositiveornegativeandwhethertheeffectsareadditivewillhavetobeevaluatedineachcase.5.10.5EffectsonotheremissionsEnsilingandprocessingincreasestheuseoffuelandresultsinadditionalCO2emis-sionsascomparedtograzingonfreshherbage.Moreover,reducedNDFdigestibi-lityduetoprocessingcanleadtoincreasedmanureemissionsofCH4(Knappetal.,2014).Therefore,awhole-farmLCAanalysis(Beaucheminetal.,2008)isneeded.5.10.6Productivityandthequalityofmeat,milk,manure,crop,andairNomajorconcernsinthisregardbecausedecreaseddigestibilityisgenerallymorethancompensatedbyincreasedintake,resultinginanincreasedintakeofdigestiblenutrients.DecreasedNDFdigestibilitycouldreducemilkfatproduction(Boadietal.,2004).5.10.7SafetyandhealthaspectsFinegrindingcanincreasetheriskofruminalacidosis(Boadietal.,2004).Itwouldhavetobecarefullymanagedbyadaptingtheanimalsgraduallyandmonitoringtheintakeofindividualanimals.5.10.8AdoptionpotentialEasytoadoptinnon-grazingsystems.Foragepreservationmethodsthatoptimizethenutritionalqualityoffeed,andhenceanimalperformance,arerecommended.Thesestrategies(especiallyensiling)arealreadyadoptedinmanypartsoftheworld.However,thegreaterneedformachineryorcontractingservicesleadstoadditionalcosts.5.10.9ResearchrequiredWhilestorageandprocessingofforagehasbeenshowntodecreaseCH4yield,itisnotcleariftheCH4perunitofanimalproductisalsodecreasedasthereislimitedliteratureonthissubject.Studiesneedtoconsiderwhole-farmCO2eqemissionsasadecreaseinentericCH4productionmayincreaseemissionselsewhereinthefarmingsystem.Sincethiscanvarywidelybetweensystemsandregions,studiesparameterizinglocalproductionsystemsareneededtodeveloppredictivemodels.70Mitigationofmethaneemissions5.11FORAGES:INCREASEDFORAGEDIGESTIBILITY5.11.1DescriptionIncreasingforagedigestibilityleadstoimprovedanimalperformance,decreasingtheemissionsofCH4perunitofproduct.5.11.2ModeofactionForagesaremoredigestiblewheninavegetativephenologicalstageofmaturity.Inpastoralsystems,foragedigestibilitycanbeincreasedbyoptimizinggrazingmanage-mentsothatthepre-grazingherbalmassandheightarenotexcessive.ThedigestibilityofOMisoftenhigherforlowthanforhighherbalmassswards.Thedigestibilityofforagesconservedashayorsilagecanbemaximizedbycuttingandpreservingatavegetativephenologicalstage.Treatmentswithalkalis,urea,fibrolyticenzymesandlignolyticfungihavealsobeeninvestigatedtoseewhethertheyincreasethedigesti-bilityofmatureforages(Adesoganetal.,2019).Increasingforagedigestibilityraisesanimalproductivity,forageintakeanddigestion.Responsestoincreasedforagedigesti-bilityintermsofabsoluteCH4productioncanbevariable,butabsoluteCH4produc-tionusuallyincreaseswithgreaterDMIandincreasedOMfermentationintherumen.Whenmeasuredinvitro,concentrationsofneutraldetergentfibreandindigestiblefibrereduceCH4production,whileconcentrationsofwater-solublecarbohydratesandOMdigestibilityoftheforageincreaseCH4production(Weibyetal.,2022).Thus,selectingforagesthatincreaseinvitroCH4productionwilllikelydecreaseCH4inten-sity(CH4perkgofDMI,milkormeat).TheamountsofadditionalCH4producedwhenfeedinglow-fibre,highlydigestibleforagesarerelativelylowerthanthosepro-ducedbytheanimalproduct(Beauchemin,McAllisterandMcGinn,2009).5.11.3EfficacyGreaterforagedigestibilitycanincreaseabsoluteCH4emissions,butitgenerallyresultsinlowtomoderatedecreasesinCH4emissionintensity(Beaucheminetal.,2020).DairycowsgrazingonswardsdifferinginpregrazingherbalmassproducedsimilaramountsoftotaldailyentericCH4percow,buttheincreaseinmilkproduc-tionwithlowherbagemassresultedina10percentlowerentericCH4intensity(Muñozetal.,2016).Cowsfedfreshherbagegrasscutafterashorterregrowthperiodproducedmorefat-andprotein-correctedmilkandthesametotalamountofCH4,butCH4intensitywas12percentlowerwiththeshortergrassregrowthperiod(Warneretal.,2015).Warneretal.(2016)comparedgrassensiledatthreestagesofmaturity,andreportedthatensilinglessmaturegrassresultedingreaterDMintensity,DMdigestibilityandmilkproduction.AbsoluteCH4productionwas6percentgreaterwiththeearliestcutgrass,butCH4intensitywas24percentlower.Macomeetal.(2018)evaluatedgrassensiledatfourdifferentstagesofmaturity,andconcludedthatCH4yield,CH4productionperkilogramofingesteddigestibleOMandCH4intensityofdairycowswere16,24and21percentlower,respectively,fortheyoungestcomparedtotheoldestcutgrass.5.11.4PotentialtocombinewithothermitigationstrategiesFromapracticalpointofview,improvedforagedigestibilityiseasytocombinewithotherCH4mitigationstrategiesatthefarmlevel.Whetherbiologicalresponsesareadditive,orwhetherpositiveornegativeinteractionsexistremainstobeinvestigated,whenincreasedforagedigestibilityiscombinedwithotherCH4mitigationstrategies.71Methaneemissionsinlivestockandricesystems5.11.5EffectsonotheremissionsEmissionsofGHGotherthanentericCH4willbealteredbygrazing(e.g.changesinstockingrates)orchangesincuttingmanagementthataffectthedigestibilityofconservedforages.Earlierherbagecuttingforensilageorhay-makingwillresultinalowergrassbiomassavailable,thusaffectingthefossilfuelemissionsofCO2perkilogramofDMconserved,althoughlessfossilfuelperhectaremaybeneededtoharvestandensileorbaletheforage.Downstreamemissionswillalsobeaffected,asagreaterdigestibilitywilldecreasetheoutputofmanureaswellaschangingitscomposition,andperhapsdecreasetheemissionsofCH4frommanureaccordingly.Nitrogenexcretioninurineandfecesmayalsobeaffected,asforagescontainmoreNatvegetativestages.Bestpracticesingrazingandbettermovementofanimalsonpasturecanreducetheheterogeneousdistributionofmanure,thusreducingN2Oemissions.AnLCAwillbeneeded,andlocalresearchisrecommendedtoestablishreliablepracticesforeachregion.5.11.6Productivityandthequalityofmeat,milk,manure,crop,andairAnimalproductivityisexpectedtoincreaseasforagequalityincreases,whereasmanureoutputisexpectedtodecrease.Changesinmanurecompositionanddegra-dationcharacteristicsaswellasitsmethanogeniccapacityneedtobeinvestigated.Higherstockingratescanresultinincreasedammoniaemissionsfrommanuredepositedtosoils,whichleadstoairqualitydegradation.5.11.7SafetyandhealthaspectsTherearenosafetyconcernsforanimals,humans,foodortheenvironment.Noapprovalsfromgovernmentagenciesarerequired.5.11.8AdoptionpotentialIncreasingforagequalitywithresultingincreasesinanimalproductivityisregardedfavourablybyproducers(Knappetal.,2014).However,cuttingearlyreducestheoverallforagebiomassformakinghayorensilinganditwillincreasecosts,whichcanmakeitunattractivetosomeproducers,unlessoverallbenefitsinproductionandprofitabilitycanbedemonstrated.Localresearchtodeterminetheoptimalcut-tingstagesisrecommended.Demonstrationsystemsatmodelcommercialfarmsmaybeneededforthewidespreadadoptionofeconomicallybeneficialsystems.5.11.9ResearchrequiredTherequiredknowledgeregardingthebiologicalresponsesandthenecessarytech-nologiesexistandareavailable.MoreresearchisneededonhowforagecharacteristicsaffectCH4emissions.Localresearchisrecommendedforestablishingoptimalcut-tingtimesforforages,onesthatmaximizeanimalproductionandfarmprofitability.Lifecycleassessmentsconductedattheregionallevelareneeded.Suchresearchwillalsohelptoestablishemissionfactorsspecifictoeachtypeofgrasslandandpasture.5.12FORAGES:PERENNIALLEGUMES5.12.1DescriptionIncreasingtheproportionoflegumeforages(e.g.alfalfa)inruminantdiets.72Mitigationofmethaneemissions5.12.2ModeofactionThehighlyvariablenutritiveprofileofforagesaffectsentericCH4production.Atthesamephysiologicalstageofmaturity,legumeforagescontainlessneutraldeter-gentfibre(NDF)thangrasses,andalthoughthefibreinlegumesismorelignified,thedeclineinfibredigestibilitywithadvancingmaturityismuchgreaterforgrassesthanforlegumes.Fibrethatismoredigestibleresultsinaruminalfermentationthatdecreasestheacetatetopropionateratioandmethanogenesis.Inaddition,legumescontainsecondarycompoundsthatdecreaseCH4production(i.e.condensedtan-ninsandsaponins;seeSection5.25andSection5.26),althoughconcentrationsofthesecompoundsarehighlyvariable(MacAdamandVillalba,2015;AboagyeandBeauchemin,2019;Kozłowskaetal.,2020).Thereisinterestintannin-containingtropicallegumessuchasLeucaenaleucocephalaandDesmanthusspp.(Suybengetal.,2019).Lastly,animalperformanceisoftenincreasedwiththeinclusionoflegumesinruminantdiets,whichdecreasesCH4intensity.5.12.3EfficacyThefactthatthereductioninCH4productionduetothedietaryinclusionoflegumesdependsonthequalityoftheforagesbeingcomparedmakesitdifficulttoquantify,asdifferencesinfeedintakeanddigestibilityrendertheresultsconfusing.Fortemperateforages,ameta-analysis(n=112treatmentmeans)byArchimèdeetal.(2011)reportednodifferenceinCH4betweenlegumesandC3grasses.Inotherstudiescomparingtemperateforages,reductionsinCH4productionduetofeedinglegumesratherthangrasseshavebeennon-existentorinconsistent(Chavesetal.,2006;Dinietal.,2012;Hassanatetal.,2013,2014;Arndtetal.,2015b).Whenitcomestoforagesgrowninwarmerenvironments,Archimèdeetal.(2011)reportedthatlegumesproducedlessCH4perkilogramofintake(DM,-19percent;OM,-24percent;digestibleOM,-26percent)thanC3orC4grasses.However,thoseresultswerenotsubstantiatedbyKennedyandCharmley(2012)whoreportedthatCH4forcattlefedtropicalgrassesaccountfor5.4to7.2percentofGEI(10.9–13.4percentofdigestibleenergyintake),whereasfortropicalgrass–legumemixtures,thevalueswere5.4to6.5percentofGEI(8.6–13.0percentofdigestibleenergyintake).ThenotableexceptionwasthelegumeLeucaenaleucocephala,whichdecreasedCH4yieldby11percentwhenitsinclusionratewasdoubled;similareffectswerenotobservedforotherlegumespecies(KennedyandCharmley,2012).Thus,theuseoflegumesmaybeaCH4mitigationstrategyinareaswithwarmerclimateswherethedigestibilityofgrassesdeclinesrapidlywithincreasingmatu-rity,themitigationeffectbeinghighlydependentonforagespeciesandquality.Whenthenutritivevalueofthediet(digestibility,CP)increaseswiththeincor-porationoflegumes,animalperformancewouldbeexpectedtoincrease,therebydecreasingCH4intensity.5.12.4PotentialtocombinewithothermitigationstrategiesItcanbeeasilycombinedwithotherstrategies,especiallythosewithdifferentmodesofaction.5.12.5EffectsonotheremissionsPerenniallegumeforagesbiologicallyfixN,whichreducestheamountofNfertil-izerusedandconsequentlytheCO2emissionsfrommanufacturingN-containing73Methaneemissionsinlivestockandricesystemsfertilizers(Rochonetal.,2004;Lüscheretal.,2014).BiologicalfixationofNalsoincreasestheNavailableforassociateandsubsequentcrops(Schultze-Kraftetal.,2018).Thenitrogenfixedbylegumeforagesisstillsubjecttolosses,andthuscontributestoN2Oemissionswhentheirresiduesdecay(Guyaderetal.,2016),althoughemissionsofN2Obylegumesarelowercomparedtothosegeneratedbygrassswards(Lüscheretal.,2014).Perennialforagescanincreasesoilcarbonstor-age(Littleetal.,2017),helpingtorehabilitatedegradedsoils,especiallyintropicalareas(Schultze-Kraftetal.,2018).Changesinthedietaryforagesourcecanaffectthephysicochemicalcharacteristicsofmanure.Forexample,CH4emissionsfrommanureslurrywerelowerwhenfeedingdairycowsalfalfacomparedwithmaizesilage(Masséetal.,2016).FossilfuelCO2emissionsfromtheuseoffarmequip-mentarealsolowerforperennialcomparedwithannualforages,suchasmaizesilage(Hawkinsetal.,2015).Foragelegumesgenerallyhavehighnutritivevalue(digestibleenergyandcrudeprotein),whichcandecreasetheuseofpurchasedsup-plementsandofassociatedcostsandemissions(Schultze-Kraftetal.,2018).5.12.6Productivityandthequalityofmeat,milk,manure,crop,andairTheeffectsonanimalproductivityofincreasingtheproportionofforagelegumesinthedietarehighlydependentontheproductionsystemandspecificforages,andthuscannotbebroadlyquantified.Rochonetal.(2004)estimatedpositiveeco-nomicbenefitsfromlegumeandlegume-grasssilagescomparedwithgrasssilageintheUnitedKingdomandtheEuropeanUnion.Johansen,LundandWeisbjerg(2018)conductedameta-analysisoftemperateforagesindairycows’dietsandcon-cludedthat,overall,legume-baseddietsresultedinhigherDMIandmilkyieldthangrass-baseddietsbuttherewasnodifferenceinfeedconversionefficiency.Milkfatandmilkproteinconcentrationswereloweronlegume-baseddietscomparedwithgrass-baseddiets.However,thereweredifferencesinDMIandenergy-correctedmilkamongthelegumes,andthusnotalllegumesareequallyeffective.Bothanimalandforageproductivityneedtobeconsideredwhenconductingasystemanalysis.5.12.7SafetyandhealthaspectsNomajorconcerns.Grazedcloversandalfalfacancausebloat(timpanism),butthisaspectisknownandcangenerallybewellmanagedbyfarmers.Itshouldbecon-sideredthatsomelegumescontaintannins,whichifconsumedinexcesscandepressdigestibility.Acceptedbyregulatoryofficials.5.12.8AdoptionpotentialTheadoptionpotentialishigh,buthighlydependentonclimate,soilandthegrow-ingenvironment.Aregionalapproachusinglifecycleassessmentisneeded,priortorecommending.MayhavegreaterCH4mitigationpotentialintropicalareas,wherethedigestibilityofgrassesrapidlydeclineswithincreasingmaturity,andwherecon-centrationsofsecondaryplantcompoundsarerelativelyhigh.Legumesfixnitrogenandthusdecreasetheneedfornitrogenousfertilizers,althoughtheycanincreasetheneedforapplyingphosphorus.5.12.9ResearchrequiredLifecycleassessmentstudiesthatconsiderclimate,soiltype,landuseandproduc-tionsystemsarenecessarytodeterminetheoptimumuseoflegumesindifferent74Mitigationofmethaneemissionslocations.Theseassessmentsshouldcompareanimalproductivityunderdifferentforagemanagementsystemstoidentifytheoptimumlegumeforinclusion,onewhichminimizesemissionintensity.Forageproductivityandpersistencemustalsobeconsidered.Thereisaneedforcontrolledanimalresearchstudiesthataccountfordifferencesinintake,digestibilityandplantsecondarycompoundstoexaminethetrueCH4mitigationpotentialoftropicalandtemperatelegumes.Plantsecond-arycompoundsindifferentlegumespeciesshouldbequantified,factoringinthestageofmaturityoftheplantsandthedurationoftheirstorage.5.13FORAGES:HIGH-STARCHFORAGES5.13.1DescriptionUseofforageswithhigh-starchconcentration(i.e.wholeplantcereals,sorghumandmaize).5.13.2ModeofactionWithhigh-starchforagethereisanincreaseinstarchandadecreaseinthefibreconcentrationofthediet,resultinginarumenfermentationthatpromotespro-pionateproduction(Arndtetal.,2015a),whichcompeteswithmethanogenesisformetabolichydrogen.ItmayalsodecreaserumenpH(Hassanatetal.,2013),whichinhibitsmethanogens.TheseforagescanenhanceanimalperformanceduetoahightotaldigestiblenutrientcontentandagreaterDMI(Benchaaretal.,2014;Gislonetal.,2020).5.13.3EfficacyMethaneproductionmayincrease,remainstableordecrease,dependingonchangesinDMI(increasingtheenergydensityofthedietcanincreaseintakewhendietsarelimitedbyrumenfill).Upto15percentlessCH4fordietscontainingmaizesilagecomparedwithsomeotherforageshasbeenreported(Hassanatetal.,2013;Benchaaretal.,2014;Gislonetal.,2020).However,theeffectsonCH4perunitofanimalproductivityarehighlyvariable,andmaydifferaccordingtonutritionalvaluesofharvestedforages(Arndtetal.,2015b).Theefficacyofhigh-starchforagesinreducingCH4dependsonthestageofmaturityofthevariousforages(i.e.timeofharvest)andtherelativedifferencesinstarchconcentration.5.13.4PotentialtocombinewithothermitigationstrategiesItcanbeeasilycombinedwithotherstrategies,especiallythosewithdifferentmodesofaction.5.13.5EffectsonotheremissionsAchangeinforagesourcewillimpactotheremissions,thereforepromotinghigh-starchforagesasaCH4mitigationstrategyneedstobeassessedatthefarmscaleusingLCA.Forageproductionsystemsarehighlyvariableanddependentonfarmsiteconditions(e.g.soiltypeandfertility,water,climate)andmanagementpractices.Thesefactorsaffectforageyieldandnutritivevalue,fieldemissions,animalperfor-mance,andmanurecharacteristicsandemissions.Rotz,MontesandChianese(2010)reportedthatincreasingtheratioofmaizesilagetoalfalfasilageindairycowdietsresultedinNbeingusedmoreefficiently,whichbroughtaboutasmalldecreaseinexcretedmanureNthatreducedtheemissionofN2Ofromcropland.Maizesilage75MethaneemissionsinlivestockandricesystemsproductionledtofewerCO2emissionsfrommachineryandfuelcomparedwithalfalfa,withthenetresultofa13percentdecreaseinCO2eqemissionsperkilogramofmilk.Incontrast,Uddinetal.(2021)reportedonlya2.5percentdecreaseintheCO2eqperkilogramofmilkformaizesilagecomparedwithalfalfasilageinthedietoflactatingdairycows.However,carbonstorageinsoilswasnotconsideredbyRotz,MontesandChianese(2010)orUddinetal.(2021).Littleetal.(2016)showedthat,althoughreplacingalfalfasilagewithmaizesilageinthedietoflactatingdairycowsloweredYmby10percent,differencesbetweenthetwoforagesystemsforCO2eqemissionsperkilogramofmilkwereminimal.Furthermore,theperennialforagehadgreaterpotentialtostoresoilcarbonthanthemaizesilagerotation,illustratingtheimportanceofconsideringallemissionsourcesandsoilcarbonchangespriortorecommendinghigh-starchforagestodecreaseentericCH4production.5.13.6Productivityandthequalityofmeat,milk,manure,crop,andairTheeffectsofhigh-starchforagesonanimalproductivitydependonthenutritivevalueofforages.Thechemicalcompositionanddigestibilityofmaizesilagehybridsishighlyvariable(FerrarettoandShaver,2015;Zardinetal.,2017),asisthecasewithmostforages.Ameta-analysisof547treatmentmeansformaizesilagedietsindicatedthatmilkyieldpertonneofDMwashighlypositivelycorrelatedwithstarchcontent(r=0.65)andNDFdigestibility(r=0.49)andnegativelywithNDFcontent(r=-0.72)(García-Chávezetal.,2020).TheuseofmaizesilagecandecreasetheNcontentofdiets,andhasbeenassociatedwithgreaterNuseefficiencyinani-mals,decreasedmanureNexcretion,andreducedammonia-NandN2Oemissionsfrommanure(Ardntetal.,2015a).TheeffectsonCH4emissionsfrommanurearenotwellknown.5.13.7SafetyandhealthaspectsNomajorconcernsandacceptedbyregulatoryofficials.5.13.8AdoptionpotentialMaizesilageisalreadywidelyusedinthedietsofbeefanddairycattlearoundtheworld,wherethegrowingconditionsarefavourable.Maizeisawarm-seasoncrop,anditisthusnotagronomicallysuitableinmanylocationsacrosstheglobe.Otherhigh-starchforagessuchassmall-graincereals(barley,oat,triticaleandwheat)arewidelygrownintemperatelocations,whilesorghumismoresuitableforsemi-arid,warmerclimates.5.13.9ResearchrequiredFeedinghigh-starchforagestoreduceentericCH4emissionsisnotrecommendedunlessaccompaniedbyanLCAindicatingthatthenetemissionsofmeatandmilkproductionarealsodecreased.Thegreatestpotentialofhigh-starchforagestoreducetotalCO2eqemissionsmaybewhenusedtoreplaceanotherannualforagecrop.Asforagequalitydirectlyaffectsanimalproductivity,furtherresearchshouldexaminethepotentialforusinglocallyadaptedhigh-starchforagestoincreaseanimalproduc-tivityandlowertheCO2eqemissionintensityofanimalproducts.Theresearchneedstotakeintoaccountlocalagronomicalandanimalproductionconditions.Acompari-sonoftheyieldsofforagesandthefeedingvaluetheyproduceperhaisneeded.76Mitigationofmethaneemissions5.14FORAGES:HIGH-SUGARGRASSES5.14.1DescriptionTheuseofhigh-sugargrasses(mainlycultivarsofperennialryegrass,LoliumperenneL.)withanelevatedwater-solublecarbohydrate(WSC)concentration.TheWSCcontentsaretypicallyincreasedinhigh-sugargrassesto250g/kgofDM,butcanbeashighas350g/kgofDM(Lovettetal.,2006b;Riveroetal.,2020).TheWSCconcentrationismainlyincreasedattheexpenseofCPand,insomecases,NDFcontent.TheconcentrationofWSCvarieswithcultivar,stageofmaturityandforagemanagement(Lovettetal.,2006b;Riveroetal.,2020).5.14.2ModeofactionThegreaterconcentrationofreadilyavailablecarbohydratesdecreasestheacetatetopropionateratioinrumenfermentation,andconsequentlyCH4productionisreduced(Riveroetal.,2020).High-WSCgrassesalsoimprovetherateoffermenta-tionandrumenmicrobialproteinsynthesis,withlessammonia-Nabsorbedandexcretedasureaintheurine.ThebalanceofcarbonandNintherumenisimproved,leadingtoenhancedNutilizationbythemicroorganisms.5.14.3EfficacyInvitrostudiesgenerallyreportlessCH4forhigh-versuslow-sugargrasses(Lovettetal.,2006b;Wangetal.,2020),butinvivoresultsareinconsistent.Ellisetal.(2012)estimatedthatanincreaseinWSCconcentrationof40g/kgofDMormoremayberequiredtoalterinvivoCH4production.ThemitigationpotentialalsodependsontheconcomitantchangesinCPandNDFconcentrationanddigestibility.Usingamodel-lingapproachinwhichhigh-sugargrasseswereincorporatedintodairycowdiets,Ellisetal.(2012)concludedthatCH4(g/dayandpercentageofGEI)actuallyincreased,especiallywhenWSCincreasedattheexpenseofCP.YetthesimulatedCH4intensitydecreasedbyupto17percentwhenDMIincreasedduetofeedinghigh-sugargrasses.Zhao,O’ConnellandYan(2016)fedfreshperennialryegrasstosheepandreportedmoderatecorrelations(r=0.44to0.54)betweenWSCconcentrationandvariousexpressionsofCH4production.However,withdriedforages,therewasnodiffer-enceinCH4production,yieldorintensityfordairycowsfedhigh-versuslow-sugargrasses(193versus103gWSC/kgDM;Staerfletal.,2012b).ItappearsthattheCH4mitigationpotentialofhigh-sugargrassesmaybereducedwhenconservedashay.5.14.4PotentialtocombinewithothermitigationstrategiesCanbeeasilycombinedwithotherstrategies,especiallythosewithdifferentmodesofaction.Thetypeofinteraction(negative,positiveoradditiveeffects)willneedtobeexaminedineachcase.5.14.5EffectsonotheremissionsHigh-sugargrasscultivarshavebeenshowntodecreasethetotalNexcretion,andparticularlytheproportionofNexcretedinurine(Staerfletal.,2012b;FoskolosandMoorby,2017).Consequently,ammoniaandN2Oemissionsarereduced.AlifecycleassessmentofmilkproductionindicatedthatthetotalCO2eqperkilogramofmilkwasreducedby3percentwhendairycowswerefedonhigh-sugarcomparedwithconventionalryegrasspastures(Soteriadesetal.,2018).77Methaneemissionsinlivestockandricesystems5.14.6Productivityandthequalityofmeat,milk,manure,crop,andairIntheory,anincreasedsupplyofreadilyfermentablecarbohydratesshouldraiseanimalproductivityinamannersimilartosupplementationinthecaseofcon-centrates.Increased(+9percent)DMIduetoincreaseddigestibilitywasreportedfordairycowsinearlylactationthathadbeenfedfreshhigh-sugargrass(243vs161gWSC/kgofDM;Moorbyetal.,2006).Ameta-analysisbyEllisetal.(2012)reporteda3.3percentaverageincreaseinDMIwithincreasedWSCconcentration(+39g/kgofDM)ofgrassleadingtoincreasedmilkyield.However,amorerecentmeta-analysisindicatedthat,onaverage,feedingdairycattlehigh-sugargrassesdidnotincreasemilkproduction,althoughurinaryNexcretionwasdecreasedby26percent(FoskolosandMoorby,2017).Additionally,thelowerCPconcentra-tionofhigh-sugargrassesmaynegativelyaffecttheproductivityofhighproducingruminantsifproteinrequirementsarenotmet.Forexample,milkproductionwas18percentlowerwhendairycowswerefeddriedhigh-sugarcomparedwithcon-trolledryegrass(193vs103gWSC/kgDM),possiblybecausethedietswerenotisonitrogenous(158vs254gcrudeprotein/kgofDM,respectively;Staerfletal.,2012b).Theproductivityandotheragronomiccharacteristicsofhigh-sugargrasseswillalsohavetobeconsidered,astheymayimpacttheareaofgrasslandnecessarytosustainacertainlevelofproduction.Anydifferencesinpersistencecouldaffecthowsoonapastureneedstoberesown,whichwillaffecttheemissionsofCO2andN2Oassociatedwiththeuseoffossilfuelsandfertilizers.5.14.7SafetyandhealthaspectsNomajorconcernsandacceptedbyregulatoryofficials.5.14.8AdoptionpotentialPerennialryegrassiseasytoestablishandmanageinagronomicallysuitableareas.Itgrowswellinawiderangeofsoilfertilityconditions,withhigh-forageyieldsanddigestibility.However,itsproductivityandnutritionalcomponentsaregreatlyaffectedbyseason,fertilizationrateandcultivar(Rivera,CharaandBarahona,2019).Theadoptionpotentialofhigh-sugarryegrasscultivarsisconsiderableintemperateareaswhereryegrassiscommonlygrown.Priortorecommendinghigh-sugargrassesforCH4mitigation,theclimate,soil,thegrowingenvironmentandyieldpotentialmustbeconsidered.Perennialhigh-sugargrassesarecurrentlynotavailableintropicalorsubtropicalareas.5.14.9ResearchrequiredMostoftheresearchtodateonhigh-sugargrasscultivarshasbeenlimitedtotheUnitedKingdom,theKingdomoftheNetherlandsandNewZealand,andanexpandedgeographicalanalysisisthusrequired.Furtherinvivostudiesareneededtoquantifytheeffectsofhigh-sugargrassesonCH4production,yieldandanimalperformanceforvariousproductionsystems.WhetherCH4mitigationeffectsdif-ferforpastureversusconservedhigh-sugargrassshouldbeexamined.Amorein-depthunderstandingofthechemicalcompositionanddigestibilityofhigh-sugargrassesisalsolacking.Finally,LCAstudiesthatconsiderclimate,soiltype,landuseandproductionsystemsareneededtodeterminetheoptimumuseofhigh-sugargrassesindifferentgeographicallocations.78Mitigationofmethaneemissions5.15FORAGES:PASTURESANDGRAZINGMANAGEMENT5.15.1DescriptionGrasslandsareimportantsourcesoffeedforruminantsandprovidesecurelive-lihoodsandeconomicopportunitiesforruralcommunities(Charáetal.,2017;Mottetetal.,2018).Grazingsystemsvarywithclimate,plantspecies,soiltypesandlivestock,andincludeseason-longcontinuousgrazing,rest-rotationgrazing,deferred-rotationgrazingandintensivelymanagedgrazing.Thesesystemsmanagepasturestoprovideforageresourcesforanimalsbybalancinglivestockdemandwithforageavailability(bothintermsofquantityandquality),whilepromotingrapidpastureregrowthduringthegrazingseasonaswellaslong-termpastureper-sistence.Adequategrazingmanagementcanimproveherbagequantityandqual-ity,leadingtoincreasedanimalproductionperhectare(Congioetal.,2018;Savianetal.,2018),withincreasedsoilcarbonstocksanddecreasedCH4intensity(Guyaderetal.,2016;deOliveiraSilvaetal.,2016;Makkar,2018;Savianetal.,2018).TheuseofpasturesforsustainableproductionandtheproductionofanimalproteinsourcescontributestoFAO’sSustainableDevelopmentGoals.Inadditiontotraditionalpasture-basedsystems,silvopastoralsystems(SPS)thatincorporatetreesandshrubsinpasturesincreasetheamountofbiomassperunitofareaandprovideotherecosystemandbiologicalservices,includingincreas-ingbiodiversity,firecontrolandwatermanagement(Murgueitioetal.,2011).Silvopastoralsystemspromoteasustainableintensificationoflandwithoutusingfossilfuels,whileincreasingbiodiversity,wateruseefficiencyandbiomassproduc-tion,andrespectinganimalwelfare(Mauricioetal.,2019).TheuseofSPScanbeaviableoptionespeciallyinLatinAmerica.Vandermeulenetal.(2018)showedthatSPSwithmultipurposeshrubsandtreeswerebeneficialfortheecosystemwhilethewoodyfodderimprovedruminalproteindigestion,reducedparasiticinfesta-tionanddecreasedCH4emissions,butlimitationssuchastoxinscanrestricttheiruse.Mauricioetal.(2019)demonstratedthatSPSbasedondifferentforagespecies,shrubsandtreesenhancedthecapacitytoproducemeatandmilkwithouttheuseofgrain.5.15.2ModeofactionThisstrategyisbasedontheintensificationofgrazingsystems.Theintentionistoimproveherbagequalityandquantitythroughgrazingmanagementsystemsthatpromoterapidregrowth.Thesesystemsconsiderpre-grazingandpost-grazingswardheight,maximizeherbagenutritionalqualityandincreasedigestibleOMintakebygrazingruminants,andimprovelanduse(Muñozetal.,2016;Gregorinietal.,2017;Congioetal.,2018;Savianetal.,2018).5.15.3EfficacyGrazingmanagementcanlowerentericCH4yieldandintensity,butCH4produc-tionisnotexpectedtochange,althoughitmayincreaseifDMIisincreased,andiftheexpandedforageproductionentailsgreaterstockingrates.TheextenttowhichgrazingmanagementlowersCH4intensityvariesextremely,dependingontheproductionsystemandlocalconditions.Forexample,rotationalgrazingbasedonswardpre-andpost-grazingheightsincreasedthedigestibleOMintakeofsheepgrazingonItalianryegrass(Loliummultiflorum),thusreducingCH4intensityby79Methaneemissionsinlivestockandricesystems17percent,althoughtheabsoluteCH4productionwasnotaffected(Savianetal.,2018).Fordairycattle,managingtheswardheightoftropical,non-irrigatedele-phantgrass(PennisetumpurpureumSchum.cv.Cameroon)decreasedCH4inten-sityby21percentduetoincreasedmilkproduction,althoughitdidnotaltertheabsoluteCH4production.Inthecaseofbeefcattle,CH4intensity(g/kgcarcass)was10percentlowerforheavyversuslight,continuousgrazing,althoughthesoilcarbonsequestrationwaslowerforheavygrazing(Alemuetal.,2017).Pasturespeciesmayalsocontainphytocompoundssuchascondensedandhydro-lysabletanninsthatreduceentericCH4production(Vandermeulenetal.,2018;Stewartetal.,2019;Ku-Veraetal.,2020).Thepresenceofshrubsorlegumeforages(e.g.Macrotylomaaxillare)inpasturelandscanimprovethenutritionalqualityofthedietwhilereducingCH4emissionsduetothepresenceoftannins(Limaetal.,2020).TheinclusionofdiversifiedforagespeciesinpasturescanthereforeincreasethequantityofbiomassforanimalswhiledecreasingentericCH4emissions.5.15.4PotentialtocombinewithothermitigationstrategiesItisexpectedthatgrazingmanagementanduseofSPSwouldaddtotheeffectsofotherCH4mitigationstrategies.5.15.5EffectsonotheremissionsGrazingmanagementaffectstheCO2eqintensityofbeefproductionbyinfluencingdietquality,animalperformanceandsoilcarbonreserves.ThetreespeciesintheSPScanaffectsoilCH4sinks(Borken,XuandBeese,2003)duetocomplexmechanismshavingtodowiththechemicalcomposition,moistureandmicrobiologyofthesoil(Dunfield,2007).Higherfeedintakemayincreasemanureemissionsunlessfor-agedigestibilityisalsoincreased.However,theintensificationofanimalproductionwouldbeexpectedtodecreasethetotalCO2eqemissionsperunitoflivestockprod-uct(Capper,CadyandBauman,2009);therefore,anLCAisneededwhenassessingthestrategy’sefficacy.5.15.6Productivityandthequalityofmeat,milk,manure,crop,andairInmoststudies,improvedgrazingmanagementhasimprovedanimalproduction,duetoincreasedDMIandimprovedforagequality.Forexample,optimizinggraz-ingefficiencyandherbagequalityinthestudyconductedbyCongioetal.(2018)improvedmilkproductionefficiencyby51percent,whiledecreasingCH4emis-sionintensityandCH4yieldby20percentand18percent,respectively.GreatermilkproductionefficiencyincreasedCH4emissionsperhectareby29percent.Theauthorsconsideredthatstrategicgrazingwascosteffective.5.15.7SafetyandhealthaspectsNomajorconcernsandacceptedbyregulatoryofficials.5.15.8AdoptionpotentialItispossibletoimplementimmediatelytheimprovedmanagementofpasturesinextensiveandintensivelivestocksystems.Theselectionofforages,shrubsandfod-derspeciestobeusedneedstobetailoredtoeachregionandgrazingmanagementsystem.Thehighcostofimplementingrotationalsystems(fences,watertroughs)limitsadoptionprospects.Nevertheless,pasturemanagementcanbeimplemented80Mitigationofmethaneemissionsatfarmlevel,issuitableforallgrazingruminantcategories,andhashighfarmerandconsumeracceptance.Therearelimitationsthatneedtobeovercome,suchastheneedforexternalinputs(e.g.fertilizer),potentialdecreasesinbiodiversityinsomecasesandanegativeimpactonanimalwelfare(e.g.heatstress).ImplementingtheSPSisanobjectiveoftheGlobalAgendaforSustainableLivestock(GASL,2021).5.15.9ResearchrequiredAholisticapproachinvolvingmultidisciplinaryresearchteamsandstakeholders(ruralextensionservices,associations,cooperativesandfarmers)isneededtoimprovepasturesthatpromotecarbonsequestrationandCH4sinksinsoils,reduceexternalinputs(energy,fertilizers)andimproveanimalwelfare.Alifecycleassess-mentofpasture-basedsystemsneedstoencompasssoilcarboninadditiontoaccu-rateestimatesofentericCH4emissionsandexcretaaswellasotheraspectsofthelandscapeandenvironment.Long-term,regionallyfocusedresearchisneeded.Extensionservicessupportedbypublicpolicies(i.e.paymentforenvironmentalservices)maybeneededtoencourageadoption.5.16RUMENMANIPULATION:IONOPHORES5.16.1DescriptionDietarysupplementationwithionophorestoimprovefeedefficiencyanddecreasetheacetate/propionateratiointherumen,therebymitigatingentericCH4emissions.5.16.2ModeofactionIonophoresarecarboxylicpolyethersubstancesthatinterferewiththeiontrans-portacrosscellmembranesofgram-positivebacteriaandprotozoa.Ionophoresmodifytheiontransportfluxacrosscellmembranesofmicroorganisms,increas-ingtheconcentrationofprotons(H+)inthecytoplasm(DuffieldandBagg,2000;Duffield,RabieeandLean,2008a,2008b;HersomandThrift,2012;Azzaz,MuradandMorsy,2015).Formaintainingcellequilibrium,thebacterialcellsuseenergytoextrudeH+,whichresultsinreducedgrowthandthedeathofcells(DuffieldandBagg,2000).Duetothestructureofcellmembranes,ionophoresaremainlyactiveagainstgram-positivebacteriaandprotozoa(Beaucheminetal.,2009;HersomandThrift,2012;Azzaz,MuradandMorsy,2015),buttheydonottargetmethanogensdirectly(Mathisonetal.,1998;Beaucheminetal.,2009).Byshiftingthebacterialpopulationoftherumen,ionophoresmodifytheVFAproductionfromacetate(H2source)topropionate(H2sink),thusleadingtoreducedmethanogenesis(Mathisonetal.,1998;DuffieldandBagg,2000;Duffield,RabieeandLean,2008a;Hristovetal.,2013;Azzaz,MuradandMorsy,2015).Anincreasedfeedefficiency(HersomandThrift,2012;Hristovetal.,2013)alsoreducesCH4emissionintensity.Thepotentialfortherumenmicrobestoadapttoionophoresisnotclear,withsome(Mathisonetal.,1998;Beaucheminetal.,2009)butnotall(Appuhamyetal.,2013)reportsindicatingtime-limitedeffects.5.16.3EfficacyIntheirmeta-analysisof22publishedstudies,Appuhamyetal.(2013)demon-stratedthatmonensinreducedYm,theCH4conversionfactor,by0.5percentunits(5.97vs5.43forcontrolandtestgroups,respectively)inbeefcattle,withdietshighinNDFconcentrationshowingthegreatesteffects.However,therewasnoeffect81MethaneemissionsinlivestockandricesystemsofmonensinontheYmvaluefordairycows.Differentdosesweretestedinthebeefanddairystudies.Whenadjustedtothemonensindose,theCH4mitigationeffectsweresimilarfordairycowsandbeefsteers(-12+6g/dand-14+6g/d,respectively).WhenfactoringinDMIdifferences,monensinreducedYmindairycowsandbeefsteersby0.23+014and0.33+0.16,respectively.Thedurationofthetreatmentperioddidnotsignificantlymodifythemonensineffect.5.16.4PotentialtocombinewithothermitigationstrategiesUsingcombinationsofionophoresortherotationalfeedingofionophoresmayhelpavoidmicrobialadaptation(Mathisonetal.,1998).Ithasagoodpotentialtocomplementotherstrategieswithdifferentmodesofaction.Nointeractioneffectswereobservedwhenmonensinwascombinedwith3-nitrooxypropanolinthedietsfedtobeefcattle(Vyasetal.,2018).5.16.5EffectsonotheremissionsWithimprovementsinfeedconversionefficiencyduetomonensin,thequantityofOMexcretedinthemanuremightbereduced,furtherreducingCH4emissionsfromafarm.AnitrogenmetabolismimprovedthroughionophoresreducesurinaryNexcretionandassociatedpotentialemissionsofNH3andN2O.Themonensindoseinthedietbeingsmall,theincreaseinCO2emissionsfromtheuseoffossilfuelsinmonensinmanufactureandtransportisconsequentlythoughttoberathersmall.5.16.6Productivityandthequalityofmeat,milk,manure,crop,andairIonophoresareusedtoimprovefeedefficiencyandtheproductivityofbeefcattleanddairycows(HersomandThrift,2012).Ionophoresalsoimprovefeedefficiency(astheyreducefeedintakebyabout0.3kg/dayandincreasemilkyieldby0.7kg/dayinthecaseofmonensinfedtodairycows;monensinbeingthemostextensivelystudiedionophore[Duffield,RabieeandLean,2008b]),whichleadstoagreaterproductionforthesameamountoffeedconsumed(Mathisonetal.,1998;DuffieldandBagg,2000;Duffield,RabieeandLean,2008b;Beaucheminetal.,2009;HersomandThrift,2012;Hristovetal.,2013).Theuseofionophoresmayaffectthefattyacidprofileofmilkthroughreducingtheshortchainfattyacidsandstea-ricacidwhileincreasingtheconjugatedlinoleicacid(Duffield,RabieeandLean,2008b).Inadditiontoproductivitybenefits,ionophoresmayalsoimproveruminanthealth,particularlyastheydiminishtheriskofsub-clinicalketosis(DuffieldandBagg,2000;Duffield,RabieeandLean,2008a),subacuteacidosis(Appuhamyetal.,2013)andbloat(DuffieldandBagg,2000;Duffield,RabieeandLean,2008a,2008b;Appuhamyetal.,2013).5.16.7SafetyandhealthaspectsTheconcentrationofionophoresinthedietshouldbelimitedtoavoidtoxicity(Novilla,1992;Hall,2000)and,aswithanyfeedadditive,careshouldbeexertedduringhandling.Theuseofmonensinissubjecttoapprovalbyregulatoryagen-cies,anditisbannedinsomecountries,includingintheEuropeanUnion.Ithasbeenquestionedwhetherthewidespreaduseofionophorescontributestothecross-resistancetootherantibiotics(Wong,2019).82Mitigationofmethaneemissions5.16.8AdoptionpotentialWhentheuseofionophoresisauthorized,theadoptionpotentialcanbehighinproductionsystemswheredairycowsandbeefcattlearesupplementedwithmineralsorcompoundfeeds.Ionophoresaresupplementedviafeed(HersomandThrift,2012)andthereforedonotnecessitatespecificinvestmentsonthefarm.Theimprovementsinanimalperformanceprovideeconomicbenefitsthatgenerallyoff-setthecostoftheionophore.Ionophorescanalsobeprovidedintheformofslow-releasecapsules,whichcanbeusefulformoreextensivepasture-basedsystems.5.16.9ResearchrequiredTheuseofionophoresinbeefanddairycattlefeediswellknownanditscommer-cialapplicationiswidespread.Numerousstudiesandmeta-analyseshavedemon-stratedthebenefits;however,itisrecommendedthatmeta-analysesbeupdatedtoincludemorerecentlypublishedinformation.5.17RUMENMANIPULATION:CHEMICALINHIBITORSOFMETHANEPRODUCTION5.17.1DescriptionSeveralchemicalcompoundsinvestigatedsincethe1960sinhibittheformationofCH4inrumenfermentationwhenpresentinthedietinsmallconcentrations.Theinvestigationalcompound3-nitrooxypropanol(3-NOP),whichiscommer-ciallyavailableinsomecountries,isdiscussedseparatelybelow(5.18).Studiesinvestigatingtheuseofchemicalinhibitorsofmethanogenesisinpreruminantani-malsarediscussedinSection5.19.5.17.2ModeofactionChemicalinhibitorstargetmethanogensbutnotallofthemthroughdirectlyinhibitingmethanogenesis.Halogenatedmethaneanalogues–chloroform,bro-moform,iodoform,bromochloromethane(BCM),carbontetrachlorideandothers(Bauchop,1967;Treietal.,1971;Lanigan,1972)–inhibitthelaststepofmetha-nogenesisbyreactingwithvitaminB12toblockthecobamide-dependentmethyltransfer(Woodetal.,1968).CoenzymeManaloguesbromoethanesulfonate(BES;Gunsalus,RomesserandWolfe,1978)and3-NOP(Duinetal.,2016)alsoinhibitthelaststepofmethanogenesisbyblockingmethyl-coenzymeMreductase.Hydroxymethylglutaryl-SCoAinhibitorsmevastatinandlovastatininhibitthesynthesisofmembranelipidsinarchaea(MillerandWolin,2001).Itwasspeculatedthat9,10-antraquinonemaydisruptelectrontransferandhinderATPgenerationinmethanogens(Garcia-Lopez,Kung,Odom,1996).Directinhibitionofmethano-gensbyotherchemicals,suchaspyromelliticdiimide(MartinandMacy,1985),andhalogenatedcompounds2,2,2-trichloroacetamide(Treietal.,1971)andhemiacetalofchloralandstarch(Trei,ScottandParish,1972),amongothers,isevidencedbytheaccumulationofH2,buttheirexactmechanismsofactioninthemethanogencellhasnotbeendemonstrated.5.17.3EfficacyIntworecentmeta-analysesofinvivostudies(Venemanetal.,2016;Arndtetal.,2021),chemicalinhibitorsofmethanogenswerefoundtocausethestrongestdecreaseinabsoluteCH4productionofallthevariousanti-methanogenicstrategies.83MethaneemissionsinlivestockandricesystemsInsomeinvivostudies,absoluteCH4productionwasinhibitedbymorethan90percentcomparedwithcontrolledtreatments(MathersandMiller,1982;McCrabbetal.,1997;Mitsumorietal.,2012).ThehighlyspecificmethanogenesisinhibitorBESisverypotentinvitro,butitseffectslastedforonly3daysinvivo(Immigetal.,1995).Conversely,along-terminhibitionofmethanogenesisinvivobydifferentchemicalcompoundshasbeenobservedinotherstudies(e.g.Treietal.,1971;Trei,ScottandParish,1972;Clapperton,1974,1977;Daviesetal.,1982;Tomkins,ColegateandHunter,2009).5.17.4PotentialtocombinewithothermitigationstrategiesThehighspecificityofthesecompoundsmakesitpossibletofindadditiveeffectswhentwoormorecompoundswithdifferentmechanismsofactionarecombinedand,likewise,whencoupledwithotheranti-methanogenicstrategiesthathavedifferentmechanismsofaction.Differentmethanogensareinhibitedbychemi-calcompoundstodifferentextents(Ungerfeldetal.,2004;Duinetal.,2016),andthuscombiningorrotatingdifferentinhibitorsofmethanogenesisisaninterestingresearchdirection.Invitroexperimentsasaproofofconceptarerecommended(e.g.ZhangandYang,2012;PatraandYu,2013).5.17.5EffectsonotheremissionsManufacturingandtransportingthesechemicalcompoundsresultinemissionsoffossilfuel-generatedCO2.However,becausetheirdietaryconcentrationisverylow,thesignificanceoftheseemissionsonthedailyCO2eqproductionorintensitybasisisverylow.5.17.6Productivityandthequalityofmeat,milk,manure,crop,andairIngeneral,inhibitingmethanogenesiswithchemicalcompoundsdoesnotaffectani-malproductivity(Ungerfeld,2018;Arndtetal.,2021).DigestibilityisnotaffectedbutDMImostlydecreases(Ungerfeld,2018).Theamountandchemicalcomposi-tionofmanureisprobablylittleaffected,butthepassageofchemicalinhibitorstomanurehasnotbeenevaluated(exceptfor3-NOP).5.17.7SafetyandhealthaspectsCompoundssuchashalogenatedCH4analoguescanbetoxictotheanimal,arevolatileandcandepletetheozonelayeroftheatmosphere.TheconcentrationofBCMinmuscle,fatandoffalofsteersfedBCMwaswithinmaximumlimitsinAustralia,althoughpotentiallossesduetovolatilitywerenotconsidered(Tomkins,ColegateandHunter,2009).Thechemicalinhibitors’toxicity,theirpassagetoani-malproductsandvoidingintotheenvironmentmustbecarefullyexaminedbeforethesecompoundscanberecommendedandmarketed.5.17.8AdoptionpotentialChemicalinhibitorscanallowastrongandconsistentdecreaseinentericCH4emissionswithminimaleffectsonotherGHGemissionsbuttheresearchisyettodemonstratethisclearly.Theinclusionofaninhibitorinadietisboundtoincreasecostsandwillbeunattractivetoproducers,unlessaccompaniedbythehigherpricethatproductsassociatedwithalowercarbonfootprintcommand.Itmayalsobepossibletotakeadvantageofagreaterenergyretentionandchanges84Mitigationofmethaneemissionsinrumenandanimalmetabolismtoincreaseanimalproductivity,butmoreresearchisneededtoexplorethosepossibilities.Chemicalinhibitorsmaynotbesuitableforgrazingsystemsinwhichanimalsarenotsupplemented,unlessslow-releaseformsadequateforthosesystemscanbeformulatedoralowerefficacyaccepted.Approvalbygovernmentagenciescanbelengthyandexpensive.Consumersmaybereluctanttoacceptthem,butweareunawareofpublishedconsumersurveysonthismatter.5.17.9ResearchrequiredThisresearchareaisofmuchinterestduetothehighestefficacyobserved.Methanogenenzymesarebeingscreenedforthedevelopmentofnewchemicalinhibitors(Carboneetal.,2018),andadditionalinhibitorsarebeinginvestigated(e.g.ZhangZ.-W.etal.,2019a,2019b).Atthesametime,thereareolderreportsinwhichsomecompoundswereshowntohavelong-lastingeffectsonmethano-genesis;wearenotawareoffurtherinvestigationsregardingthetoxicity,passagetoanimalproductsordamagetotheenvironmentofthosecompounds,andthoselinesofresearchwereprobablyabandonedbecauseofthedifficultiesinvolvedinhandlingthesecompoundsinthefeedorfoodchain.Moreover,furtherresearchtounderstandchangesinrumenmicrobiomeandwholeanimalmetabolisminducedbyinhibitionofmethanogenesisisrecommendedinordertooptimizetheresultsoftheinterventions(Ungerfeld,2018;UngerfeldandHackmann,2020).5.18RUMENMANIPULATION:3-NITROOXYPROPANOL(3-NOP)5.18.1Description3-nitrooxypropanol(3-NOP)isaCH4inhibitordevelopedandcommercializedbyDSMNutritionalProducts(Basel,Switzerland).Thismononitrateesterof1,3-propanediolhastheHOCH2CH2CH2ONO2formula(Duinetal.,2016;Yu,BeaucheminandDong,2021).5.18.2Modeofaction3-NOPisasmallmoleculewithashapesimilartothatofmethyl-coenzymeM(methyl-CoM;Duinetal.,2016).Methyl-CoMisasubstrateofcoenzymeMreductase(MCR)inthelaststepofmethanogenesis.Asananalogueofmethyl-CoM,3-NOPselectivelybindsintotheactivesiteofMCRinapositionsimilartonaturalligandmethyl-CoMandinactivatesMCRbyoxidizingtheactivesitenickel+1incofactorF430.Additionally,3-NOPiscleavedbyanelectrontransferprocessintonitriteand1,3-propanediol,whichalsoinactivatesMCR.Itisworthnotingthatthetotalityofthemodeofactionisreversibleuponremovalof3-NOP(Duinetal.,2016).Asaresult,CH4productionisinhibitedandtheflowofmetabolichydrogeninrumenfermentationshiftsfromacetateandCH4towardspropionate,butyrateandvalerate(Romero-Perezetal.,2014;Schildeetal.,2021).5.18.3EfficacyThereisagrowingnumberofscientificpublications(greaterthan50)describingitsefficacyfordairyandbeefcattleinarangeofdifferentdietsandmanagementsystems,withseveralreviewsandmeta-analyses(Dijkstraetal.,2018,Jayenagaraetal.,2018;Kimetal.,2020;Arndtetal.,2021;Yuetal.,2021;Kebreabetal.,2023).Thisextensive85Methaneemissionsinlivestockandricesystemsbodyofpublisheddata(invitro,short-termandlong-termstudies),inconjunctionwithstudiesrununderthespecificguidelinesandrequirementsneededtoregister3-NOPinEurope,allowedtheevaluatingpaneltoassessandconcludethat3-NOPhasthepotentialtobeefficaciousinthecaseofallruminantspecies(Bampidisetal.,2021).Themeta-analysesbyDijkstraetal.(2018)andKimetal.(2020)establishedlineardecreasesinCH4productioncorrespondingto3-NOPdosage.Inthemeta-analysisbyDijkstraetal.(2018),meanresponsesweregreaterindairy(38.2±3.33percentand34.9±3.43percentforCH4productionandyield,respectively)com-paredwithbeefcattle(26.1±2.76percentand21.1±2.99percentforCH4produc-tionandyield,respectively).Thosemeta-analyseswerelatelyupdatedfordairycowstoincludethemostrecentstudies(Kebreabetal.,2023)andtotakeintoaccounttheeffectofdietcomposition(NDF,EEandstarchcontent).Modelsincludingonlythe3-NOPdoseindicatedadecreaseof32.7,30.9and32.6percentforCH4production(g/d),yield(g/kgDMI)andintensity(g/kgenergy-correctedmilk),respectively,atanaverage3-NOPdoseof70.5mg/kgDM.Theresponseto3-NOPdeclinedwithincreasingdietaryNDFandEE.Althoughmostlong-termstudieshaveshownthat3-NOPeffectivenessremainedconstant,acoupleofstudiesreportedthat3-NOPeffectivenessdeclinedslightlyovertime,whichmightberelatedtothelowdosageused(Yu,BeaucheminandDong,2021).5.18.4PotentialtocombinewithothermitigationstrategiesGoodpotentialtocombinewithotherstrategiesthathavedifferentmodesofaction.IncrementalmitigationeffectsonCH4yieldwerereportedfor3-NOPwhencom-binedwithunsaturatedlipids(Zhangetal.,2021),higherconcentrateproportion(Schildeetal.,2021)andmonensinionophores(Vyasetal.,2018).5.18.5EffectsonotheremissionsTheemissionsfromproducing3-NOPinsmall-scaleconditionswerereportedas48to52kgCO2eq/kg3-NOP(Alvarez-Hessetal.,2019;FengandKebreab,2020.Thiswouldrepresentapproximately6gCO2eq/kgdietDM,assumingadoseof118mg3-NOP/kgDM.Forexample,foradairycowdailyconsuming25kgDMandemitting274gofCH4(~100kgperyear),theincreaseinCO2eqemis-sionsduetofeeding3-NOPwouldrepresentabout2percentofthebasalCO2eqemissionsfromentericCH4(calculationsnotshown),notcountingemissionsofCO2eqfrommanure,N2Oandfossilfuels.Nkemka,BeaucheminandHao(2019)showednoresidualeffectsoffeeding3-NOPtobeefcattleonmanureCH4emis-sionswhenusedinananaerobicdigester.Owensetal.(2020)likewiseestablishedinfieldconditionsthatmanurefromcattlefed3-NOPhadunchangedemissionspat-terns.Tofurtherstudyemissionsuponmanurespreading,Weberetal.(2021)con-ductedalab-scalestudyusingsoilsamendedwithmanurefromcattlefed3-NOPandconcludedthatGHGemissionsweredependentonsoiltexture.Forcoarse-texturedsoil(blackChernozemic),GHGemissionsweregreaterwhenamendedwithmanurefromcattlefed3-NOPcomparedwithcontrolmanure(mainlyduetoincreasedN2Oemissions),butthiseffectwasnotobservedforothersoiltypesorwhenthemanurewasfirstcomposted.Thisaspectthusneedsfurtherstudy.86Mitigationofmethaneemissions5.18.6Productivityandthequalityofmeat,milk,manure,crop,andairStudiesreportnonegativeeffectsof3-NOPondigestibility,withpossiblesmallincreasesindigestibilityinsomecases(Zhangetal.,2012;vanGastelenetal.,2020).Accordingtomostdairystudies,supplementingdietswith3-NOP(40to80mg/kgDM)hasnotimprovedanimalperformance(Arndtetal.,2021;Jayanegaraetal.,2018),nordiditaffectDMI,milkyield,milkcomponentyieldorfeedefficiency.Itdidslightlyincreasebodyweightgain(Haisanetal.,2014;Hristovetal.,2015a;vanGastelenetal.,2020),andsmallchangesinmilkcomponents(Jayanegaraetal.,2018;Schildeetal.,2021)werenotedinsomestudies.Dependingonthedietandthedoseof3-NOP(100to200mg/kgDM),mostbeefcattlestudiesreportadecreaseinDMIof2to6.5percent(Alemuetal.,2020,2021),withnonegativeeffectsonanimalperformance(Alemuetal.,2020,2021;Vyasetal.,2016,2018),exceptwhenfeedingahigh-graindietwithahigh3-NOPdose(200mg/kgDM).Anenhancedgaintofeedratio(by2.5percentto5percent)wasreportedinsome(Alemuetal.,2021;Vyasetal.,2016,2018)butnotallbeefcattlestudies(K.Beauchemin,personalcommunication).Theimpactof3-NOPonrumenfermentationwasassessedforbothbeefanddairycattle.Adistinctshifttowardsgreaterpropionateandbutyrateconcentrationandareductionintheacetatetopropionateconcentrationratiouponsupplementationwith3-NOPwereobserved(Jayanegaraetal.,2018).Ithasbeenhypothesizedthatthisshiftmightleadtohigherenergyandglucoseavailabilityfortheanimal(Ungerfeld,2018;Ungerfeld,BeaucheminandMuñoz,2022).RuminalpHwasalsoshowntobehigherwith3-NOPsupplementation,indicatingareducedriskofrumenacidosis(Jayanegaraetal.,2018).5.18.7SafetyandhealthaspectsThesafetyof3-NOPforanimaluseandhumansconsumingmeatandmilkfromanimalsfed3-NOPwasassessedanditsuseapprovedonthebasisofanextensivesetofstudies,initiallybyregulatoryofficialsinBrazilandChile,followedbytheEuropeanUnion(Bampidisetal.,2021),Argentina,Australia,Pakistan,Switzerland,TürkiyeandUruguay,anditiscurrentlybeingassessedbyregulatoryofficialsinotherjurisdictions.TheEUmarketauthorizationprocessforfeedadditivesdeclaredtheproducttobesafefordairycowsandcowsintendedforreproduction,theconsumerandtheenvironment,whenadministereduptothemaximumrecom-mendeddoseof88mg3-NOP/kgcompletefeed(withaDMcontentof88percent)(Bampidisetal.,2021).3-NOPisrapidlyhydrolysedintherumenpost-dosing(2to3hours;Thieletal.,2019a)to1,3-propanediolandnitrate,whicharelow-toxicitycompoundsnaturallyoccurringintherumenofnon-3-NOPsupple-mentedcows.1,3propanediolisfurtherhydrolysedandusedinenergymetabolismwith3-NOPcarbonincorporatedintocarbohydrates,aminoacidsandfattyacids(Thieletal.,2019a).Inlactatinggoats,3-NOPwasshowntobeextensivelymetabo-lizedtoCO2,withlessthan5percentofradioactivityofdosed14C-labelled3-NOPexcretedviaurineandfeces,withminutequantitiesinmilklactose(Thieletal.,2019a).3-NOPanditsmetabolitesarenotexpectedinmilkfatorproteinbecauseoftheirhighwater-solubilities.Residuesinbeefmeatwereshowntobeminuteornon-existent(Thieletal.,2019a).Thieletal.(2019b)reportedthatinrats3-NOPanditsmetabolitesposenomutagenicandgenotoxicpotential.Basedonextensiveexami-nationofgenotoxicity,Bampidisetal.(2021)didnotruleoutthat3-NOPmaybe87Methaneemissionsinlivestockandricesystemsgenotoxic.However,thosefindingswerenotconsideredrelevantforthesafetyofthetargetspeciesandconsumers,because3-NOPisrapidlymetabolizedintherumenandpost-absorption.Bampidisetal.(2021)statedthatfeeding3-NOPexposestheconsumerto3-nitrooxypropionicacid(NOPA),anon-genotoxiccompound.However,NOPAisanintermediarymetaboliteofabsorbed3-NOPinrodents,andnotamainendproductfor3-NOPmetabolismandelimination.Furthermore,inruminants,rumenmetabolismof3-NOPsharplydecreasestheplasmaconcentra-tionofNOPAafteringestionof3-NOPincomparisontorodents.Evenwithdosesof3-NOPattwicethemaximalrecommendeddose,thepassageofNOPAtothemilkofdairycowswasalmostalwaysundetectableoroccurredataverylowcon-centration,andhumanintakeofNOPAfrommilkproducedbycowsfed3-NOPwasestimatedtobenegligibleandsafeforconsumers(Bampidisetal.,2021).Justasotherfeedadditives,3-NOPshouldbehandledwiththenecessarycare,bothinthesupplychainandbyfarmersandfarmworkers,toavoidpotentialirritation.5.18.8Adoptionpotential3-NOPisalreadyapprovedforuseinBrazil,ChileandtheEuropeanUnion,andtheauthorizationprocessisongoingforothermarkets.Itiscommerciallyavailableinsomemarketsandhasclearpotentialforadoptionbyconfinementsystemsusingtotalorpartialmixedrations.Initscurrentform,3-NOPmaynotbesuitableforgrazingruminantsbecauseitismosteffectivewhenmixedintotherationsuchthatitisconsumedthroughouttheday(unlessalowereffi-cacyorhigherdosingisaccepted),thusmatchingthefermentationoffeedandproductionofCH4.Preliminarystudiesusingaprototypeslow-releaseformof3-NOPhaveprovensuccessful(Muetzeletal.,2019)butwillrequirefurthertest-inginlargerscalestudies.3-NOPrequiresapprovalbyregulatoryofficials.Someadvantagesofusing3-NOPinrationsareitsloweffectivedose(1-2g/d),highspecificitytowardsmethanogens,relativelysustaineddecreaseinCH4overlongperiodsoftime(i.e.months)andsafety.Including3-NOPinanimaldietswillresultinincreasedfeedcosts,aswillbethecaseformanyotherCH4inhibitors,andunlessthereisanincrementinthepriceofanimalproductsproducedwithalowercarbonfootprint,oraconsistentimprovementinanimalperformance,pro-ducersmaynotreadilyadopttheinclusionof3-NOPindiets.Asurveyregardingfarmerorsectorexperiencewith3-NOPindairydietswillstartintheKingdomoftheNetherlandsin2022.Wearenotawareoftheexistenceofsurveysregardingconsumeracceptanceof3-NOP.5.18.9ResearchrequiredResearchisneededtodevelopastableformof3-NOPforgrazinganimalsoraslow-releaseformthatcouldbefedlessfrequently.Theoptimumdoseindietsthatvaryinchemicalcompositionneedsrefining.Theefficacyofusing3-NOPinlong-termbeefanddairycowstudiesundervariousconditionsrequiresadditionalvalidation.Studiesthatcombine3-NOPwithothermitigationstrategiesarelacking.FurtherevaluationoftheGHGemissionsfrommanureofanimalsfed3-NOPisneeded,althoughnonegativeimpactsondigestibilityhavebeenobserved.Moreresearchisalsoneededtoachieveabetterunderstandingofthechangesintherumenandthewholeanimalmetabolismthatresultfrominhibitingrumenmethanogenesis,whichmaypotentiallytranslateintoanimprovedanimalproductivity.88Mitigationofmethaneemissions5.19RUMENMANIPULATION:IMMUNIZATIONAGAINSTMETHANOGENS5.19.1DescriptionVaccinationagainstrumenmethanogens.5.19.2ModeofactionStimulationoftheruminant’simmunesystemtoproduceantibodiesagainstmetha-nogens.Antibodiesaredeliveredtotherumenviasaliva.5.19.3EfficacyTheeffectsonCH4yieldhavebeenmildornon-existentinsheep(Wrightetal.,2004;Leslie,AspinandClark,2008;Williamsetal.,2009)andgoats(Zhangetal.,2015).TheantibodiesagainstmethanogensdecreasedCH4productioninmixedrumencultures,accordingtoanon-peerreviewedstudy(BakerandPerth,2000),andeffectswerevariableasstatedbyanothermixedrumenculturesstudy(Cooketal.,2008).GrowthandCH4productionofapurecultureofMethanobrevibacterruminantiumwereinhibitedbyantibodiesagainstmethanogens(Wedlocketal.,2010).Invivostudieswithsheephaveshownthatvaccinationwithamodelmetha-nogenantigenincreasedconcentrationsofantibodiesinsaliva,estimatedtoresultinupto104moleculesofantigen-specificIgGpermethanogencellintherumen(Subharatetal.,2016).5.19.4PotentialtocombinewithothermitigationstrategiesItappearsfeasible,butexperimentshavenotbeenconductedtoinvestigatesyner-gismsamongmitigationstrategies.Providedthatvaccinescanbeefficacious,theuseofotheradditivesthatdirectlytargetmethanogenscouldamounttoduplicativemitigation.5.19.5EffectsonotheremissionsItseemslikelythatCO2emissionsfromfossilfuelsgeneratedbymanufacturing,packaging,transportingandstoringvaccineswouldbeminimal.Itisassumedthatdigestibilityandnutrientexcretionwillnotbeimpactedbyvaccination.5.19.6Productivityandthequalityofmeat,milk,manure,crop,andairTherehavebeennoeffectsofmethanogensvaccinesonDMintakeandbodymassgain(Wrightetal.,2004;Williamsetal.,2009).Anon-peerreviewpublicationclaimedgreaterDMIandwoolgrowthinsheepvaccinatedagainstmethanogens(BakerandPerth,2000).Ifanefficaciousvaccinethatclearlydemonstratesareduc-tioninCH4emissionsisdeveloped,theeffectsonanimalperformanceandproductqualitywouldneedtobethoroughlyevaluated,asotherCH4-mitigatingmeasuresdonotnormallyhavesuchbeneficialeffects.5.19.7SafetyandhealthaspectsUnknownatpresent,butitcanbesupposedtoposealowrisk,asantibodiesnatu-rallyoccurinanimaltissueseatenbyhumans.Oncedeveloped,thevaccineswouldneedtogothroughappropriateregulatoryapprovalprocesses.89Methaneemissionsinlivestockandricesystems5.19.8AdoptionpotentialThisentericCH4mitigationstrategyisattractiveforextensivegrazingproduc-tionsystemsthatusesupplementsinfrequentlyornotatallandthathavealimitedpotentialforintensification.ThefactthatitisunlikelytosignificantlyaffecttheemissionsofotherGHGs,thatitwouldbeeasyandsafetoapply,notrequirespe-cializedtechnicalskillsandlikelybeacceptabletogovernmentagenciesandcon-sumersalsomakesitinteresting.Ifproveneffective,thedevelopmentofvaccinesagainstmethanogensisperhapsthemostdesirableapproachforcontrollingCH4emissionsduetoextensiveruminantproductionsystems.5.19.9ResearchrequiredThisCH4mitigationstrategyisatpresentataproof-of-conceptstage,asshownbyantibodyresponsesthatthevaccinationinducedinserum,salivaandrumenfluid(Wrightetal.,2004;Zhangetal.,2015;Subharatetal.,2015,2016).Theidentifica-tionofmembrane-associatedandsurface-exposedproteinspresentinabroadrangeofrumenmethanogensthatcanserveasantigensisnecessarytodevelopasuccess-fulvaccine.Thegenomesequencingofrumenmethanogenshasbeenusefultoiden-tifypotentialantigens(Leahyetal.,2013;Wedlocketal.,2013).Vaccinationagainstmethanogenshasbeenshowntoinducetheproductionofantibodiesinsalivaandtheirdeliverytotherumen(Subharatetal.,2015,2016).Ithasbeendemonstratedthatantibodiesagainstmethanogenshavesomestabilityinrumenfluid(Subharatetal.,2015)andthattheyagglutinatemethanogensinvitro(Wedlocketal.,2010).However,eventhoughindividualstepsinthedevelopmentofvaccinesagainstmethanogensseemtohavebeensuccessful,invivoeffectsonCH4productionhavesofarbeenminorornon-existent(Baca-Gonzálezetal.,2020).Itissomewhatsurprisingthattherearemorepeer-reviewedstudiesinwhichCH4productionhasbeenexaminedinvivo(Wrightetal.,2004;Leslie,AspinandClark,2008;Williamsetal.,2009;Zhangetal.,2015)thaninmixedcultures(Cooketal.,2008).Vaccinationdidnotaffecttheabundanceofmethanogensbutitincreasedtheirdiversity,suggestingthatthelackofeffectsonCH4productionmightbeduetoalackofbroad-spectrumvaccinesusedagainsttherumenmethanogeniccommunity(Williamsetal.,2009).Moreworkisrequiredtoselectappropriateantigenspresentacrossdiverserumenmethanogensclades,determinetheirefficacyagainstcultivablerumenmethanogensaswellasinvitromixedbatchandcontinuouscultures,developadequateadjuvantsandassessthepersistenceofimmuneresponsesacrossruminantpopulations.5.20RUMENMANIPULATION:BROMOFORM-CONTAININGSEAWEEDS(ASPARAGOPSISSP.)5.20.1DescriptionSomeredseaweeds(macroalgae)inhibitmethanogenesisduetotheircapacitytosynthesizeandaccumulatehalogenatedcompounds,suchasbromoformanddibromochloromethane(Machadoetal.,2016).Tworedseaweeds,AsparagopsistaxiformisandAsparagopsisarmata,haveshownhighinhibitoryeffectsonCH4productioninvitroandinvivo(Kinleyetal.,2016;Lietal.,2016;Roqueetal.,2019a,2021;Stefenonietal.,2021).90Mitigationofmethaneemissions5.20.2ModeofactionTheanti-methanogenicpropertyofAsparagopsisisduetoitscontentofhalogenatedcompounds,ofwhichbromoformisthemostabundant(Machadoetal.,2016).HalogenatedCH4analoguesreactwithvitaminB12toblockthecobamide-dependentmethyltransfer(Woodetal.,1968)intomercaptoethanesulfonate(coenzymeM)toproducemethyl-coenzymeM,whichisitselfthemethyldonorinthelaststepofmethanogenesis(HarmsandThauer,1996).5.20.3EfficacyInvivostudieswithsheep,steersanddairycowsreporteddose-dependentdecreasesbetween9to98percentofCH4productionwhensupplementingthedietwithAsparagopsis(Lietal.,2016;Kinleyetal.,2020;Roqueetal.,2019a,2021;Stefenonietal.,2021).Asevereinhibitionofmethanogenesis(>50percent)wasobservedwith1percentorlessAsparagopsisinthediet(Lietal.,2016;Kinleyetal.,2020;Roqueetal.,2019a,2021).TheefficacyofAsparagopsisonCH4mitigationdependsonitsconcentrationofbromoform,whichrangedfrom3.28to39×10-3µg/kgDMIindifferentstudies(Kinleyetal.,2020;Roqueetal.,2019a,2021).Additionally,AsparagopsismaybemoreeffectiveatdecreasingCH4productioninhigh-concentratethaninhigh-foragediets(Roqueetal.,2021).Stefenonietal.(2021)concludedthattheefficacyofAsparagopsisgraduallydiminishedprobablyduetoinstabilityandlossofbromoformfromtheproductwithtime,ratherthananadaptationoftherumenmicrobes,althoughthisshouldbeinvestigatedfurther.However,Roqueetal.(2021)didnotreportalossofefficacyfromtheproductoverafive-monthperiod.5.20.4PotentialtocombinewithothermitigationstrategiesHowtheuseofAsparagopsiscombineswithothermitigationstrategieshasnotbeenexperimentallyexamined,butitisexpectedtohavepotentialwhendifferentbioac-tivecomponentsormodesofactionareinvolved.CombiningitwithotherCH4mitigationstrategiesmayallowdecreasingtheconcentrationofbromoforminthediettoalleviatepotentiallydetrimentaleffectsonDMI,healthandsafety(seesec-tion5.20.7below).5.20.5EffectsonotheremissionsTheCO2eqemissionsfromgrowing,harvesting,processing(drying),storingandtransportingAsparagopsisonalargescaleneedtobeconsideredinanLCAtodeter-minethenetimpactonGHGintensityofmeatandmilkproduction.Theassess-mentofozone-relatedenvironmentalimpactsprobablydeservesconsiderationasbromoformisdocumentedasanozone-depletingsubstance(Papanastasiou,McKeenandBurkholder,2014);inapreprintcurrentlyunderreview,thepoten-tialglobaldepletionofstratosphericozonewasestimatedtoberelativelysmallforAsparagopsisgrowthconditionsinAustralia(Jiaetal.,2022).5.20.6Productivityandthequalityofmeat,milk,manure,crop,andairAdietarysupplementationwithAsparagopsisreducedfeedintakeinmost(Roqueetal.,2019a,2021;Stefenonietal.,2021;Muizelaaretal.,2021),butnotall(Kinleyetal.,2020),experiments.AfeedsupplementcontainingAsparagopsiswasrejectedbysomesheep(Lietal.,2016)anddairycows(Muizelaaretal.,2021)thathadbeenfedhighlevelsofseaweed.Asparagopsisincreased(Kinleyetal.,2020)ordidnotaffect91Methaneemissionsinlivestockandricesystems(Roqueetal.,2021)bodymassgainofsteers,althoughinbothstudiesfeedefficiencywasimprovedduetothereductionoffeedintake.TheinclusionofAsparagopsisinthediethadnoeffectsoncarcassormeatquality(Kinleyetal.,2020;Roqueetal.,2021).Milkproductionwasdecreasedduetosupplementationwith1percentDMAsparagopsisbecauseoftheDMIreduction(Roqueetal.,2019a;Stefenonietal.,2021).TheeffectofAsparagopsisonmanureGHGemissionisunknown.5.20.7SafetyandhealthaspectsLong-termoralexposureofanimalstobromoformcancauseliverandintes-tinaltumors.ItisthereforeclassifiedintheUnitedStatesasaGroupB2,prob-ablehumancarcinogen(EnvironmentalProtectionAgency[EPA],2000).Bromoformresidueswerenotdetectedinmeat,fat,organsorfecesfromsheepandbeeffedAsparagopsis(Lietal.,2016;Kinley,etal.,2020;Roqueetal.,2021),butanaccumulationofiodineinmeatwasreported(Roqueetal.,2021).Roqueetal.(2019a)andStefenonietal.(2021)didnotfindanypassageofbromoformtomilkfromdairycowsfedAsparagopsis,althoughMuizelaaretal.(2021)withnocon-trolanimalsintheirstudy,reportedpassageofbromoformtomilkinnon-adapteddairycows.AccordingtoStefenonietal.(2021),iodineandbromideaccumulatedinmilkfromcowsfedAsparagopsis.TheruminalmucosaofanimalsthatconsumedAsparagopsisshowedpathologicalsignsinsheep(Lietal.,2016)andcows(Muizelaaretal.,2021).Asinthecaseofotherfeedadditives,Asparagopsisshouldbehandledwiththenecessarycare.5.20.8AdoptionpotentialBromoform-containingseaweedwillnotbereadyforadoptionuntilvariouschal-lengesaresuccessfullymet,notablythepotentialsafetyrisksitposesforanimalsandhumans.Sofar,invivostudieshaveusedwild-harvestedAsparagopsiswithvari-ablebromoformcontent(Vijnetal.,2020).Asuccessfuladoptionwillrequirecon-sistentlygrowingandprocessingseaweedspeciestoaccumulatehalogenatedcom-poundsandmaintaintheirconcentrationswhentransporting,handlingandfeed-inganimals.Bromoformandotherhalogenatedalkanesareaconcernforanimal,foodandenvironmentalsafetythatwillneedtoberesolved,ifthisstrategyistobeadopted.ThepassageofbromoformtomilkinanimalsfedAsparagopsisisinconsis-tent(Roqueetal.,2019a;Stefenonietal.,2021;Muizelaaretal.,2021).Animalandfoodsafetyconcernsrelativetothetransferandaccumulationofiodineandbro-mideinmilkandmeatwillalsoneedtobeexaminedandsuccessfullyaddressedforseaweed-basedmitigantstobeadopted.Iftheinclusionofbromoform-containingseaweedsatlowlevelscouldberegardedasacceptable,theadministrationofpurebromoforminotherforms(e.g.slowrelease)couldequallybeconsidered.Thiscri-terionmightbeextendedtoothermethanogenesis-inhibitinghaloalkanes,suchaschloroformandbromochloromethane,althoughanencapsulatedformofbromoch-loromethanewasdeemedunlikelytobeapprovedforcommercialuseinAustralia(Tomkins,ColegateandHunter,2009),astheproductionofbromochloromethanewasbannedin2002undertheMontrealProtocolbecauseofitsozone-destroyingproperties.Thus,itcurrentlycannotbeconsideredforuseasaCH4mitigationtechnology.Finally,aswithanyotheradditive,theinclusionofAsparagopsisinani-malfeedwillinvolveanextraexpenditure,andthereforeitscost-effectivenessmustbeconsidered.92Mitigationofmethaneemissions5.20.9ResearchrequiredMoreinvivoresearchisneededtodetermineCH4mitigationandproductivitychangesunderdifferentdietandmanagementconditions.Effectivemethodsforgrowing,pro-cessingandstoringAsparagopsis,howtoimproveitspalatabilityandthebestdeliverymethodswillneedtobeestablished.GrowthconditionsthatpromotebromoformarekeytoAsparagopsisefficacy,andyetthepotentialthreatthatbromoformposestoanimals,farmers,consumersandtheenvironmentareaconcernwhenitcomestoitsuse.Themetabolicfateofbromoformingestedbyanimalsandthedistributionofingestedbromideamongthedifferentexcreta(viafeces,urine,milkandexhala-tion)needstobeestablished.Thebromide-containingcompoundspresentinmilk(Stefenonietal.,2021)shouldbeidentifiedtodeterminepossibleriskstoconsumers.Safetyissuesassociatedwithiodineandheavymetalsalsoneedtobeaddressed.ItisrecommendedtostudythecombinationofAsparagopsiswithotherCH4mitigationstrategies.Bromoformvolatilizationfromproductionsitesshouldbeprevented.5.21RUMENMANIPULATION:OTHERSEAWEEDS5.21.1DescriptionSeaweeds(macroalgae)otherthanAsparagopsismayinhibitmethanogenesisduetothepresenceofspecificbioactivecomponents,buttheresearchonthesealternativeseaweedsislimitedmainlytoinvitrostudies(asreviewedbyAbbottetal.,2020).5.21.2ModeofactionSeaweedshaveahighlyvariablechemicalcomposition,dependingonthespecies,timeofcollectionandgrowthenvironment.Theanti-methanogenicpropertyofthesealternativeseaweedsmaybeduetolowconcentrationsofbromoformandnumerousotherbioactiveconstituents,includingpolysaccharides,proteins,peptides,bacterio-cins,lipids,phlorotannins(similartocondensedtanninsfoundinterrestrialplants;onlyfoundinbrownseaweeds),saponinsandalkaloids(Moraisetal.,2020;Abbottetal.,2020).ThesecompoundsareknowntodecreaseCH4productionbysuppress-ingarchaeaandprotozoa,resultinginashiftinrumenfermentationpathways,andinsomecasesanundesirabledecreaseinsubstratedegradability.Someoftheseseaweeds–includingLaminariadigitata(brown),Macrocystispyrifera(brown),Pterocladiacapillacea(red),Rhodymeniacalifornica(red),Ulvaintestinalis(green)andUlvaspp.(green)–producebromoform,buttheystorelessinbiomassthanAsparagopsis(CarpenterandLiss,2000).5.21.3EfficacySeveralseaweedshavebeenidentifiedashavingahighinvitroCH4mitigationpotential(>50percentdecrease):Cladophorapatentiramea(green),Cystoseiratri-nodis(brown),Dictyotabartayresii(brown),Gigartinaspp.(red),Padinaaustralis(brown)andUlvaspp.(green)(Duboisetal.,2013;Machadoetal.,2014;Maiaetal.,2016).RedandbrownseaweedsseemtohavegreatereffectsonCH4produc-tionthangreenseaweeds(McCauleyetal.,2020).Invivoefficacyisnotknownandneedstobeinvestigated.5.21.4PotentialtocombinewithothermitigationstrategiesGoodpotentialtocombinewithotherstrategieswithdifferentbioactivecompo-nentsormodesofaction.Negativeinteractionsmightoccurifcombinedwithcom-poundsthathavesimilarmodesofaction.93Methaneemissionsinlivestockandricesystems5.21.5EffectsonotheremissionsTheCO2eqemissionsofgrowing,harvesting,processing(drying),storingandtrans-portingseaweedatalargescaleneedtobeconsideredinanLCAtodeterminethenetimpactonGHGintensityofmeatandmilkproduction(McCauleyetal.,2020).TheimportanceofupstreamemissionsofCO2woulddependonthepercentageofseaweedinclusioninthediet.Thereisalsothepossibilityofpurifyingorextractingseaweedbioactives,whichwoulddecreaseemissionsrelatedtodryingandtransporta-tion.ThepotentialfixationofCO2throughphotosynthesiswasdeemedtocontributetowardsmitigationintheemissionofGHGs(McCauleyetal.,2020);however,thisislikelyaminorbenefitasmostoftheCO2wouldbereleasedintotheatmospherebytheanimalsorhumansconsuminganimalproducts,asisthecasewithotherfeedstuffs.5.21.6Productivityandthequalityofmeat,milk,manure,crop,andairThenutritivevalueofseaweedsvariesconsiderablydependingontheircomposi-tionandanimaladaptation,anditwouldneedtobeevaluatedinvivoforanysea-weedfoundtohaveanti-methanogenicpotential.Lowdoses(<2percentofDM)maynotaffecttherationintake,digestibilityortheamountofmanureexcreted;however,phlorotannin-containingseaweedsmayshiftnitrogenexcretionfromurinetofeces(Antayaetal.,2019).Proteindigestibilitywaslowerforabrownthanforaredseaweed(Abbottetal.,2020).Ahigh-mineralconcentrationlimitsthedigestibleOMconcentrationinmanyseaweeds.Beneficialeffectssuchasanimprovedimmuneandantioxidativestatusandtheinhibitionofpathogenshavebeenreported(Makkaretal.,2016),butthisisprobablyhighlyspecies-dependent.Byincreasingthecontentofbeneficialfattyacids,seaweedmaypositivelyaffectthequalityofanimalproducts(McCauleyetal.,2020).5.21.7SafetyandhealthaspectsSeaweedstendtoconcentrateminerals,specificallyheavymetalssuchasarsenicandcopper,aswellasiodineandnitrate;therefore,thesafetyandhealthimpactsneedtobedeterminedforeachseaweed(Makkaretal.,2016;Abbottetal.,2020;McCauleyetal.,2020;Moraisetal.,2020).Ahigh-iodineconcentrationwasfoundinthemilkofcowsfedthebrownseaweedAscophyllumnodosum(Antayaetal.,2015),afindingthatlimitstheadoptionpotentialfordairycows.Healthproblemswerereportedinsheepaccustomedtoconsuminglargeamountsofseaweedsincoastalareas(Makkaretal.,2016).Potentialtoxicityandresiduesinmeatandmilkwilldependonthecontentoftoxicmineralsandthelevelofseaweedinclusioninthediet.5.21.8AdoptionpotentialTheprospectsforimmediateadoptionarelow,butthereisagoodpotentialforadop-tioninthefuture,especiallyincoastalareaswithnativeseaweedswherethesemaybeconsumedwet.Otherwise,seaweedneedstobedriedrapidly,beforeitbecomesmouldy.Low-temperaturedryingreducestheinactivationofbiochemicalcom-pounds(Makkaretal.,2016).Asustainableproductionofseaweedswillberequiredtomeetthedemand(Abbottetal.,2020).Poorpalatabilityduetohigh-saltcontentandtoxicitymaybelimitations,particularlywhenofferedasfreechoicetoanimalsortograzingruminants.Itmightbemoreeffectivetoincorporateseaweedsintoa94Mitigationofmethaneemissionstotalmixedrationorextractthebioactivessuchthattheycanbeusedasafeedaddi-tive.Adoptionwillbecontingentoncost-benefitanalysisandregionalavailability.Governmentagencies’approvalwilldependonthecontentofpotentiallytoxicmine-rals,whichmayhavetobeanalysedfrombatchtobatchunlessaconsistentcom-positioncanbeshown.Inclusionofseaweedsinruminantdietsmaybeacceptabletoconsumers,providedthereisnoriskoftoxicityandnooff-flavoursinmeatormilk.5.21.9ResearchrequiredSeveralaspectsstillneedtobeconsideredifseaweeduseistoreduceentericCH4emissions(Vijnetal.,2020).SubstantialinvivoresearchisneededtoestablishtheCH4mitigationpotentialandtheenvironmentalimpactsofseaweedfarming.Bioactivecompoundsandthegrowthconditionsthatpromotethesebioactivesarekey.Productpalatability,bestadditivedeliverymethod,qualitycontrolandthepotentialtoextractbioactivecompoundswillneedtobedetermined.Safetyissuesassociatedwithhighconcentrationsofcertainbioactives,iodineandheavymetalsneedtobeaddressed.Acomparisonwithsynthetically-derived,identicalbioactivecompoundsneedstobecarriedout.5.22RUMENMANIPULATION:DEFAUNATION5.22.1DescriptionSomerumenmethanogensareectosymbionts(Vogels,HoppeandStumme,1980)orendosymbionts(Finlayetal.,1994)ofprotozoa,whichsupplythemwithH2andformate.Ithasbeenproposedthattheeliminationofprotozoawouldcausethelossoftheirmethanogenicsymbionts,resultinginadecreaseinCH4productionintherumen.Protozoacanbeeliminatedfromtherumenbyusingchemicalsorlipids,byfreezingrumencontentsorbyisolatingnewbornanimals(Newboldetal.,2015).Inthissection,wediscussdefaunationtargetingtheeliminationofrumenprotozoa,ratherthanthedecreaseinprotozoalnumbersthroughtheadditionofphytochemi-calssuchassaponinsandtannins,orionophoressuchasmonensin.Thoserumenmanipulationstrategiesarediscussedinothersections.5.22.2ModeofactionProtozoadonotdisposeofmetabolichydrogeninpropionateproduction(Goopy,2019),andtheremovalbysymbioticmethanogensoftheH2andformatethattheyproducefavourscarbohydratefermentation.Ithasbeenestimatedthatprotozoa-associatedmethanogenscontributebetween9and37percentofCH4producedinrumenfermentation(Newbold,LassalasandJouany,1995;Newboldetal.,2015).Sincethepresenceofprotozoaisnotstrictlynecessarytorumenfunctionandani-malsurvival(Morgavi,etal.,2010;Newboldetal.,2015),theireliminationhasbeenproposedasameansofdecreasingentericCH4productionthroughthesimulta-neousremovalofsymbioticmethanogens.Defaunationdoesnothaveacleareffectontheabundanceoftotalmethanogens(Huws,WilliamsandMcEwan,2020),butprotozoa-associatedmethanogensseemtobemoreactiveCH4producersthanfree-livingmethanogens(JamiandMizrahi,2020).Protozoamayalsofavourmethano-gensbyprotectingthemfromoxygentoxicity(Morgavietal.,2010).95Methaneemissionsinlivestockandricesystems5.22.3EfficacyFromasummaryofinvivoandinvitroexperiments,Hegarty(1999)concludedthateliminatingprotozoaresultedinanaveragedecreaseof13percentinCH4produc-tion,whichwasnotsolelyduetotheremovalofprotozoa-associatedmethanogens.Meta-analysesofinvivoexperimentswithcattle,sheepandgoatsbyMorgavietal.(2010),Newboldetal.(2015)andLietal.(2018)foundthatdefaunationcauseddecreasesinCH4productionof10to11percent,althoughthiswashighlyvariable.Themeta-analysisbyVenemanetal.(2016)reportedanaverage17percent(4to29percentconfidenceinterval)decreaseinCH4yield.Therecentmeta-analysisbyArndtetal.(2021)concludedthatdefaunationresultedindecreasesof10and20percentinabsoluteCH4productionandyield,respectively.Linearrelationshipsbetweenproto-zoalnumbersandCH4yieldhavebeenestablished(Morgavietal.,2010;Guyaderetal.,2014).Themeta-analysisbyLietal.(2018)suggestedalong-termadaptationofCH4productiontodefaunation.Similarly,Morgavietal.(2012)showedanumericaldecreaseinCH4productionofwethersdefaunatedintheshortterm,andanumericalincreaseinCH4productioninwethersthathadbeendefaunatedformorethantwoyears.Incontrast,previousworkhadnotfoundevidenceofalong-termadaptationtodefaunation(Morgavi,JouanyandMartin,2008).5.22.4PotentialtocombinewithothermitigationstrategiesNotmuchisknownabouttheinteractionsofdefaunationwithotherCH4mitiga-tionstrategies.Ithasbeenproposedthatdefaunationaffectednitratesupplementa-tion,withnitratedecreasingCH4yieldinfaunatedsheepbutnumericallyincreas-ingitindefaunatedanimals(Nguyen,BarnettandHegarty,2016).Withregardstochemicalinhibitorsofmethanogenesis,itwasspeculatedthatfree-livingrumenmethanogensmaybemoreresistantto2-bromoethanesulfonatethantoprotozoalsymbionts,conferringondefaunatedrumenfluidresistancetothisinhibitorofmethanogenesis(Ungerfeldetal.,2004).5.22.5EffectsonotheremissionsBecauseitcanimprovetheefficiencyofNutilizationanddecreaseNeliminationinurine(Eugène,ArchimèdeandSauvant,2004;Newboldetal.,2015),defaunationmaydecreasetheemissionsofN2OfromNvoidedintotheenvironmentinanimalurine.Thefibreexcretedinmanuremayincrease,asdefaunationhasbeenshowntodecreasefibredigestibility(Eugène,ArchimèdeandSauvant,2004;Newboldetal.,2015;Lietal.,2018).5.22.6Productivityandthequalityofmeat,milk,manure,crop,andairThemeta-analysesbyEugène,ArchimèdeandSauvant(2004),andNewboldetal.(2015),reportedbeneficialeffectsofdefaunationonweightgain,feedconversionefficiencyandwoolproduction,witheithernoeffectsonDMI(Eugène,ArchimèdeandSauvant,2004)ordecreasedDMI(Newboldetal.,2015).Themeta-analysisbyArndtetal.(2021)didnotsuggestanyeffectsofdefaunationonDMIorweightgain.Therewereconsistentdecreasesinrumen,overalltractOMandNDFdigestibility,rumenVFAandammoniaconcentration,anincreasedmicrobialnitrogenproduc-tion,andashiftinnitrogenexcretionfromurinetofeces(Eugène,ArchimèdeandSauvant,2004;Newboldetal.,2015;Lietal.,2018).Decreasesinfibredigestibility96MitigationofmethaneemissionscanpartiallyaccountforthedecreaseinCH4productioncausedbydefaunation(Firkinsetal.,2020).Beneficialeffectsonanimalperformanceweremoreimpor-tantwithhigh-forage,low-qualitydiets(Eugène,ArchimèdeandSauvant,2004).ProtozoalnumberswerefoundtoassociatepositivelywithDMIandNDFdigest-ibility(Guyaderetal.,2014).AccordingtoNewboldetal.(2015),defaunationdecreasesbiohydrogenationofpolyunsaturatedfattyacids.5.22.7SafetyandhealthaspectsThroughengulfingstarchgrainsandmetabolizinglactate,protozoacanhelpmain-tainamorestablerumenpHwhenfeedinghighlyfermentablediets,therebypre-ventingacidosis(Eugène,ArchimèdeandSauvant,2004;Newboldetal.,2015).Thereisnoevidencetosuggestthatdefaunationcouldharmtheanimal’shealthandtheenvironment,orputthosewhoconsumeanimalproductsatrisk.5.22.8AdoptionpotentialDefaunationresultsinmilddecreasesinCH4emissions.Moreover,defaunatingandmaintainingdefaunatedanimalsinproductionsettingsposesachallenge.Consequently,defaunationhasnotbeenrecommendedasaCH4mitigationstrategyforpracticalreasons(Hristovetal.,2013a;Newboldetal.,2015;Huws,WilliamsandMcEwan,2020).5.22.9ResearchrequiredTherearedifferencesamongprotozoawithregardtotheirassociatedmethanogensandcontributiontoCH4production,aswellastheircellulolyticcapacity(Morgavietal.,2010;Firkinsetal.,2020)andbacterialpredatoryactivity(Newboldetal.,2015).TargetingspecificallytheorderVestibuliferidahasbeensuggestedasaresearchdirectionbecauseofthehigh-CH4producingandlow-fibredegradingactivityofthisorder(Huws,WilliamsandMcEwan,2020),butthiskindof“fine-tuning”proto-zoalmanipulationstrategiesarenotavailableatpresent.Thereisaneedforfurtherrefinementintheunderstandingofhowdifferentprotozoaltaxaaffectmethanogen-esis,intra-ruminalnitrogenrecycling,fibredigestion,utilizationofsolublecarbohy-drates,oxygenscavenging,aswellastheirrumensequestrationandpassage(FirkinsandMackie,2020).5.23RUMENMANIPULATION:ALTERNATIVEELECTRONACCEPTORS5.23.1DescriptionDietarysupplementationwithorganicandinorganiccompoundsthatdrawelec-tronsawayfrommethanogenesistowardsalternativehydrogenotrophicpathwaysinrumenfermentation.5.23.2ModeofactionOrganicalternativeelectronacceptorsarecarboxylicacidintermediatesofpath-waysinrumenfermentationthateitherincorporatemetabolichydrogenthemselves(fumarate,whichisreducedtosuccinateinthepropionaterandomizingpathway)orcanbemetabolizedintocompoundswhichincorporatemetabolichydrogen(malate,whichisdehydratedtofumarate;acrylate,whichcanbeesterifiedtoacrylyl-CoAandincorporatedintothepropionatenon-randomizingpathway;crotonate,97Methaneemissionsinlivestockandricesystemswhichcanbeesterifiedtocrotonyl-CoAandincorporatedintobutyrateforma-tion)(Russell,2002;CarroandUngerfeld,2015;UngerfeldandHackmann,2020).Importantly,theresultingelectronsinks(propionateandbutyrate)areabsorbedthroughtherumenwallandhaveanutritionalvalueforruminants.Inorganicalternativeelectronacceptorsarestronganionswhichdissociatewhenaddedassaltstothedietand,whentheyarereduced,drawelectronsawayfromCH4formation.Acompletenitratereductionyieldsprimarilyammonium,whichcanbeincorporatedintomicrobialNorabsorbedthroughtherumenwall.Nitratereductionviaintermediatenitritealsoexertsadirectinhibitionofmethanogens(Hulshofetal.,2012;Lathametal.,2016;Yangetal.,2016).Thereductionofsulfateyieldshydrogensulfide,whichcanbeexpelledasagas(dissimilatoryreduction)orincorporatedintomicrobialaminoacidsandcofactors(assimilatoryreduction;Drewnoski,PoggeandHansen,2014).ForaddedalternativeelectronacceptorstodrawmetabolichydrogenawayfromCH4formation,theirreductionhastobethermodynamicallymorefavourablethanmethanogenesisfortheinvivorumenconcentrationofallmetabolitesinvolved(Cord-Ruwisch,SeitzandConrad,1988;UngerfeldandKohn,2006).5.23.3EfficacyThemodeofactionofalternativeelectronacceptorsimposesatheoreticallimita-tionontheirefficacy,duetothestoichiometryofmetabolichydrogenincorpora-tionintheirreduction.Forexample,thereductionof1moleoffumarateto1moleofsuccinateincorporates1moleofreducingequivalents([2H]),whichtheoreti-callywouldsuppresstheformationof0.25moleofCH4throughhydrogenotro-phicmethanogenesis(CO2+4H2→CH4+2H2O)(CarroandUngerfeld,2015).Forexample,adecreaseofonly10percentinCH4productionofacowproducing328g/d(~500L/d)ofCH4wouldrequiretheanimaltoingest1.4kg/dofsodiumfumarate,i.e.aconsiderablepartofitsdiet(Newboldetal.,2005).Furthermore,meta-analysesofinvitroexperimentshaveshownthatthedecreasesinCH4pro-ductionwerebelowthetheoreticalexpectationforfumarateandmalate,becausefumarateandmalatewereapparentlypartiallyconvertedtoacetateratherthantopropionate,thusreleasing[2H]insteadofincorporatingit[Ungerfeldetal.,2007;UngerfeldandForster,2011].Invivoresultsoffumarateandmalatesupplementa-tionhaveproducedvariableresults,rangingfromnoeffectsinsomestudiestomildandmoderatedecreasesinCH4production(i.e.10to23percent)inothersCarroandUngerfeld,2015).Woodetal.(2009)reportedpronounceddecreasesinCH4production,beyondwhatthestoichiometricalreductionoffumaratetosuccinatewouldresultin.Itispossiblethattheinclusionofanelevatedlevel(10percentasfed)ofhighlyfermentablefumaricacidinthedietfurtherdecreasedCH4produc-tionandshiftedfermentationtopropionate(Janssen,2010),beyondwhatfumaratereductionwouldallowfor.Stoichiometrically,4molesofhydrogenareredirectedtowardsthereductionof1moleofnitrate,equivalentto258.7gCH4perkilogramofnitrate.Theconsump-tionof173g/dofsodiumnitratefullyreducedtoammoniumwoulddecrease10percentofCH4emittedfromacowproducing328g/dofCH4(~500L/d;calcula-tionsnotshown).Thisidealstoichiometryiscomplicatedbytheincompletereduc-tionofnitrate,whichwouldresultinalowerCH4decrease,andthedirecttoxicityofnitratereductionviaintermediatenitriteonmethanogens,whichwouldincreasethe98MitigationofmethaneemissionsmitigationofCH4production.NitratesupplementationconsistentlydecreasesCH4productioninvivoinlong-termexperiments(LeeandBeauchemin,2014),includ-ingexperimentslastingaslongas407consecutivedays(Granja-Salcedoetal.,2019).Fromtheirmeta-analysis,LeeandBeauchemin(2014)reportedalineardecreaseof8.3gofCH4perkilogramofDMintake,pergramofnitrateingested,perkilogramofbodymassandperday.Inasubsequentmeta-analysis,Fengetal.(2020)reportedthatthemeandoseofnitratesupplementationof16.7g/kgDMdecreasedCH4produc-tionby13.9percentonaverage,althoughthisdependedonthenitratedose,typeofanimal(greaterefficacyindairythaninbeefcattle)andDMI.ItsefficacydiminishedwithincreasingDMI.Onaverage,theirfindingsamountedto364gofsodiumnitratedecreasingCH4productionby10percentinacowconsuming24kgofDMperdayandproducing328gofCH4perday,i.e.about50percentoftheoreticalmitigationefficiency(calculationsnotshown).Mitigationefficiencycanbegreaterinindividualstudies,e.g.Hulshofetal.(2012)achieved87percentefficiency.5.23.4PotentialtocombinewithothermitigationstrategiesTheadditionoffumarate(Tatsuokaetal.,2008;Ebrahimietal.,2011)ormalate(Mohammedetal.,2004)toinvitroincubationsinwhichmethanogenesiswasinhibitedhelpedredirectaccumulateddihydrogentowardspropionateformation;incontrast,theadditionofbutyrateprecursorsaselectronacceptorsdidnotrelieveaccumulationofdihydrogenthroughenhancingbutyrateformation(Ungerfeldetal.,2006).TheadditionoffumaratetothedietofgoatsdidnotinteractwiththeforagetoconcentrateratiowithrespecttoCH4production(Yangetal.,2012).InorganicelectronacceptorsnitrateandsulfatehadadditiveeffectsonCH4decrease(vanZijderveldetal.,2010).Nitrateadditiontendedtonegativelyinteractwithlinseedoil(Guyaderetal.,2015)butitinteractedsynergisticallywithcanolaoil(Villaretal.,2020).NitrateinteractednegativelywithdefaunationonCH4pro-duction(Nguyen,BarnettandHegarty,2015).Theadditionofnitrite-reducingbacteriumPropionibacteriumacidipropionicididnotinteractwithnitrateaddi-tion(norhadaneffect)onCH4emissionsfromsheep(deRaphélis-Soissan,2014).Tocompensateforthesmallreductioninfeedconsumption,dietscontainingnitratecouldbeassociatedwithsupplementationinoilsandfats,increasingboththeenergydensityandthemitigationpotentialofthediet.5.23.5EffectsonotheremissionsEmissionsofCO2fromfossilfuelsassociatedwithmanufacturing,orextractingandisolatingfumarateandmalatefromnaturalsources,maybeconsiderableduetothesecompounds’srelativelyimportantdietaryconcentrationsthatareneededtoexertaneffectonCH4emissions.Malateisnaturallypresentinsomeforagesatvegetativestages(Callawayetal.,1997);selectingvarietieswithhighandsustainedmalatecon-tentanddesirableagronomictraitscouldpreventadditionalCO2emissions.Apartfromtheemissionsrelatedtothemanufactureofnitratesalts,nitratecanbepartiallyreducedtoN2Ointherumen(deRaphélis-Soissanetal.,2014;Petersenetal.,2015).UnlessitissupplementedtoaN-deficientdiet,nitrateshouldisonitro-genouslyreplaceanotherNsourceinordernottoincreaseNvoidedintotheenvi-ronment,whichcanpotentiallyincreaseN2Oemissions(Beaucheminetal.,2020).99Methaneemissionsinlivestockandricesystems5.23.6Productivityandthequalityofmeat,milk,manure,crop,andairMalatecanhelppreventacuteacidosisbystimulatinglactateutilizationbySelenomonasruminantium,aswellasamelioratesubclinicalacidosis.Malateandfumaratedecreasedbiohydrogenationoflinoleicandlinolenicacidsinvitro,andincreasedtheproductionofrumenicacid,whichmaypotentiallyimprovethenutritionalqualitiesofanimalproducts.MoststudieshavenotshownanyeffectsofamoderateinclusionofmalateonDMI,whereasfumarateeffectshavebeenmoreinconsistent,withdecreasedDMIinsomestudiesandlackofeffectsinothers.Malatesupplementationdidnotaffectweightgainormilkproductioninsomestudiesandimprovedtheminothers,whilefumaratesupplementationhasnotaffectedmilkproduction(CarroandUngerfeld,2015).Overall,thebenefitsofnitratesupplementationonanimalproductivityhavenotbeendemonstrated(Yangetal.,2016),exceptwhennitrateisaddedtonitrogen-deficientdiets(Nguyenetal.,2015).Wangetal.(2018)foundthatreplac-ingureabynitrateonanisonitrogenousbasisinalowproteincontentdietincreasedmicrobialNproductionandmilkyield,whichmayberelatedtoadditionalmicro-bialATPgenerationresultingfromnitratereduction(Yangetal.,2016).5.23.7SafetyandhealthaspectsFumarateandmalatearenaturalintermediatesofrumenfermentation.Theyareregardedassafe,andregisteredasanimalfeedingredientsintheEuropeanUnionandtheUnitedStates(CarroandUngerfeld,2015).Nitratefermentationviainter-mediatenitriteisabsorbedthroughtherumenwallandentersbloodcirculation,reactingwithhemoglobintoproducemethaemoglobin,whichcannotcarryoxy-gen.Nitratepoisoningcanbefatal,butitispossibletoadapttherumengraduallytoanincreaseinthereductionrateofnitritetoammonium(LeeandBeauchemin,2014;Yangetal.,2016).Tracesofnitratehavebeenfoundintissues(Doreauetal.,2018)andmilk(Guyaderetal.,2016)ofanimalsfednitratebuthavenotbeendeemeddangeroustoconsumers.TheinclusionofnitrateinanimalfeedsisnotapprovedintheUnitedStatesandinCanada(Beaucheminetal.,2020).InAustralia,carboncreditscanbeobtainedbyfeedingnitratetobeefcattle(https://www.legislation.gov.au/Details/F2015C00580).Thehighdietarysulfateresultsinhydrogensulfideproduction,whichcancausepolioencephalomalacia(Drewnoski,PoggeandHansen,2014).5.23.8AdoptionpotentialFeedingfumarateandmalatetoruminantsislargelylimitedbycost,giventhelevelofinclusioninthedietneededtoobtainaneffectonCH4mitigation,andtheirinconsistenteffectsonanimalperformance.Nitratesupplementationrequiresthegradualadaptationofanimalsandcanonlyberecommendedforfarmsinwhichfeedintakeiscarefullymanaged.Inaddition,thenitratecontentofherbageandforagesneedstobetakenintoaccounttopreventexcessivelyhighlevels.ThepotentialriseinN2OemissionsasaresultoffeedingincreasedNlevelsshouldbecarefullyassessed.Itwasestimatedthatsupplementingnitrateinsteadofureaasanon-proteinNsourcewouldbemorethantwiceasexpensive(Callaghanetal.,2014).100Mitigationofmethaneemissions5.23.9ResearchrequiredInvivoexperimentswithcombinationsofmethanogenicinhibitorsandfumarateormalatetoexaminetheincorporationofaccumulateddihydrogenintopropionateproductionwouldbeofinterest.Theselectionofgrasseswithamalatecontentthatstayshighthroughoutmaturitycanbeapossiblerouteofsupplementation.Effortstodecreasenitriteaccumulationbyaddingnitrite-reducingbacteriahavebeensuccessfulininvitroexperiments(Saretal.,2005a,2005b),buttheyonlynumericallylowerednitriteandmethaemoglobinconcentrationinplasmainvivo(deRaphélis-Soissanetal.,2014).Moreeffortstoexaminedifferentdosesandfrequenciesofadministra-tionofnitrite-reducingbacteriaarerecommended,inadditiontoisolatingnewnitritereducersfromtherumenenvironment.5.24RUMENMANIPULATION:ESSENTIALOILS5.24.1DescriptionEssentialoilsarecomplexmixturesofvolatilelipophilicsecondarymetabolites,traditionallyextractedfromplantsbyboilingwaterandsteamdistillation;othermethodsincludesolventextraction,supercriticalCO2extractionandexpressionextraction.Theyarespecifictoplantsandresponsibleforeachplant’scharacter-isticflavourandfragrance(BenchaarandGreathead,2011).Essentialoilscanbeextractedfrommanypartsofaplant,suchastheleaves,flowers,stem,seeds,rootsandbark(Benchaaretal.,2008).Whenextractedandconcentrated,essentialoilsmayexertantimicrobialactivitiesonawidevarietyofmicroorganisms,includingbacteria,protozoaandfungi(DeansandRitchie,1987;Sivropoulouetal.,1996;Chao,YoungandOberg,2000).Inadditiontoplantsources,essentialoilscanbechemicallysynthesizedforcommercialuse.Chemically,essentialoilsarevariablemixturesofterpenoids,mainlymonoterpenesandsesquiterpenes,althoughditer-penesmayalsobepresent,aswellasavarietyoflowmolecularweightaliphatichydrocarbons,acids,alcohols,aldehydes,acyclicestersorlactones,andexception-allyN-andS-containingcompounds,coumarinsandhomologuesofphenylpro-panoids(DormanandDeans,2000).5.24.2ModeofactionMostessentialoilsexerttheirantimicrobialactivitiesbyinteractingwithprocessesrelatedtothebacterialcellmembrane,includingelectrontransport,iongradients,proteintranslocation,phosphorylationandotherenzyme-dependentreactions(Ultee,KetsandSmid,1999;DormanandDeans,2000).Gram-positivebacteriaappeartobemoresusceptibletotheantibacterialpropertiesofessentialoilsthangram-negativebacteria.Theresistanceofgram-negativebacteriatotheantimicrobialactivityofessentialoilsisduetoanouterlayersurroundingtheircellwallthatactsasapermeabilitybarrier,limitingtheaccessofhydrophobiccompoundsofessentialoils(Burt,2004).However,phenoliccompounds(e.g.thymolandcaravacrolcontainedinsomeessentialoils,suchasthymeandoregano)caninhibitthegrowthofgram-neg-ativebacteriabydisruptingtheoutercellmembrane(Helanderetal.,1998).Itseemsthatthesmallmolecularweightofessentialoilsallowsthemtopenetratetheinnermembraneofgram-negativebacteria(Nikaido,1994;DormanandDeans,2000).Ruminalgram-positivebacteriaareinvolvedinfermentationprocessesthatproduce,101Methaneemissionsinlivestockandricesystemsamongotherendproducts,acetate,butyrate,formate,lactate,hydrogenandammonia(RussellandStrobel,1989).MostofthesefermentationprocessesarecoupledwiththeproductionofCH4,areductivesteprequiredforthedisposingofreducingequi-valentsmainlyproducedbythisgroupofbacteria(OwensandGoetsch,1988).Ontheotherhand,gram-negativebacteriaareinvolvedinthefermentationpathwaysassociatedwiththeproductionofpropionateandsuccinate(RussellandStrobel,1989;Russell,1996).Whenthisgroupofbacteriaispredominantintherumen,rumenfermentationpatternsshifttowardsmorepropionate(H2-usingpathway)andlessacetate(H2-producingpathway)production,thusreducingtheavailabilityofhydro-genforruminalmethanogenesis(BergenandBates,1984).Neithermethanogensnorprotozoa,whichareinasymbioticrelationshipwithmethanogens,appeartobesensitivetotheactivityofessentialoils(BenchaarandGreathead,2011).5.24.3EfficacyAnumberofessentialoils(e.g.oregano,thyme),garlicoilanditsderivativeshavebeenshowntodecreaseCH4productioninvitro(Cobellis,Trabalza-MarinucciandYou,2016).TheadditionofMootralasaningredientat9.9or18.0percentofthesub-strateincubatedinsemi-continuousculturesmarkedlydecreasedCH4productionby95percentandbymorethan99percent,respectively,whilethetotalVFAproductionwasincreased(Egeretal.,2018).InanotherRusitecstudy,theinclusionofMootral–oncemoreatlevelsofadietaryingredient(17.7percentDM)–insemi-continu-ouscultures(Rusitec)eliminatedCH4productionafter4daysofMootralinclusion,followingwhichCH4productionresumed.Yetagain,thetotalVFAproductionstronglyincreasedwithMootralinclusion(Bredeetal.,2021).However,theresultsfrominvivostudieshavebeenlessconclusive(BenchaarandGreathead,2011).Essentialoilswithahighcontentofphenoliccompounds(e.g.thymol,carvacrol),garlicanditsactivecompounds(alliin,diallylsulphidesandallicin)appeartobeeffec-tiveforCH4reductioninvitrowhenaddedathighlevelsrelativetofeedsubstrate,althoughtheirefficacywasnotconfirmedorlesspronouncedinvivo(Klevenhusenetal.,2011;Benchaar,2020,2021).Commercialproductscontainingvariousessen-tialoilshavebeenshowninaverylimitednumberofstudiestohaveapotentialtodecreaseCH4production.Forinstance,acommercialproductoforeganooil(OregoStim®,Anparioplc,Nottinghamshire,UnitedKingdom)fedtolactatingdairycowswasreportedtoreduceCH4yieldby22percent(Kollingetal.,2018).Feeding15gperdayofacommercialproductcontainingcitrusextractandallicinfromgarlic(MootralGmbH,Switzerland)tofeedlotsteersreducedentericCH4yieldby23percent,butonlyinthefinalweek(week12)ofthestudy(Roqueetal.,2019b).Inanotherstudy,MootraldidnotaffectCH4emissionsbyfeedlotsteersinweek8ofthetrial,butdecreasedthetotalCH4andCH4yieldinweek29by26and30percent,respectively,whenfeedingahigh-concentratediet(Bitsieetal.,2022).A10percentdecreaseinCH4yieldwasreportedforacommercialmixtureofcoriander,eugenol,geranylacetateandgeraniol(Agolin®Ruminant;AgolinS.A.,Bière,Switzerland)whenfedtodairycowsattherateof1gperday(Belancheetal.,2020).Basedontheliteratureavailabletodate,itappearsthatessentialoilsandtheircompoundsmayholdpromiseforCH4mitigation,butfurtherworkonanimalfeeding–especiallylong-termstudies–isrequiredtodeterminetheirefficacy.102Mitigationofmethaneemissions5.24.4PotentialtocombinewithothermitigationstrategiesOpportunitiesexisttocombinethiswithothermitigationstrategiesthathavedif-ferentorsimilarmechanismsofaction.Forinstance,giventhatessentialoilshavenoeffectonprotozoa,combiningthesesubstanceswithotherphytocompoundsknownfortheirantiprotozoalactivity(e.g.saponins)mayincreasetheirmitigatingaction.Monensinisknownforitsinhibitoryeffectonruminalmethanogenesis,duetonegativeeffectongram-positivebacteria,whichincreasespropionateproductionattheexpenseofacetate.Thusitscombinationwithessentialoils,whichalsoinhibitthesamegroupofbacteria,mayenhancethereducingeffectonCH4production.Giventhatmostessentialoilsdonotactdirectlyonmethanogens,theircombina-tionwithotherdirectinhibitors(e.g.chemicalinhibitors)couldcontributetothemitigatingeffects.5.24.5EffectsonotheremissionsSomeessentialoilsandtheircompoundshavebeenreportedtoreducedietarypro-teindegradationinvitro,althoughinvivostudieshavebeeninconsistent(Cobellis,Trabalza-MarinucciandYu,2016).IfthisisaccompaniedbyareductioninurinaryNexcretion,thepresenceofN2Oandammoniamaypotentiallybereduced.5.24.6Productivityandthequalityofmeat,milk,manure,crop,andairIngeneral,feedingessentialoilstoruminantsdoesnotaffectanimalproductivityorproduct(milk,meat)quality(Benchaar,HristovandGreathead,2009).Adverseeffectsofessentialoilsonfeeddigestionwerereported(Benchaar,HirstovandGreathead,2009;Cobellis,Trabalza-MarinucciandYu,2016)and,ifsucheffectsoccurinanimals,itwouldhaveanegativeimpactonproductivity.Thereisapoten-tialforthetransferofcompoundspresentinessentialoils(e.g.terpenes)tomilk(Lejonklevetal.,2013)andmeat(deOliveiraMonteschioetal.,2017),whichcanpositivelyornegativelyaffectthequalityandorganolepticpropertiesofmeatandmilk.Theamountandchemicalcompositionofmanureareunlikelytobeaffected,butifthefeeddigestionintherumenisdepressed,theamountofmanureexcretedandassociatedemissionscouldincrease.5.24.7SafetyandhealthaspectsLittleisknownabouthowsafetheuseofessentialoilsisinruminantnutrition.Atthedosesgenerallyrecommendedbythefeedindustry,theprobabilityofessen-tialoilsbeingtoxictoanimalsislow.However,cautionshouldbeexercised,espe-ciallyiftheessentialoilsarefedathighdoses.Forexample,anumberofessentialoilcomponents(e.g.carvacrol,cinnamaldehyde,eugenol,thymol)havebeenregisteredbytheEuropeanCommissionforuseasflavouringsinfoodstuffs.Yetessentialoilcompoundssuchasestragoleandmethyleugenolweredeletedfromthelistin2001duetotheirgenotoxicproperties(Burt,2004).Theuseofessentialoilsasfeedaddi-tivesinlivestockproductionmustalsobesafeforthefeedmanufacturingpersonnelandfarmworkers.Thesesubstanceshavebeenreportedtobepotentiallyirritatingandmaycauseallergicdermatitis(Burt,2004),whichsuggeststhatcautionmustbetakenbyuserswhenhandlingsuchfeedadditives.103Methaneemissionsinlivestockandricesystems5.24.8AdoptionpotentialBecausetheyareplant-derivedproducts,essentialoilsareconsideredsaferthanantibioticsorchemicaladditives.Essentialoilshaveawidespectrumofantimicro-bialactivity,whichmakesitdifficulttotargetspecificmicrobialgroupsandcanadverselyaffectfeeddigestionintherumen.Inaddition,ithasbeenreportedthatmicrobialpopulationsareabletodegradeessentialoilsoradapttothemovertime.Itremainschallengingtoidentifyessentialoilsthatselectivelyinhibitrumenmetha-nogenesis,withlastingeffectsandwithoutdepressingfeeddigestionandanimalproductivity.Becauseessentialoilsarehighlyvolatile,mostcommercialproductsarecoatedandformulatedinawaythatcontrolsthereleaseoftheactiveingredientonceaddedtotheanimal’sdiet.However,thelong-termstabilityofproductsandtheneedforcontrolledstorageconditionscanbelimitingfactors.Finally,unlessthereareclearproductivitybenefits,theadditionalcostsinvolvedmaydiscouragesomefromadoptingthisparticularstrategy.5.24.9ResearchrequiredThepotentialofessentialoilstomitigateentericCH4emissionhasbeenmostlyexaminedinvitroandthereisaneedtoconductmoreinvivostudiestodeterminetheefficacyofessentialoils.Therangeofessentialoilsavailableisextensive(>3000)andmoreworkisrequiredtoidentifytheonesthataremosteffectiveinreducingentericCH4production.Manyoftheconcentrationsthathaveshowneffectsinvitroaretoohighforinvivoapplications,andthusmoreresearchiswarrantedatoptimaldoses,underspecificdietaryconditionsthatlendthemselvestoCH4miti-gationwithoutnegativelyaffectinganimalproductivity.Moreover,thefavourableeffectsobtainedinvitromaybeduetomicrobialadaptationinvivo.Consequently,additionallong-termanimalstudiesareneededtoinvestigatetheextenttowhichmicrobesadapttothesesubstances.Furtherworkisalsorequiredtoassessthetransferofessentialoilsintoanimalproductsandthepotentialimpactthismayhaveonthequalityofanimalproducts.5.25RUMENMANIPULATION:TANNINEXTRACTS5.25.1DescriptionAdietarysupplementoftannin-richextracts.5.25.2ModeofactionTanninsexerttheiranti-methanogeniceffectsbymodifyingtherumenmicrobialcommunityanditsfunction.AccordingtoAboagyeandBeauchemin(2019),seve-ralmechanismshavebeenproposedtoexplaintheanti-methanogenicactivityoftannins,includingdirectlyinhibitingmethanogensandtheprotozoalpopulationassociatedwithmethanogens;decreasinghydrogenproductionbyinhibitingfibro-lyticbacteriaandfibredigestibility;andactingasanalternativehydrogensinktomethanogenesis.5.25.3EfficacyTanninsderivedfromvegetablesourcescanbeclassifiedintocondensed(CT)andhydrolysable(HT)tannins.Whentanninsareextracted,bothtannintypescanbepresentatdifferentconcentrations,dependingontheplantpartfromwhichtheextractwasobtained,theplant’smaturitystageandgrowingconditions.The104Mitigationofmethaneemissionsanti-methanogeniceffectoftannin-containingfeedsisvariableduetofactorssuchastheplantsource,structuralcomplexity(CTandHThavehighandlowmolecularweights,respectively),dose,thetypesofbasaldietsandruminantspecies(Mueller-Harvey,2006;Jayanegara,etal.,2012;AboagyeandBeauchemin,2019).Feedingpurifiedtannin-richextractscomparedtonon-extractedtannins(i.e.tanninspresentinwholeplantsoragro-industrialby-products)couldlimithowothercompoundsconfoundwiththeanti-methanogenicactivityoftannins.Ameta-analysisofinvitroandinvivostudiesshowedthatCH4productiondecreasedwithincreasingdietarytanninlevels,withamoreconsistent,discernibleeffectobservedwhentannininclu-sionwasgreaterthan20g/kgdietaryDM(Jayanegara,LeiberandKreuzer,2012).Studiesconductedoncattle,sheepandgoatshaveshowneffectiveanti-methanogenicactivitywhensupplementingHT-richextractsfromAcaciamearnsii(Carullaetal.,2005;Staerfletal.,2012a;Alves,Dall-OrsolettaandRibeiro-Filho,2017;Denningeretal.,2020),CTfromSericealespedezawiththeadditionofquebrachoextract(Liuetal.,2019)oracombinationofHTandCTextractsfromchestnutandquebracho(Duvaletal.,2016;Aboagyeetal.,2018).Intheseinvivostudies,thedecreaseinCH4emissionrangedfrom6to45percentandtheCH4mitigationeffectswereobservedinbothforage-andconcentrate-baseddiets.However,severalstudieshavereportednoeffectsonCH4emissionswhensupplementingCTextractsfromquebrachoandMimosatenuiflora(Beaucheminetal.,2007;Ebertetal.,2017;Limaetal.,2019)orHTextractsfromchestnutandvalonea(Śliwińskietal.,2002;Wischeretal.,2014).Nonetheless,supplementingtannin-richextractsisapromisingCH4mitigationstra-tegyandthereisevidencetosuggestthatfeedingtanninscouldexhibitlong-termCH4mitigatingeffects(Staerfletal.,2012a;Duvaletal.,2016;Aboagyeetal.,2018).5.25.4PotentialtocombinewithothermitigationstrategiesCombiningtanninextractswithotherCH4inhibitorsappearsfeasible,butincon-sistentadditiveeffectsonCH4reductionhavebeenreportedinsomestudies.TheadditiveeffectsonCH4mitigationhavebeendemonstratedwhenatanninextractfromSwieteniamahoganiwascombinedwithaSapindussaponinextractinvitro(Jayanegaraetal.,2020),andwhensupplementingatanninextractfromAcaciamearnsiiwithcottonseedoilindairycows(Williamsetal.,2020).However,studiesconductedinsheepandgoatshavereportednoadditiveanti-methanogeniceffectwhenatanninextractfromAcaciamearnsiiwascombinedwithnitrate(Adejoroetal.,2020),whenaMimosatenuifloraextractwascombinedwithsoybeanoil(Limaetal.,2019),andwhentanninsfromSericealespedezaplusquebrachoextractwerecombinedwithmonensin,soybeanoilorcoconutoil(Liuetal.,2019).5.25.5EffectsonotheremissionsIftanninsupplementationdecreasesfibredigestibility,theexcretionoffermentableOMwouldbeexpectedtoincrease,whichmightincreaseCH4lossesfrommanure(Gerberetal.,2013b).However,Staerfletal.(2012a)showedthatfeedingacaciatan-ninextractsreducedfibredigestibilitywithoutaffectingCH4emissionfrommanure.TanninshavebeenshowntoinhibitmanureCH4emissionwheningestedoraddeddirectlytomanure(Whitehead,SpenceandCotta,2013;Phametal.,2017).Theanti-methanogeniceffectofingestedtanninsmaythuspersistinmanure.Inaddition,numerousstudies(especiallythoseinvolvinghigh-proteindiets)havedemonstratedthattanninsbindandinteractwithdietaryproteinsintheGIT,whichimprovesN105MethaneemissionsinlivestockandricesystemsutilizationanddecreasesurinaryNlosses(Mueller-Harvey,2006;Waghorn,2008;AboagyeandBeauchemin,2019).Consequently,manureammoniaandN2Oemis-sionsarelowered(Powell,AguerreandWattiaux,2011;Duvaletal.,2016).5.25.6Productivityandthequalityofmeat,milk,manure,crop,andairTannin-containingfeedscanbelesspalatablebecausetanninsbindtosalivarygly-coproteins,whichresultsinanastringenttaste(Mueller-Harvey,2006).Moreover,feedinghighconcentrationsoftannins(i.e.>50g/kgDM)canactivateantinutri-tionalpropertiesthatnegativelyimpactintake,fibreandproteindigestibility,andanimalperformance(AboagyeandBeauchemin,2019).Supplementingpurifiedtanninextractsratherthannon-extractedtanninscanlimittheinteractionbetweentannincharacteristicsandthenutritionalcompositionofthediet,therebyreducingtheconfoundingeffectonanimalperformance(AboagyeandBeauchemin,2019).Toavertthenegativeeffectsoftannins,feedingalowtomoderatethresholddosehasbeenrecommended(i.e.<30to50g/kgDMdiet),asthiscanimproveanimalperformance(weightgainandmilkyield),preventbloat,enhanceNutilization,controlintestinalparasitesandmitigateentericCH4emissions(Mueller-Harvey,2006;Waghorn,2008;PatraandSaxena,2011).Lastly,thedietarysupplementationoftanninscanimprovethefattyacidcomposition,oxidativestability,andthesen-soryqualitiesofmeatandmilk(Salamietal.,2019;Frutosetal.,2020).5.25.7SafetyandhealthaspectsComparedwithCTs,HTsaremoresusceptibletomicrobialhydrolysisinthegut,sincethemetabolitestheyproducemayhavepotentiallytoxiceffectsontheanimalpost-absorption(Reed,1995;McSweeneyetal.,2001).Feedinghighconcentra-tions(i.e.>50g/kgDMdiet)ofHTsmaycausesuchadverseeffectsaslivernecro-sis,kidneydamage,hemorrhagicgastroenteritisandevenmortality(Reed,1995).FeedingahighconcentrationofCTmayalsoaffecttheintestinalmucosa,therebydecreasingtheabsorptionofessentialnutrientssuchasaminoacids,whichcouldinturnincreasetheriskoftoxicitytoplantcompoundssuchascyanogenicglycosides(Reed,1995).Thenegativeeffectoftannins,particularlyHTs,canbepreventedthroughgradualadaptationandcontinuousfeedingorfeedinglowerconcentrations(i.e.<50g/kgDMdiet)(AboagyeandBeauchemin,2019).Tanninshavenotbeenshowntoposeasafetyrisktoanimalproductsdestinedforhumanconsumption.5.25.8AdoptionpotentialTanninsaresecondarymetabolitesnaturallypresentinplants.Theproductionoftanninextractsisscalableandsometanninextracts(e.g.extractsfromtara,mimosa,quebracho,gambier,pineandchestnutplants)arecurrentlyproducedonacom-mercialscalefordifferentapplicationsinthewood,dyeing,leatherandwineindus-tries(Fraga-Corraletal.,2020).Tanninextractscanbeeasilyincorporatedintothedietsofanimalsinintensiveandconfinedfeedingsystems.Tanninsaresafetoapplyandthisstrategydoesnotrequirespecializedtechnicalskillstobeimplemented;careshouldbetakennottoapplyexcessivedosesthatcouldcompromisedigesti-bilityandnutrientutilization.Becausetheyareplant-based,inmostjurisdictions,tanninextractsaresubjecttoalessonerousregulatoryapprovalprocesscomparedwithchemicalfeedadditives,despitetherebeingsomerisksofnegativesideeffects.106Mitigationofmethaneemissions5.25.9ResearchrequiredMoreresearchisrequiredtoelucidatehowthestructuralcomplexityofHTandCTextractsinfluencestheiranti-methanogenicactivity,andtoidentifytheoptimumconcentrationofspecificsourcesoftanninextractsforreducingCH4emissionwithouthavinganegativeimpactonanimalperformance.FuturestudiesshouldalsofocusondevelopinganeffectivecombinationoftanninextractswithotherCH4inhibitors,whichcouldexhibitadditiveandlong-termentericCH4-mitigatingeffects.TheeffectthatsupplementaltanninshaveonCH4emissionsfrommanureneedstobeclarifiedfordifferenttypesofbasaldiets,andthemechanismofsuchanti-methanogeniceffectsneedstobeunderstood.TheabilityoftanninstoreduceNlossesandN2OemissionsindicatestheneedforanLCAapproachwhenimple-mentingthisCH4mitigationstrategy.5.26RUMENMANIPULATION:SAPONINS5.26.1DescriptionThedietarysupplementationofsaponin-containingplantsorsaponin-richextracts.5.26.2ModeofactionTheanti-methanogeniceffectofsaponinsismainlyduetotheirabilitytoinhibittheprotozoapopulationintherumen(whichindirectlydecreasestheprotozoa-associatedmethanogens).Saponinsalterruminalfermentationbypromotingtheproductionofpropionateandreducingtheavailabilityofhydrogenformethano-genesis(Jayanegara,WinaandTakahashi,2014;PatraandSaxena,2009a).Moreover,theanti-methanogenicactivityofsaponinscouldbedirectlyrelatedtoadecreaseintheactivityandnumberofmethanogens(PatraandSaxena,2009a).5.26.3EfficacyTheCH4-mitigatingeffectofsaponinsishighlyvariabledependingonthesource,chemicalstructureanddosageofsaponins,dietcomposition,andtheadaptationofrumenmicrobestosaponins(GoelandMakkar,2012;PatraandSaxena,2009b).MostinvitroandinvivostudieshaveshownthatSapindussaponins,teasapo-nins,Quillajasaponins,Yuccasaponins,lucernesaponinsandSesbaniasaponinsdecreasedCH4production,althoughsomestudieshavereportednoeffects(PatraandSaxena,2009a;GoelandMakkar,2012;Jafari,etal.,2019).Ameta-analysisofinvitrostudiesfoundthatCH4productiondecreasedwithincreasinglevelsofsaponins,andthattheanti-methanogeniceffectivenessofsaponinsourceswasasfollows:Yucca>tea>Quillaja,inrecedingorder,althoughnostatisticaldifferencebetweenthemwasobserved(Jayanegara,WinaandTakahashi,2014).Thevariabi-lityoftheanti-methanogeniceffectofsaponinsmaybepartlylinkedtothetransientnatureoftheiranti-protozoalactivity(Wina,MuetzelandBecker,2005),duetotheinactivationofsaponinsthroughthedeglycosylationtosapogeninsbyrumenmicrobes(Newboldetal.,1997;Teferedegneetal.,1999).Thus,maintainingtheanti-protozoalactivityofsaponinsintherumenwouldbeastrategyforimprovingtheconsistencyoftheiranti-methanogeniceffects.Themaintenanceofanti-proto-zoalactivitycouldbeachievedbycombiningsaponinswithglycosidaseinhibitorstoavoiddeglycosylation(Ramos-Moralesetal.,2017b),ormodifyingthechemi-calstructureofsaponinstopreventenzymaticcleavageformicrobialdegradation(Ramos-Moralesetal.,2017a).107Methaneemissionsinlivestockandricesystems5.26.4PotentialtocombinewithothermitigationstrategiesSaponinsmaybecombinedwithotherCH4inhibitorsthathavecomplementarymechanismsofactiononmethanogenesis.However,somestudiessuggestthatthissynergisticanti-methanogeniceffectmaydependonthesaponinsource.InvitrostudieshaveshownthatsupplementingalowdoseofQuillajasaponinsinforage-andconcentrate-baseddietsexhibitedanadditiveCH4mitigatingeffectwhencom-binedwithgarlicoil,nitrateorboth,withoutadverseeffectsonfeeddigestionandrumenfermentation(PatraandYu,2013,2014,2015a,2015b).Moreover,additiveanti-methanogeniceffectswereobservedinvitroforQuillajasaponincombinedwithnitrateandsulfate(PatraandYu,2014)andforSapindussaponincombinedwiththetanninextractofSwieteniamahogani(Jayanegaraetal.,2020).However,noadditiveCH4-mitigatingeffectwasfoundwhenteasaponinwascombinedwithsoybeanoil(Maoetal.,2010)orfumarate(Yuan,etal.,2007)insheepdiets.5.26.5EffectsonotheremissionsSaponinscouldreducerumenNH3concentrationandimproveNuseefficiency,possiblyduetotheirNH3-adsorptionpropertyandanti-protozoalactivitywhichreducesproteolysisanddeaminationofdietaryproteinsintherumen(Wina,MuetzelandBecker,2005;PatraandSaxena,2009a).Consequently,feedingsapo-nins–particularlyYuccasaponins–hasthepotentialtoreduceNH3emissionsfrommanure,althoughthiseffecthasbeeninconsistentinsomestudies(LiandPowers,2012;Sunetal.,2017;Adegbeyeetal.,2019).Moreover,thepositiveeffectofsapo-ninsinimprovingNuseefficiencycouldreduceNlossesandN2Oemissionsfrommanure(Yurtsevenetal.,2018).5.26.6Productivityandthequalityofmeat,milk,manure,crop,andairTheinclusionofsaponinsatanappropriatelevelinthedietmightnothavenegativeeffectsonanimalperformance.Ameta-analysisofinvitrostudiesfoundthatthedietaryinclusionofhighersaponinlevelsdidnothaveadverseeffectsonfeeddiges-tionandrumenfermentation(Jayanegara,WinaandTakahashi,2014).Althoughthebenefitsofsaponinsonanimalproductivityareuneven,theiranti-protozoaleffectcouldincreasetheefficiencyofmicrobialproteinsynthesisandenhancethesupplyofmetabolizableprotein,thusimprovinganimalperformanceespeciallyforroughage-baseddiets(Wina,MuetzelandBecker,2005;PatraandSaxena,2009a).Moreover,thereareindicationsthatdietarysaponinscouldhaveantioxidantandanti-inflammatoryactivitiesthatcouldreduceoxidativestress,improveimmunityandanimalhealth(Zhou,etal.,2012;Wangetal.,2017),andhenceindirectlycon-tributetoloweremissions.Additionally,supplementingdietarysaponinscouldpotentiallyimprovethefattyacidprofileandoxidativestabilityofruminantmeat,althoughonlylimitedimprovementshavebeenobservedinthequalityofmilk(VastaandLuciano,2011;Szczechowiaketal.,2016;Toraletal.,2018).5.26.7SafetyandhealthaspectsSaponinshavenotbeenshowntoposearisktohumansconsuminganimalpro-ductsthathavebeenfedsaponins.However,saponins(mostlysteroidalsaponins)fromsomeplantscanbetoxictoanimals,causingphotosensitizationfollowedbyliverandkidneydegenerationaswellasintestinalproblemssuchasgastro-enteritisanddiarrhoea(Wina,MuetzelandBecker,2005).Anoverviewoftoxic108Mitigationofmethaneemissionssaponin-containingplantshasbeenprovidedbyWina,MuetzelandBecker(2005).Nonetheless,saponinsmightbesubjectedtolessstringentregulatoryapprovalthanchemicalinhibitorsbecausetheyareplantderived.5.26.8AdoptionpotentialSupplementingruminantdietswithsaponin-containingplantsorextractscouldreadilybeadoptedasastrategy.Theproductionofsaponinextractsisscalableandsomesaponinextracts(e.g.YuccaandQuillajabarksaponins)havebeencommer-ciallyproducedforapplicationinthepharmaceutical,foodandcosmeticindustries(Güçlü-ÜstündağandMazza,2007).Atleastonepatentexistswhichinvolvestheuseofsaponinsinruminantfeeding(Aounetal.,2003).Saponinsaresafetoapplyanddonotrequirespecializedtechnicalskillsfortheformulationofdiets.5.26.9ResearchrequiredYucca,teaandQuillajasaponinshaveshownpotentialforreducingCH4emissionsbutfurtherstudiesarerequiredtoestablishtheoptimumdosageandtheirinterac-tionwithbasaldietswithaviewtoimprovingourunderstandingoftheiranti-methanogeniceffectsinthelongterm.ThecombinationofQuillajasaponinwithothermethanogenesisinhibitors(particularlynitrate)promisestoachieveagreateranti-methanogeniceffect,butinvivostudiesarerequiredtoconfirmtheirsyner-gisticCH4-mitigatingeffectsinruminants.ThepotentialofsaponinstoreduceNlossesfromanimals,manureNH3andN2Oemissionsfrommanurerequiresfur-therinvestigation.Potentialinteractionofsaponinswithotheremissions(NH3andN2O),apartfromentericCH4,suggeststhatthisCH4mitigatingstrategyshouldbeexaminedusingLCA.5.27RUMENMANIPULATION:BIOCHAR5.27.1DescriptionThedietarysupplementationwithbiochar.Biocharisformedasaresultofthepyrolysis(350–600°Cwithlimitedoxygen)ofvariousbiomasssources.5.27.2ModeofactionIthasbeenproposedthatbiocharenhancesbiofilmformation(Leng,2014)andhydrogentransferamongmemberswithinthemicrobialcommunities(Chenetal.,2014).ThetransferofdihydrogentoacceptorsotherthanCO2couldresultinareductionofentericCH4emissions.5.27.3EfficacyTheadditionofbiocharat2percentofdietaryDMsuggestedthatitcouldlowerCH4emissionsfromanartificialrumensystem(Saleemetal.,2018),butsubse-quentstudiesusingothersourcesofbiomassfailedtodetectanyimpactofbiocharonCH4emissionsincontinuousculturesystems(Tamayaoetal.,2021a,2021b).SubsequentstudiesalsofailedtodetectanyimpactofbiocharonCH4emissionsinfinishingbeefcattle(Terryetal.,2019;Sperberetal.,2021).Biomasssourcesaswellaspyrolysisconditionsandthesecondarytreatmentofbiocharwithacidicoralkalisolutionsmayaffecttheefficacyofbiochar.Sincebiocharappearstobelargelyindigestiblebymixedrumencultures(Tamayaoetal.,2021),reductionsinCH4emissionscouldberelatedtoadepressionindigestibility,providedthatbiocharconstitutesasignificantproportionofthediet.109Methaneemissionsinlivestockandricesystems5.27.4PotentialtocombinewithothermitigationstrategiesSynergisticresponsestocombiningbiocharwithbiofat,anindustrialby-productofcashewnutshells,havebeenshowntoreduceCH4emissionsinvitro(Saenabetal.,2020),butasynergisticinteractionwithothermitigationstrategieshasnotbeenreportedinvivo.5.27.5EffectsonotheremissionsDependingonpyrolysisconditionsandemissionscapture,theformationofbiocharcanreleasevariableamountsofCO2,CH4andN2O(Sparreviketal.,2015).Addingbiochartoruminantdietsmayincreasethelevelofrecalcitrantcarboninmanureaswellasincreasingstablecarbonlevels(Romeroetal.,2021)andreducingN2Oemis-sionsfromsoils(Shakooretal.,2021).Incontrast,thedirectadditionofbiochartostoredliquidmanurewasfoundtoincreaseGHGemissions(Liuetal.,2021).5.27.6Productivityandthequalityofmeat,milk,manure,crop,andairBiocharhasbeenshowntoimprovefeedefficiencyinlambs(Mirheidarietal.,2020)andcarcassqualityinbeefcattle(Terryetal.,2020),buttheseresponsesdonotappeartobeaccompaniedbyreductionsinentericCH4emissions.5.27.7SafetyandhealthaspectsBiocharhasbeenusedasafeedcolouringagentandasachelatoroftoxinswithinthedigestivetractoflivestock.Biomasssourcesshouldbeassessedforthepresenceofheavymetals,polychlorinatedbiphenyls,dioxinsorotherpotentialtoxicants,beforebeingusedasfeedstockfortheproductionofbiocharthatcouldbefedtolivestock.5.27.8AdoptionpotentialBiocharisavailableonthemarketandproducedonanindustrialscaleasasoilamendmentforuseonfarmsandinurbangardens.ItdoesnotasyetappeartohaveentericCH4mitigationpropertiesbut,shoulditbeshownatsomepointtoreduceGHGemissionswithintheoveralllivestockproductioncycle,biocharisavailableonthemarket.Theeaseofhandlingwouldimproveifitwereadministeredinapel-letedform,andcaremustbetakenowingtotheexplosivepotentialofbiochardustwithinconfinedspaces.5.27.9ResearchrequiredBiocharappearstohavelimitedpotentialtolowerentericCH4emissions.AlternativebiomasssourcesforpyrolysisandsecondarychemicaltreatmentsofbiocharcouldstillbeexploredtodeterminetheirpotentialtoreduceruminalCH4emissions.Additionalworkshouldfocusontherolethatbiocharcanplayinalter-ingthechemicalcompositionofmanure;forexample,byincreasingthelevelofstablecarbon,itcancontributetofosterOMaccumulationandtheretentionofmanurenutrientswithintheplantrootprofile.TheuseofbiochartolowerGHGemissionsfromlivestockshouldbeexploredfromanLCAperspective,takingintoconsiderationallemissionsandsinksthroughouttheproductionchain.Basic,long-termresearchcouldseekabetterunderstandingofhowchannellingdihydrogentodifferenthydrogenotrophicmicrobialgroupsiscontrolled.110Mitigationofmethaneemissions5.28RUMENMANIPULATION:DIRECT-FEDMICROBIALS5.28.1DescriptionDirect-fedmicrobials,orlivemicrobialadditives,areviablemicroorganisms(e.g.fungi,yeasts,bacteria)thatcanmodifyrumenfermentationwheningestedbyaruminant.Forpresentpurposes,wewillfocusexclusivelyonthedecreaseofCH4production,despitetherebeingotherobjectives,suchasstabilizingrumenpHandimprovinglactateutili-zationorfibredigestion.5.28.2ModeofactionTherecanbevariousmodesofaction.Generally,thepurposeofalivemicrobialadditiveistoredirectmetabolichydrogenawayfromCH4productionandtowardsanalternativeproductoffermentationnutritionallyusefultothehostruminantanimal.Thismaybeachievedthroughtheincorporationofdihydrogenintopath-waysotherthanmethanogenesis,thestimulationofpathwaysthatdonotpro-ducedihydrogen,orthroughanaerobicCH4oxidation(Jeyanathan,MartinandMorgavi,2013).Foralivemicrobialadditivetobesuccessful,theaddedmicro-organismmustfollowathermodynamicallyfeasiblepathwayandtheaffinityforthereactionsubstratesmustbehigh.Supplyingadditionalenzymeactivityintheformofalivemicroorganismtoathermodynamicallynon-spontaneousprocessisineffective.Forexample,ahydrogenotrophshouldhavealowdihydrogenthresh-oldandahighaffinityfordihydrogentocompetewithhydrogenotrophicmetha-nogens(Ungerfeld,2020).Anotherpossibilitywouldbetouselivemicrobialaddi-tivesthatproducebacteriocinscapableofdirectlyinhibitingmethanogens(Gilbertetal.,2009;Jeyanathan,MartinandMorgavi,2013).5.28.3EfficacyTheeffectsofyeasts,Aspergillusoryzae,andoflacticacidbacteriaonrumenfermen-tationandCH4productionhavebeeninconsistent,andthereforetheyhavenotbeenselectedtodecreaseCH4production(Jeyanathan,MartinandMorgavi,2013;Weimer,2015).Astrategythathasbeeninvestigatedisthestimulationofpropionateproduc-tionasapathwayincorporatingmetabolichydrogen(Jeyanathan,MartinandMorgavi,2013;Elghandouretal.,2015).SomestrainsofpropionibacteriahavecausedmilddecreasesinCH4productionininvitrobatchcultures(Alazzehetal.,2012).Mamuadetal.(2014)andKimetal.(2016)observedstrongerdecreasesinCH4productionininvitrobatchcultureswiththeadditionoffumaratereducers.InvivoexperimentswithpropionibacteriafoundanumericaldecreaseinCH4productionwithahigh-forage(Vyasetal.,2014a)butnotwithamixed(Vyasetal.,2015)orahigh-concentratediet(Vyasetal.,2014b).ApatentclaimsthatacombinationofaPropionibacteriumandLactobacillusrhamnosusstrain32causeda25percentdecreaseinCH4productionoflactatingHolsteincowsfedamixeddiet,withnoeffectobservedinthosefedadiethigherinstarch(Bergeretal.,2014).Addingnitrateandsulfate-reducingbacte-riahasalsobeenfoundtodecreaseCH4productioninvitro(Jeyanathan,MartinandMorgavi,2013).Theuseofreductiveacetogens,whichhavetheabilitytoreduceCO2withdihydrogentoproduceacetate,ininvitrorumenfermentation,hadminimalornoeffectsonCH4productionunlessaccompaniedbyachemicalinhibitorofmethano-genesis(Nollet,DemeyerandVerstraete,1997;LeVanetal.,1998;Lopezetal.,1999).ThemagnitudeofCH4oxidationintherumenwasestimatedtobeminimal(Jeyanathan,MartinandMorgavi,2013).111Methaneemissionsinlivestockandricesystems5.28.4PotentialtocombinewithothermitigationstrategiesThephysicochemicalmodeofactionoflivemicrobialadditiveschartspossiblecom-binationswithotherCH4mitigationstrategies.Livemicrobialadditivescanenhancetheflowofmetabolichydrogenthroughdesirable,thermodynamicallyfeasiblemetabolicpathways,whoseratesareconstrainedbyenzymekinetics(Ungerfeld,2020).Forexample,inhibitingmethanogenesiswithchemicalcompoundsinbatchculturesallowedreductiveacetogenesisconductedbyaddedreductiveacetogenstobefunctional(Nollet,DemeyerandVerstraete,1997;LeVanetal.,1998;Lopezetal.,1999).Invariousstudiesofinvitrocultures,nitrite-andnitrate-reducingbacteriahavebeensuccessfullytestedwithaddednitratetodecreaseCH4produc-tion,enhancetherateofnitritereductiontoammonium,andpreventtheaccu-mulationofnitrite(Jeyanathan,MartinandMorgavi,2014),asthereductionsofnitrateandsulfatearethermodynamicallymorefavourablethanmethanogenesisintherumen(UngerfeldandKohn,2006).InanexperimentconductedwithsheepfednitrateasaCH4-mitigationstrategy,supplementationwiththenitrite-reducingbacteriumPropionibacteriumacidipropionicidecreasedplasmanitriteconcentra-tiononlynumerically(deRaphélis-Soissan,2014).Usinglivesuccinateorpropio-nateproducerscouldalsoimprovetheconversionoffumarateormalateaddedtopropionate.5.28.5EffectsonotheremissionsGrowing,storingandtransportinglivemicrobialadditiveswouldgeneratesomeCO2emissionsfromfossilfuels.Theimpactontheanimal’sefficiencyofusingNwouldhavetobeevaluated.Overall,theadditionalemissionsofCO2eqwouldpresumablybelow.5.28.6Productivityandthequalityofmeat,milk,manure,crop,andairInvivoresultsofCH4mitigationexperimentswithdirect-fed(live)microbialaddi-tivesarescarce.Vyasetal.(2014a,2014b,2015)didnotreportanyeffectsofaddingpropionibacteriatodifferentdietsonDMIorweightgain.Bergeretal.(2014)didnotfindthataddingaPropionibacteriumaloneorincombinationwithoneoftwolactobacilliaffectedDMIortheproductionofmilkandmilkcomponentsofdairycowsinanyway.5.28.7SafetyandhealthaspectsApprovalbyregulatoryagenciesisusuallyrequired.Inordertosecureit,themicro-organisminquestionmustbecharacterizedanddescribedingreatdetail,andthepotentialforpathogenicitymustbediscarded.Livemicrobialadditiveshavebeenstudiedtopreventdisorderssuchasacidosisandtodecreasetheloadofpathogensincattle(Jeyanathan,MartinandMorgavi,2013;Elghandouretal.,2015).Thecom-mercialavailabilityanduseofprobioticstomeetthenutritionalandhealthrequire-mentsofhumansanddomesticanimalsiswidespread.5.28.8AdoptionpotentialThepotentialisgood,solongasconsistentinvivoresultscanbeobtained.Livemicrobialadditivesinparticularstandaverygoodchanceofbeingadoptedasacompanionspeciesofchemicalinhibitorsofmethanogenesis;theymaybeableto112Mitigationofmethaneemissionsimproveproductivityincertainanimalcategoriesanddietsbydirectingmetabolichydrogenaccumulatedasdihydrogentowardsdesirableproducts.Possiblechangesintheabsorptionofmetaboliteswouldimproveanimalperformance,whichwouldoffsettheadditionalfeedingcostsincurred.Preparationsoflivemicrobialaddi-tivesshouldremainviableforprolongedperiodsoftime,andbeeasytouse,storeandtransport.Withafewexceptions,livemicrobialadditivesdonotpersistintherumenandneedtobedosedfrequentlytohaveaneffectondigestionandfermenta-tion(Weimer,2015).Livemicrobialadditivesmaythusnotalwaysbesuitableforextensivebeefproductionsystems,inwhichtheanimalshaveonlysporadiccontactwiththeirhumankeepers.5.28.9ResearchrequiredThereisadearthofinvitroandinvivoresearchaddressingtheoptimizationofrumenfermentationwithlivemicrobialadditives,especiallywhencombiningthemwithotherCH4mitigationstrategiessuchaschemicalinhibitorsofmethanogenesisandalternativeelectronacceptors.Areflectiveapproachthatconsidersthepossiblephysicochemicallimitationsoffermentationpathwaysisrecommended.Iflivemicrobialadditivescanbeshowntobeconsistentlyeffective,therewillbeaneedforappliedresearchtodeterminetheoptimalfrequency,doseandmodeofadministration.Anunderstandingofphysio-logicalandmetabolicchangesthatcanoccurintheanimalwillberequiredtooptimizetheproductionandabsorptionofmetabolitessoastoimproveanimalproductivity.5.29RUMENMANIPULATION:EARLYLIFEINTERVENTIONS5.29.1DescriptionTheuseofinterventionsduringtheestablishmentoftherumenmicrobiomeinpre-ruminantanimalsaimedatdecreasingentericCH4emissionslaterinthelifetimeoftheanimal.5.29.2ModeofactionTheadultmicrobiotaisresilient,inthatitrecoversfromperturbationsafterthesecease(Weimer,2015).Incontrast,thenewbornundergoesvariousstagesofmicro-bialcolonization,andinterventionsattheearlylifestagesmaymodifyandpro-grampost-weaningandadultmicrobiotainafavourabledirection.Earlylifeeventscaninfluencethemicrobialcompositionpost-weaningthroughrumendevel-opment,microbialestablishmentandhostimmunity(Abeciaetal.,2014,2018;Yáñez-Ruiz,AbeciaandNewbold,2015;Furmanetal.,2020).Fontyetal.(2007)illustratedtheconceptofearlylifeelectronredirectionbymeansofgnotobioticlambsinoculatedwithreductiveacetogensafterbirth,aprocessinwhichreduc-tiveacetogenesiscontinuedtobethemainhydrogenotrophicpathwayforupto12monthsofage.5.29.3EfficacyAbeciaetal.(2013)supplementeddoeswiththemethanogenesisinhibitorBCMfortwomonths,aftertheygavebirthtotwingoatkids.OnekidperdoereceivedBCMforthreemonthsafterbirth.ThreemonthsaftertheadministrationofBCMwasdiscontinued,kidsthathadpreviouslyreceivedBCMstillproduced20percentlessCH4perkilogramofDMIthanthosewhohadnot,althoughthedecreasein113MethaneemissionsinlivestockandricesystemsCH4productionwaslesserthanwhentheBCMtreatmentwasstopped.Thegreat-estefficacyoccurredwhenbothkidsandtheirmothersweresupplementedwithBCM.Mealeetal.(2021)administered3-NOPtocalvesuntil14weeksofage.Atweaningat11weeks,CH4productionwas10.4percentlowerinheifersreceiv-ing3-NOPand,atoneyearofage,a17.5percentCH4decreasewasstillobservedinthosecalveswhichhadreceived3-NOPearlyinlife.Conversely,Debruyneetal.(2018)didnotfindlong-lastingeffectsonCH4pro-ductionofcoconutoilsupplementationtogoatkidsupto11weeksofageinincu-bationswithrumeninoculumfromcontrolandtreatedanimals,conductedwithrumeninoculumtakenfromthelambswhentheywere28weeksold.Saroetal.(2018)didnotfindanyeffectsofadministeringalinseedandgarlicoilmixturetolambsduringtheirfirst10weeksofageontheirCH4productionat20weeksofage,althoughthoselambsthatreceivedasecondtreatmentwiththelinseedandgarlicoilmixturedecreasedtheirCH4production.5.29.4PotentialtocombinewithothermitigationstrategiesTheremaybenegativeinteractionsbetweenthesameanti-methanogenictreat-mentsadministeredearlyandthenagain,laterinlife:rumeninoculumfrom6-and12-month-oldcalveswhichhadbeensupplementedwithextrudedlinseedfrombirthuntilfourmonthsofagerespondedlesstotheinvitroadditionoflinseedoilasaCH4mitigationadditive,comparedwiththecontrolrumeninoculumfromcalvesthathadnotbeensupplementedextrudedlinseed(Ruiz-Gonzálezetal.,2017).Ontheotherhand,Saroetal.(2018)didnotfinddifferencesintermsofCH4productionbetweensupplementingalinseedandgarlicoilmixtureattwostagesinearlylife.5.29.5EffectsonotheremissionsOtherCO2eqemissionswilllikelybeaffectedinthateachparticularinterventionmayinfluenceotherCO2eqemissionslaterinlife.However,thedegreetowhichotheremissionsmightbeaffectedisexpectedtobeconsiderablysmaller,giventhatthetreatmentwouldbeshort-livedandconductedinyounganimalswithasmallbodysize.5.29.6Productivityandthequalityofmeat,milk,manure,crop,andairTheeffectsonproductivityareprobablylargelydependentontheinterventionused.Abeciaetal.(2013)reportedgreaterweightgainsandatendencyfordecreasedconcentrateintakeingoatkidssupplementedwithBCM;theperformanceoftheanimalslaterinlifewasnotreported.Supplementationofgoatkidswithcoconutoilwasshowntodecreasebodyweightat28weeksofage(Debruyneetal.,2018).Saroetal.(2018)didnotobserveanyeffectsofsupplementinglambsduring10weekswithalinseed–garlicoilcombinationonweightgainat10and20weeksofage.Mealeetal.(2021)didnotreportanyeffectsofsupplementing3-NOPtocalvesduringtheirfirst14weeksoflifeonweightgainbetweenbirthand23weeks,orweeks57to60,althoughtherewerenumericaldifferencesinfavourofcontrolanimals,thesameaswithpreweaningconcentrateintake.Giventhatearlylifetreat-mentsareappliedforarelativelyshortperiodoftime,itispossiblethatnegativeeffectsonanimalperformance,shouldtheyoccur,mightbeoffsetbycompensatorygrowth.114Mitigationofmethaneemissions5.29.7SafetyandhealthaspectsPotentialconsequencesofearlylifeinterventionsaffectingsafetyandhealthwilldependonthestrategyused.However,therewillbewash-outperiodsofseveralmonthsduringtheanimals’growingphase,beforetheyproducemilkormeat.Furthermore,dosesofanyadditiveswouldbemuchdiminishedincomparisonwithanadultanimalofamuchgreaterbodysize,whichwouldalsodiminishpotentiallynegativeenvironmentaleffects.Therefore,itislikelythatadditivesthatcouldposeunacceptablelevelsofriskfortheenvironmentorforconsumerswhenfedtoadultanimalswouldbeacceptablewhenadministeredtonewbornanimals,providedthattheydonotharmtheyounganimal.Nonetheless,thesafetyofeachearlylifeinter-ventionwillhavetobeapprovedbyregulatoryagencies.Thelong-termefficacyofearlylifeinterventionsinadultanimalsmayalsoneedtobedemonstratedfortheirusageasamitigatingmeasuretobeapprovedbyregulatoryagencies.5.29.8AdoptionpotentialTheconceptofearlylifeinterventionsisveryattractive,giventhatthecostofapply-inglong-lastingmanipulationsforashortperiodoftimetoanimalswithasmallbodysizewouldbegreatlydiminishedcomparedtoadultanimals,inwhichmostinterventionswouldhavetobeappliedcontinuously.Inaddition,itmaybesaferforconsumersandtheenvironmenttousesmallerdosesforshorterperiodsoftimefol-lowedbylongwash-outperiods.Furthermore,thisstrategymaybeadvantageousforgrazingruminantswheresupplementationoffeedadditivesisnotpossible.Researchonearlylifeinterventionsisatanearlystage.Therearefewandcontradic-toryresultsastotheefficacyofearlylifeinterventionsindecreasingCH4produc-tionlaterinlifeandthepersistencyoftheeffectsobserved,althoughsomerecentresultsareencouraging(Mealeetal.,2021).Theefficacyofearlylifeinterventionslikelydependsontheadditiveordietarymodificationused,thedose,modeanddurationoftheadministration,andtheanimalspecies,amongotherfactors.5.29.9ResearchrequiredThepersistenceinCH4decreaseafteroneyearinanimalstreateduntil11weeksofage(Mealeetal.,2021)isofgreatinterest,buttheseresultsneedtobeconfirmedinfurtherexperiments.Thereisaneedformoreresearchtoestablishthemosteffec-tiveinterventionsandtheiroptimaldoses,modesandfrequenciesofadministration,theminimaldurationandtheendpointofeachintervention,aswellastheexpectedperiodofpersistenceoftheeffectsonCH4production.Itwillbeveryimportanttostudytheeffectsofeachearlylifeinterventiononfutureanimalperformanceandhealth,andtoidentifyandunderstandthemechanismsinvolved,suchastheper-manentchangeintheestablishmentofthecompositionoftherumenmicrobiota,anatomicalandfunctionalchangesinGITdevelopment,aswellaspossiblechangesintheimmunesystem(Yáñez-Ruiz,AbeciaandNewbold,2015).5.30RUMENMANIPULATION:PHAGEANDLYTICENZYMESACTIVEAGAINSTMETHANOGENS5.30.1DescriptionPhageandthelyticenzymestheyproducearebeinginvestigatedfortheiractivityagainstrumenmethanogensasanentericCH4mitigationstrategy.115Methaneemissionsinlivestockandricesystems5.30.2ModeofactionArchaealphageproduceslyticenzymesthatbreakdownpseudomurein,theprin-cipalcellwallcomponentofrumenmethanogens.ThisdisruptiveactivitycouldreducetheproductionofCH4intherumen.5.30.3EfficacyAnovelarchaeallyticenzyme(PeiR)displayedonbionanoparticleswasshowntoreduceCH4productioninspecificpuremethanogenculturesbyupto97percentoveraperiodof5days(Altermannetal.,2018).TheefficacyofthelyticenzymedecreasedagainstmethanogensthatweremorephylogeneticallydistantfromMethanobrevibacterruminantiumM1,theoriginalhostoftheprovirus.NoinvivoormixedculturestudieshavebeenundertakentoinvestigatetheabilityofphageortheirlyticenzymestoreduceruminalCH4emissions.5.30.4PotentialtocombinewithothermitigationstrategiesItappearsfeasible,butexperimentshavenotbeenconductedtoinvestigatesyner-gismswithothermitigationstrategies.SynergismsmaybemostlikelywithothermitigationstrategiesthatspecificallytargetthosemethanogensmoredistantlyrelatedtoMethanobrevibacterruminantiumM1,whichlacksensitivitytoadminis-teredphageorlyticenzymes.5.30.5EffectsonotheremissionsPhageorenzymeproductionwouldnecessitatetheestablishmentofmanufacturingfacilitiesthatwouldlikelyrequiretheuseoffossilfuels.Producingphageorlyticenzymesonacommercialscalecouldprovechallenging.ItisassumedthatphagewouldnotalterN2Oemissionsandtheefficiencyofmilkormeatproduction,how-ever,thereisnoevidencetosuggesteitherwaybecausethetechnologyhasnotbeenassessedoutsideofthelaboratory.5.30.6Productivityandthequalityofmeat,milk,manure,crop,andairNostudieshavebeenundertakentoinvestigatetheimpactofphageorlyticenzymesonproductivity.5.30.7SafetyandhealthaspectsThisstrategyispresumedtobelowrisksincetherearealreadysometherapeu-ticapplicationsofphageinmedicineandfoodsafety,andnoneofthe65knownarchaealviruseshavebeenlinkedtoanimalpathogenesis(WirthandYoung,2020).5.30.8AdoptionpotentialThisentericCH4mitigationstrategywouldrequireadministrationofphageorlyticenzymesonacontinuousbasis,makingthetechnologymoresuitableforusewithtotalmixeddietsandlesssuitableforextensivegrazingsystems.Thetechnologywouldbemoredesirableiflytic–asopposedtotemperatearchaeal–phagescouldbeisolated,possiblyenablingtheself-propagatingbiocontrolofruminalmethanogens.However,todatelyticphagesactiveagainstrumenmethanogenshavenotbeeniden-tified,unlikecurrentcandidatelyticenzymesthathavebeenidentifiedasaresultofthesequencingofprophagewithinthemethanogengenome(Leahyetal.,2010).116Mitigationofmethaneemissions5.30.9ResearchrequiredThisCH4mitigationstrategyisnotyetattheproof-of-conceptstage,asthetech-nologyhasnotbeeninvestigatedbeyonditsimpactonpureculturesofrumenmethanogens.Althoughitiswellknownthattherumenharboursarichanddiversevirome(Gilbertetal.,2020),thereisbutasinglepreliminaryreportoftheisolationofanintactphagewithpotentialactivityagainstmethanogens(BaresiandBertani,1984).Onlythreepseudomureinendoisopeptidaseshavebeencharacterizedfortheirpotentialactivityagainstmethanogens(Schofieldetal.,2015;Altermannetal.,2018).Intactlyticphagesareknowntoplayamajorroleintheecologyofmeth-anogenswithinotheranaerobichabitats(Danovaroetal.,2016)anditisalmostcertainthattheirroleintheecologyofrumenmethanogensisequallyvital.Moreworkisrequiredtodefinethediversityofarchaealviruses(Coutinho,EdwardsandRodriguez-Valera,2019),astheyarelikelyunderrepresentedingenomicapproachesthatcharacterizetherumenvirome.Strivingtoidentifylyticphagewithactivityagainstmethanogenscouldbethenextstepinadvancingthismitigationstrategy,althoughitislikelythatacocktailofphageswillberequiredtocoverthewholerangeofmethanogensthatresideintherumen.5.31SUMMARYTABLESInthefollowingsummarytables(Table2toTable4),wedelineatethepossibilitiesandbarriersfortheapplicationofvariousmitigationstrategiesaimingtoreduceentericCH4emissionsfromruminantsinthreemainproductionsystems:i)Confinementsystemsthatincludefeedlotsanddairiesinwhichanimalsarepennedorhousedindrylotsorbuildings.Inthesenon-grazingsys-tems,allthefeedingestedbytheanimalsisprovidedbyhumanoperators.Therecanbemanyfeedingredients,includingcerealgrains,oilseedsandmeals,conservedforages,by-products,andpremixescontainingminerals,vitaminsandadditives.Thefeedingfrequencyandmanagement(i.e.totalmixedrationorfeedcomponentsofferedseparately)isdecidedonbythefarmoperator.ii)Grazingwithnosupplementation.Inthesesystems,animalsingestexclu-sivelyplantsbygrazingpastures.Extensivebeefandsheepranchingsys-temsareanexample,althoughotherdairy,beefandsmallruminantproduc-tionsystemsbasedongrazingpastureswithoutsupplementationarealsoconsideredwithinthiscategory.iii)Mixedgrazingsystems,inwhichgrazinganimalsaresupplementedconcen-tratesand/orconservedforages.Typically,theproportionoftotalDMIbytheanimalthroughgrazingpasturesversustheproportionofsupplementedfeedvariesthroughouttheyearwiththepasturegrowthcurve.Inmixeddairysystems,lactatingcowsaretypicallysupplementedtwicedailyduringmilking.Inothermixedgrazingsystems,supplementationmaytakeplaceoncedaily,althoughthiscanvary.Itisacknowledgedthatwithineachsystemthereisamplevariationdependingonanimalspeciesandcategory,climate(tropical,subtropical,temperate),eco-zone,andsoforth.Asshowninthetables,theapplicationofeachentericCH4mitigationstrategyforeachofthethreeproductionsystemsisbasedonthefollowingqualita-tiveassessment:117Methaneemissionsinlivestockandricesystems1.Availableknowledgegeneratedbyappliedresearch,indicatingthenumberofexistingpeer-reviewedinvivostudiesinwhichtheeffectsofthemitiga-tionstrategyonentericCH4productionhavebeenreported(Column1).2.ThemagnitudeofthechangeinCH4production,bothonanabsolute(perani-malandperday)andintensity(perunitofanimalproduct)basis(Column2).3.AveragemeasuredorlikelyeffectsoftheapplicationofanentericCH4-mitigationstrategyontheemissionsofotherGHGsatotherpointsintheproductionchain.UpstreamchangesmayincludethedirectandindirectreleaseofCO2andN2Ointhegrowthandmanufactureoffeeds,feedadditivesorotherproducts.DownstreamchangesmayalsooccurintheemissionsofCH4andN2Ofrommanure.Changesincropproductionandgrazingmanagementcanaffectcarbonsequestrationinsoils.Insomecases,changesinotherGHGshavebeenfoundtobeminimal,whileinothersalifecycleassessmentisrecommendedforadefinedproductionunitsuchasafarm,regionorcountry(Column3).4.EffectsoftheapplicationoftheentericCH4-mitigationstrategyonanimalproductivity.Onlythosestudiesinwhichtheeffectsofthemitigationstrat-egyonentericCH4andanimalproductivityweresimultaneouslymeasuredandreportedareconsidered(Column4).5.PresentstageoftechnicaldevelopmentofanentericCH4-mitigationstrategy.Amitigationstrategymaybeconsideredtobefullydevelopedandavailableforadoptionatthefarmlevel,althoughfurtherresearchtooptimizeitsapplicationmaystillbeneeded.Governmentapprovalandmanufacturescalingordistributionmaystillbepending,butthoseaspectsareconsideredinthelastcolumn(asGovernmentandAccessibil-ity,respectively).Moreover,amitigationstrategymaybeinitslaststagesoftechnicaldevelopmentandclosetoitspracticalapplication.Finally,somemitigationstrategiesareatanearlystageofresearchandtheirapplicationmaypotentiallytakeplaceinthelongtermwithaconsider-abledegreeofuncertaintydependingontheoutcomesoffuturebasicandappliedresearch(Column5).6.Existingconcernswithregardstopotentialtoxicitytoanimals,humanoperators,residuesinanimalproductsandtheenvironment(Column6).7.Variousaspectsrepresentingpotentialbarrierstoadoptionofamitigationstrategywithinaparticularproductionsystem(Column7).Thosecanbehighlyvariableacrosscountries,regionsandfarms.118MitigationofmethaneemissionsTable2.Summaryofentericmethanemitigationstrategiesforconfinedruminant(dairy,beeforother)systemsInvivostudiesExpectedCH4decreaserangeEffectsonotherAnimalTechnicalRiskMainbarrierstoadoptionconductedonH=≥25%;GHGemissionsproductivityavailabilitymanagementon-farmCH4mitigatingM=15-24%;U=upstream;(meat&milkR=availableD=maxdose;1F=resistancetochange;2,3F=few(<5);L=≤15%;M=manure;production,now;safetyforC=increasedcost/lackofS=some(5-10);I=increasemaybeobserved;Mi=minimal;feedefficiency)C=closetoA=animals;financialincentives;M=many(>10)U=unknown(notexamined);Ma=majorI=increase;beingavailable;H=humans;M=animalsaremanagedV=variablechangesD=decrease;U=long-termF=food;sparingly;expected,Nc=nochange;oruncertainE=environment;A=accessibility;g/kgmeatneedsLCA;U=unknown;availabilityN=none;T=technicalsupport;2Un=unknown;V=variableU=unknownG=governmentapproval;Mitigationstrategyg/dormilkV=variableCa=consumeracceptance;S=safetyAnimalbreedingandmanagementIncreasedanimalMIL4MaIRNC,TC,A,TproductionSelectionforlowSLLMiNcUNmethane-producinganimalsIncreasedfeedMVLMaIRNC,TefficiencyC,TF,C,TImprovedanimalFILMiVRNhealthImprovedanimalFILMiIRNreproductionFeedmanagement,dietformulationandprecisionfeedingIncreasedfeedingMILMaIRNC,TC,A,TlevelC,ADecreasedforagetoMLLMaIRAC,A,TconcentrateratioStarchconcentrateMLLMaVRAsourcesandprocessingSupplementationofMMMMaIRNlipidsForagesForagestorageandSILMaIRNC,T,AprocessingC,TIncreasedforageMILMaIRNC,A,TC,A,TdigestibilityC,A,TPerenniallegumesFLLMaVR5N-High-starchforagesSLLMaV/IRN-High-sugargrassesFLLMaVR5NC,A,TPasturesandgrazingN/A------managementSpecies(useofforbs,N/A------diversemixtures)Tannin-containingSLLMaVRDforagesRumenmanipulationIonophoresMLLMiIRDC,G,CaChemicalinhibitorsM6HHMiNc/VUD,A,H,C,G,Ca,SofmethaneF,E7production3-nitrooxypropanolMHHMiNc/VC,RDC,G,Ca(3-NOP)ImmunizationagainstFLLMiNcUNC,G,Camethanogens(Cont.)119MethaneemissionsinlivestockandricesystemsBromoform-SHHMa/UVR,CD,A,H,F,EC,A,G,Ca,Scontainingseaweeds(Asparagopsissp.)OtherseaweedsFU/LUMa/UUUD,A,F,E,HC,A,G,S,CaMiUNC,A,T,G,CaDefaunationMLLIorNcmeatMaproductionR,UAlternativeelectronMLLMaR,UDC,A,G,S,CaLtoMLtoMandfeedacceptors.I.efficiencyCarboxylicacidsIorNcmeatandmilkAlternativeelectronMD,A,F,EC,A,T,G,S,CaproductionNcacceptors.II.InorganicelectronacceptorsEssentialoils8FLLMiU/NcR5DC,A,T,GC,A,T,GTanninextractsFLLMVR5DC,A,T,GSaponinsFLLMiUUUC,A,GC,A,T,GBiocharFNonetoLNonetoLMaNcRDT,G,Ca,SDirect-fedmicrobialsFLLMiNcU5NEarlylifeFUUMiV/UUD,AinterventionsPhageandlyticFUUMiUUUC,G,T,Caenzymeswithactivityagainstmethanogens1Amaximumdoseexists,althoughitmaybeunknown;2ItisacknowledgedthatResistancetochange(F)andtheneedforTechnicalsupport(T)arehighlysubjectiveevaluationsandwillvaryconsiderablyamongparticularproducers,butbothaspectsshouldbeconsideredindecision-making;3ResistancetochangebecauseoftheaversiontofinancialriskisconsideredunderCost(C);onlytheaversiontotechnicalchangeisconsideredunderResistancetochange(F);4Lowintheshorttermbutcanbehighinthelongterm;5Somearecurrentlyavailableonthemarketbutfewinvivostudieshaveshownconsistentmethanedecrease;6Manyintotal,butsomeifonlythemostinvestigatedcompoundsareconsidered;7Willdependonthechemicalnatureofthecompound;8Highlyvariablechemicalnature;needindividualevaluation.Source:Authors’ownelaboration.120MitigationofmethaneemissionsTable3.Summaryofentericmethanemitigationstrategiesforextensivepastoral/ranchingsystems(beef,dairyandother)basedongrazingwithoutsupplementationInvivostudiesExpectedCH4decreaserangeEffectsonotherAnimalTechnicalRiskMainbarrierstoconductedonH=≥25%;GHGemissionsproductivityavailabilitymanagementadoptionon-farmCH4mitigationM=15-24%;U=upstream;(meat&milkR=availableD=maxdose;1F=resistancetoF=few(<5);L=≤15%;M=manure;production,now;safetyforchange;2,3S=some(5-10);I=increasemaybeobserved;Mi=minimal;feedefficiency)C=closetoA=animals;C=increasedcost/lackM=many(>10)U=unknown(notexamined);Ma=majorI=increase;beingavailable;H=humans;offinancialincentives;V=variablechangesD=decrease;U=long-termF=food;M=animalsareexpected,Nc=nochange;oruncertainE=environment;managedsparingly;g/kgmeatneedsLCA;U=unknown;availabilityN=none;A=accessibility;Un=unknown;V=variableU=unknownT=technicalsupport;2Mitigationstrategyg/dormilkV=variableG=governmentapproval;Ca=consumeracceptance;S=safetyAnimalbreedingandmanagementIncreasedanimalFILMaIRNC,TproductionSelectionforlowFLLMiNcUNC,M,A,Tmethane-producinganimalsIncreasedfeedFVLMaIRNC,TefficiencyImprovedanimalFILMiIRNC,M,ThealthImprovedanimalFILMiNcRNF,C,M,TreproductionFeedmanagement,dietformulationandprecisionfeedingIncreasedfeedingFILMaIRNC,TlevelDecreasedforagetoN/A-------concentrateratioStarchconcentrateN/A-------sourcesandprocessingSupplementationofN/A-------lipidsForagesForagestorageandN/A-------processingIncreasedforageFILMaIRNC,TdigestibilityPerenniallegumesFILMaIR4NC,A,THigh-starchforagesN/A-------High-sugargrassesFLLMaVR4NC,A,TPasturesandgrazingFILMiIRNF,C,M,TmanagementSpecies(useofforbs,FVLMaIRNC,A,Tdiversemixtures)Tannin-containingSLLMaVRNC,A,Tspecies(Cont.)121MethaneemissionsinlivestockandricesystemsRumenmanipulationIonophoresFUUMiIRDM,CaUUChemicalinhibitorsFMiUUD,A,H,C,M,A,G,Ca,SUUofmethaneUUF,E5UUproductionUU3-nitrooxypropanolFUUMiUUDC,M,G,CaLL(3-NOP)LtoMLtoMImmunizationagainstFMiUUNC,GmethanogensBromoform-FMaUUD,A,F,H,EC,M,A,G,Ca,Scontainingseaweeds(Asparagopsissp.)OtherseaweedsFMaUUD,A,F,EC,M,A,G,SNC,M,A,TDefaunationFMiUUDC,M,A,GAlternativeelectronFMaNcmilkRacceptors.I.productionCarboxylicacidsAlternativeelectronSMaNcorImeatRD,A,F,EC,A,T,G,Sacceptors.II.productionInorganicelectronacceptorsEssentialoils6FLLMiUR4DC,M,A,T,GTanninextractsMLLMVRDC,M,A,T,GSaponinsFLLMiUUUC,M,A,T,GBiocharFUUMaURD,AC,M,T,GDirect-fedmicrobialsFUUMiUU4NC,M,A,T,GEarlylifeFUUMiUUD,AM,TinterventionsPhageandlyticFUUMiUUUC,M,T,Genzymeswithactivityagainstmethanogens1Amaximumdoseexists,althoughitmaybeunknown;2ItisacknowledgedthatResistancetochange(F)andtheneedforTechnicalsupport(T)arehighlysubjectiveevaluationsandwillvaryconsiderablyamongparticularproducers,butitisadvisedtoconsiderbothaspectsfordecision-making;3ResistancetochangebecauseoftheaversiontofinancialriskisconsideredunderCost(C);onlytheaversiontotechnicalchangeisconsideredunderResistancetochange(F);4Somearecurrentlyavailableonthemarket,butfewinvivostudieshaveshownconsistentmethanedecrease;5Willdependonthechemicalnatureofthecompound;6Highlyvariablechemicalnature;needindividualevaluation.Source:Authors’ownelaboration.122MitigationofmethaneemissionsTable4.Summaryofentericmethanemitigationstrategiesformixedgrazingwithsupplementationofconcentrates,by-productsandconservedforagesInvivostudiesExpectedCH4decreaserangeEffectsonotherAnimalTechnicalRiskMainbarrierstoconductedonH=≥25%;GHGemissionsproductivityavailabilitymanagementadoptionon-farmCH4mitigationM=15-24%;U=upstream;(meat&milkR=availableD=maxdose;1F=resistancetoF=few(<5);L=≤15%;M=manure;production,now;safetyforchange;2,3S=some(5-10);I=increasemaybeobserved;Mi=minimal;feedefficiency)C=closetoA=animals;C=increasedcost/lackM=many(>10)U=unknown(notexamined);Ma=majorI=increase;beingavailable;H=humans;offinancialincentives;V=variablechangesD=decrease;U=long-termF=food;M=animalsareexpected,Nc=nochange,oruncertainE=environment;managedsparingly;g/kgmeatneedsLCA;U=unknown;availabilityN=none;A=accessibility;Un=unknown;V=variableU=unknownT=technicalsupport;3Mitigationstrategyg/dormilkV=variableG=governmentapproval;Ca=consumeracceptance;S=safetyAnimalbreedingandmanagementIncreasedanimalSIM4MaIRNC,TproductionSelectionforlowSLLMiNcUNC,A,Tmethane-producinganimalsIncreasedfeedFVLMaIRNC,TefficiencyImprovedanimalFVLMiIRNC,ThealthImprovedanimalFILMiNcRNF,C,TreproductionFeedmanagement,dietformulationandprecisionfeedingIncreasedfeedingSIMMaIRNC,TlevelDecreasedforagetoMLLMaIRAC,A,TconcentrateratioStarchconcentrateFVLMaVCAC,A,TsourcesandprocessingSupplementationofFLLMaNcCNC,A,TlipidsForagesForagestorageandFILMaIRNC,A,TprocessingIncreasedforageMILMaIRNC,TdigestibilityPerenniallegumesFILMaUR5NC,A,THigh-starchforagesSLLMaVRNC,A,THigh-sugargrassesFLLMaVR5NC,A,TPasturesandgrazingSILMiIRNF,C,TmanagementSpecies(useofforbs,FLLMaURNC,A,Tdiversemixtures)Tannin-containingSLLMaVRDC,A,Tspecies(Cont.)123MethaneemissionsinlivestockandricesystemsRumenmanipulationIonophoresMLLMiIRDC,G,CaChemicalinhibitorsFUUMiUUD,A,H,C,A,G,Ca,SofmethaneF,E6production3-nitrooxypropanolFHHMiNcCDC,G,Ca(3-NOP)ImmunizationagainstFUUMiUUNC,GmethanogensBromoform-FUUMaURD,A,F,H,EC,A,G,Ca,Scontainingseaweeds(Asparagopsissp.)OtherseaweedsFUUMaUUD,A,F,EC,A,G,SDefaunationFUUMiUUNC,A,TAlternativeelectronFUUMaURDC,A,Gacceptors.I.CarboxylicacidsAlternativeelectronFLtoMLtoMMaNcRD,A,F,EC,A,T,G,Sacceptors.II.InorganicelectronacceptorsEssentialoils7FLLMiUR5DC,A,T,GTanninextractsFLLMURDC,A,T,GSaponinsFLLMiUUNC,A,T,GBiocharFUUMaURD,AC,GDirect-fedmicrobialsFUUMiUU5NA,C,T,GEarlylifeFUUMiUUD,AA,T,GinterventionsPhageandlyticFUUMiUUUC,G,Tenzymeswithactivityagainstmethanogens1Amaximumdoseexists,althoughitmaybeunknown;2ItisacknowledgedthatResistancetochange(F)andtheneedforTechnicalsupport(T)arehighlysubjectiveevaluationsandwillvaryconsiderablyamongparticularproducers,butitisadvisedtoconsiderbothaspectsfordecision-making;3ResistancetochangebecauseoftheaversiontofinancialriskisconsideredunderCost(C);onlytheaversiontotechnicalchangeisconsideredunderResistancetochange(F);4Mediumintheshorttermbutcanbehighinthelongterm;5Somearecurrentlyavailableonthemarket,butfewinvivostudieshaveshownconsistentmethanedecrease;6Willdependonthechemicalnatureofthecompound;7Highlyvariablechemicalnature;needindividualevaluation.Source:Authors’ownelaboration.1246.Mitigationstrategiesformethaneemissionsfromanimalhousing,manuremanagementandlandapplicationThissectionprovidesdescriptionsofcurrentstrategiestomitigateCH4emissionsduringthecollection,storageandutilizationofanimalmanures.Becausemanureisoftenstoredwithinlivestockandpoultryhousingsystems,someofthesestrategiestargetCH4emissionsfromanimalhousingsystemsaswell.NumerousstrategieshavebeenputforwardtomitigateCH4emissionsarisingfrommanure.Thesestrate-giesincludethecollectionandcaptureofbiogas(ClemensandAhlgrimm,2001),theemploymentofanaerobicdigestionsystemstomaximizeCH4productionforcollectionanduseasfuel(Clemensetal.,2006;Montesetal.,2013),frequentmanureremovalfromanimalhousingorstorage(Andersenetal.,2015),manurecooling(Nietal.,2008),manureacidification(Petersen,AndersenandEriksen,2012),theadditionofamendmentsthatinhibitCH4production(Andersenetal.,2018),theseparationofsolids,theuseofbiofiltersandscrubbers,manuremana-gementsystemsthatpromoteaerobicconditions(Montesetal.,2013),aswellaslandapplicationandlandmanagementstrategies.Environmentalfactorssuchastemperature,pH,retentiontimeandfavourableanaerobicconditionsformetha-nogenicbacteriaactivityresultinincreasedCH4production,whilethepresenceofinhibitorycompoundsorenvironmentsthatinhibitthegrowthofCH4-producingbacteria,canreduceCH4production(Andersenetal.,2018).AnaerobicdigestionfollowedbybiogascollectionandutilizationisoneofthemosteffectivemeansofreducingCH4emissionsfrommanure,providedthatfugitiveemissionsarewellcontrolled.Anaerobicdigestionreducesthecarbon(C)contentofmanure(Parajuli,DalgaardandBirkved,2018).LoweringtheCcontentofmanuremeansthatthereislessenergytosupportthedenitrifyingbacteria,whichreducesthepotentialforN2Oformationofdigestedmanureappliedtothesoil(Montesetal.,2013).Whilenotmanuremanagementstrategiesperse,CH4reductionstrategiesinvolv-inganimalnutritionandgrazingsystemshavebeenincludedinthissectionbecausetheyreducetheamountofmanureproduced,andhencetheresultingemissions.Table5providesabriefqualitativeassessmentofeachlistedstrategy,includingitsmodeofaction,efficacypotential,currentadoptionpotentialandantagonisticeffectsonN2Oproduction.ThepotentialefficacyratingsofLow,MediumandHighareprovidedinTable5,whereLowrepresentsareportedCH4-mitigationefficacyofupto33percent,Mediumrangesbetween33and66percent,whileitisgreaterthan66percentforHigh.ThisclassificationsystemfollowsthemethodoutlinedinMaureretal.(2016).Incaseswheredifferingmitigationefficacieshavebeenreported,therangeofpotentialefficacieshasbeenlisted,i.e.“LowtoMedium”or“MediumtoHigh”.Moredetailedinformationabouteachstrategy,includingquantitativeinformation,potentialchangesinammonia(NH3)emissions(increaseordecrease),andreferencepublicationsforfurtherstudyareprovidedbelowTable5.WhileadoptionpotentialratingsofLowtoHighhavebeenincludedinTable5,itisimportanttonotethatthe125Methaneemissionsinlivestockandricesystemsadoptionpotentialforastrategywithinaspecificcountyorregionmaybehigherorlowercomparedtootherareasduetolocalregulations,theavailabilityorcostofthetechnology.Wherethisisthecase,itisdiscussedinmoredepthinthesectionregard-ingthemitigationstrategyinquestion.Thelistingofcurrentlyavailablemitigationstrategiesdoesnotreflectbestmanage-mentpractices.Aspecificstrategymayworkwellinonesituationandbeapoorchoiceinanother.WhilethefocusofthisreportisonCH4,somestrategiesthatmitigateCH4resultintheformationofotherGHGemissions,suchasN2O,aswellasincreasingNH3emissions.Whenthatisthecase,itismentionedinTable5alongsidethedescrip-tionofthelistedstrategy.Itshouldalsobenotedthatsomemitigationtechniquesmaybecombinedforincreasedefficacy(e.g.anaerobicdigestionwithasubsurfacemanureinjectionlandapplication),eitheratthesamemanuremanagementstageoratfarmlevel.Table5.Mitigationstrategiesformethaneemissionsfromanimalhousing,manurestorageandlandapplicationStrategyModeofactionEfficacypotentialCurrentadoptionAntagonisticGHGBiogascollectionSystemengineeredtoHighpotentialemissioneffectsandutilizationcollectandusebiogasHighiffugitiveemissionsareNoDecreasingReductioningrowthrateofcontrolledLowtoMediummanurestoragemethanogenicbacteriaNotemperatureLowtoMediumHighReductioningrowthrateof5%CH4reductionper1HighNoManuremethanogenicbacteria°CdropintemperatureacidificationMediumMayincreaseCH4Compoundscausechangesbelowproductionfor1stweekAdditiontothemicrobialcommunity20°CreportedHighfollowingadditiontostoredofmethaneHighinhibitorstothatcaninhibitCH4HighMediummanuremanure(narasin,productionifpHisreducedHighmonensin,etc.)MediumYes.CumulativeN2ODecreasedShortenedmanurestoragetobelow6emissionsmayincreasestorageintervalreducesCH4formationinMediumtoHighMediumwithanincreasednumberEfficacyincreaseswithoflandapplicationeventsSolidsseparationstorageincreaseddosageNoCompostingandRemovalofcarbonthroughMediumaerationvolatilesolidsremovalYes.ThecompostingprocessAerobicprocesscreatesLowtoHighmaycreateN2OemissionsBiofilterandscrubbersadverseconditionsforCH4HighYes.N2OmaybeproducedManureformationinbiofilterincorporationLowandinjectionMethanotrophicbacteriaYes.N2OemissionsmayManureoxidizeCH4NegativetoHighincreaseundersomesoilapplicationdependingonsoiltimingSoilservesasaCH4sinkconditionsconditionsYes.N2OemissionsmayNutritionalSoiltemperatureandLowincreaseundersomesoilstrategiesmoisturecontentimpactconditions,buttheymaybemethanogenicbacteriaMediumdecreasedunderothersactivityNoReductionofthequantityofmanurethroughanimprovedfeedconversionrate,linkedtoincreasedfeeddigestibilitySource:Authors’ownelaboration.126Mitigationofmethaneemissions6.1BIOGASCOLLECTIONANDUTILIZATION6.1.1DescriptionThereductionofCH4emissionsfromanimalmanurestoragecanbeachievedthroughtheenhancedproductionandengineeredcollectionofCH4viabiogasfrommanure.Biogascanbecollectedintraditionalmanurestoragesorpurpose-builtanaerobicdigestionsystemstoincreasetheproductionofCH4foritsuseasanenergysource.6.1.2ModeofactionThecollectionandutilization(flaring,enginecombustionorinjectionintopipelinefordistributeduse)ofCH4replacesthedirectreleaseofCH4intotheatmosphere.6.1.3EfficacyItisworthbearinginmindthatengineeredmanureanaerobicdigestionsystemscanbeexpectedtoproduceuptotwoordersofmagnitudemoreCH4thantraditionalmanurestoragesystems(Hilhorstetal.,2002).Ifmanureisstoredinagas-tightstructurepreventingfugitiveemissions,allCH4emissionsfromstoredmanurecanbeeliminatedthroughtheuseofanaerobicdigestersystems(Clemensetal.,2006).Similarly,Maureretal.(2016)reportCH4mitigationfromanaerobicdigestiontobe“High”,meaninggreaterthan66percent.6.1.4PotentialtocombinewithothermitigationstrategiesTheuseofsomeCH4mitigationstrategies,suchasmanureacidificationandtheaddi-tionofCH4inhibitorswillreducetheconversionofcarbontoCH4throughmanureanaerobicdigestion.WhileareductioninCH4productionwillnotlessentheefficacyofbiogascollection,strategiesantagonistictoCH4productionthatareimplementedupstreamofanaerobicdigestionshouldbeavoidedwhenittakesplace.Thatsaid,thistechnologycanbeusedincombinationwithmostothermitigationstrategies.TheuseofanaerobicdigestionpriortomanurelandapplicationisreportedtoreduceN2Oemissionsfollowinglandapplicationinsomecircumstances(Chadwicketal.,2011).6.1.5EffectsonotheremissionsTheproductionofCO2isalsoincreasedduringtheanaerobicdigestionprocess,butitiscollectedandutilizedasacomponentofbiogas(Lietal.,2017).TappingCH4(asafuel)orconvertingit(upgradedCH4)canmitigatetheGHGemissions.Digestate(i.e.materialremainingfollowinganaerobicdigestion)canalsocontributetoindirectGHGcreditswithrespecttochemicalfertilizersthatitsubstitutes.6.1.6Productivityandthequalityofmeat,milk,manure,crop,andairThereisnoimpactonmeatormilkproduction.Althoughtheanaerobicdigestionofmanuredoesnotremovenutrients,itwillenduptransposingmanurenutrientsfrominorganictomorereadilyavailableorganicplantforms.Thepresenceofsul-phurinmanurewillresultinthehydrogensulfideformationinbiogas,whichhasfoulodorandhumanhealthhazard.6.1.7SafetyandhealthaspectsTheCH4containedinbiogasisflammableandsafetyproceduresmustbefollowedwhendealingwithaflammablegas.Methaneisexplosivewhenmixedwithairatconcentrationsof5to15percent.127Methaneemissionsinlivestockandricesystems6.1.8AdoptionpotentialManureanaerobicdigestiontechnologyiswelldevelopedandreadyforuse.Itiseasilyadoptedtoliquidmanureslurriesandhasalonghistoryofbeingusedforbothcattleandswinemanures.Theprincipleobstacletotheadoptionofmanureanaerobicdigestionhasbeentherelativelyhighcostofbiogasproductioncomparedtootheravailableenergysources(Beddoesetal.,2007;Torrijos,2016).6.1.9ResearchrequiredTherearenomajorresearchgaps.6.2DECREASEDMANURESTORAGETEMPERATURE6.2.1DescriptionActivecoolingofslurryareascansignificantlyreduceCH4emissions.6.2.2ModeofactionTemperatureaffectsmethanogenesisandlowertemperaturesdecreasetheactivityofmethanogensduringmanurestorage.6.2.3EfficacyReducingmanurestoragetemperaturereducesmethanogenicbacteriaactivityinstoredmanure,andthusresultsindecreasedCH4emissions(Montes,2013).LoweringthetemperatureinpigslurrystoragetankshasbeenshowntocutGHGemissionsby21percentcomparedwithuncontrolledmanurestorage(Sommer,PetersenandMøller,2004).Hilhorstetal.(2002)reportedthatreducingmanurestoragetemperaturefrom17°Cto10.2°Cresultedina66percentreductionofCH4emissionsfromswinemanureslurry.Forcattleslurry,areductionof1°Cto2°Camountedtoa5to10percentdecreaseinCH4emissions.6.2.4PotentialtocombinewithothermitigationstrategiesManurecoolingcanbecombinedwithothermitigationstrategies.6.2.5EffectsonotheremissionsManurecoolingcanalsomitigateNH3emissions(aprecursortoN2Oemissions)fromin-housemanurestorage.6.2.6Productivityandthequalityofmeat,milk,manure,crop,andairNoimpactonmeatormilkproduction.ManurecoolingcanassistinmitigatingNH3emissions.6.2.7SafetyandhealthaspectsNosafetyorhealthconcerns.6.2.8AdoptionpotentialControllingmanurestoragetemperatureistechnicallyfeasible,albeitpotentiallyexpensive(dependingonclimate).Itmaybeacost-effectiveoption,iftheexchangedheatcanbeharnessedtoproduceelectricityorheat.Decreasingmanuretempera-turetolessthan10°Cbyremovingthemanurefromthebuildingandstoringitoutside,incoldclimates,canreduceCH4emissions(Hilhorstetal.,2002).128Mitigationofmethaneemissions6.2.9ResearchrequiredMostoftheresearchhasbeencarriedoutinthecontextofreducingNH3emissionsin-house,andthemeasuredimpactonCH4emissionshasconsequentlybeenlimi-ted.AnadditionaldemonstrationofefficacythroughtheevaluationofCH4emis-sionsatthisspecificscalemaybenecessary.Coolingsystemsthatcouldbeeasilyimplementedindifferenttypesofhousingstillneedtobedeveloped.6.3MANUREACIDIFICATIONTHROUGHDIETARYMEASURES6.3.1DescriptionIncorporatingbenzoicacidinthedietofpigstodecreasethepHofmanureforNH3-andCH4-emissionmitigation(pigslurry).6.3.2ModeofactionBenzoicacidusedinthedietsofpiglets,pigsandsowsismetabolizedintheliverandexcretedafterconversionintohippuricacidbymetabolicconjugationwiththeaminoacidglycine(Bühleretal.,2006;Halasetal.,2010;Galassietal.,2011).HippuricacidhasalowpH,whichtheincreasedconcentrationinurinefurtherreduces.6.3.3EfficacyThesupplementationofdietsfedtopigsforfattening,with0.7percentofbenzoicacidduringthestarterphaseand1.7percentduringthegrowing/finishingphase,reducedurinepHby1.81and2.46pointsinthestartingandgrowing/finishingphase,respec-tively.Consequently,theslurrypHwasreducedby0.48and0.78pointsforeachofthephases,respectively(denBrok,1999).TheurinarypHwasreducedsignificantlywiththeincorporationofbenzoicacidatadoseof1percentinthedietofpigsforfattening(6.4+0.6vs7.3+0.2forthetestandcontrolanimals),whilethereductionwasnotsignificantatanincorporationrateof0.5percent(Guingand,DemersonandBroz,2005).Theadditionof1percentbenzoicacidinthedietofpigsforfatteningreducedurinarypHbyonepHunit,regardlessoftheproteinlevel–7.93vs7.09(lowproteindiets)and7.77vs6.76(highproteindiets)forthecontrolandthetestgroups,respectively–throughtheincreasedconcentrationofhippuricacidintheurine(Bühleretal.,2006).Halasetal.(2010)showedasignificantdecreaseofpHinboththeurine(6.1vs7.0forthetestandcontrolgroups,respectively)andthefeces(6.7vs7.2forthetestandthecontrolgroups,respectively),whenincorporatingbenzoicacidat0.5percentintheirdiet.Similarly,thepHoftheslurrywasreducedby0.46pHpoints(8.43vs8.89)whenadding1percentofbenzoicacidtothedietofItalianheavypigs(Galassietal.,2011).WhilefeedingbenzoicacidtopigsisclearlyeffectiveindecreasingpH,itsefficacyforreducingCH4emissionsfrommanurehasnotbeenestablished.However,benzoicacidshowspotentialasamitigationstrategygiventhatthedirectacidificationofmanureslurryusingsulphuricacidhasbeenshowntosubstantiallyreduceCH4emissions.6.3.4PotentialtocombinewithothermitigationstrategiesDuetoitsuniquemodeofaction,benzoicacidcanbeusedalongsideothermitiga-tiontechniquesleadingtothereductionofOMexcretion.ItcanalsobecombinedwithothermanuremanagementstrategiesreducingCH4,whicharenotdependentonthepHofmanure.Theuseofbenzoicacidinfeedmaynegativelyinfluenceanaerobicdigestion.129Methaneemissionsinlivestockandricesystems6.3.5EffectsonotheremissionsThereductionofurinarypHwassystematicallyaccompaniedbyareductionofNH3emissions,eitherinthehouseambienceorinexhaustair.6.3.6Productivityandthequalityofmeat,milk,manure,crop,andairInadditiontothepHreduction,benzoicacidalsoimprovedweightgainandthefeedconversionrate.6.3.7SafetyandhealthaspectsTheuseofbenzoicacidissafeundertheproposedconditionsofuseandhasbeenregisteredinvariouscountries.ThereductionofNH3emissionsinanimalhousingprovidesadditionalsafetyandwelfarebenefitsfortheanimalsandfarmers.6.3.8AdoptionpotentialSincebenzoicacidcaneasilybeincorporatedintopigfeed,itcanbeadoptedwhenfarmersareusingcompoundfeedorwhenproducingfeedonfarm.Itspositiveimpactonanimalproductivityandwelfareusuallycompensatesforthecostofincorporation.Theadoptionofthisstrategymaybelimitedbytheregistrationsta-tusofbenzoicacidindifferentjurisdictions,aswellasincertainlivestockproduc-tionsystems,suchasorganicfarming.6.3.9ResearchrequiredMostoftheresearchhasbeendoneinthecontextofreducingNH3emissionsfromfarmsandtheimpactonCH4emissionshasnotbeenmeasured.Itmightbeneces-sarytodemonstrateefficacythroughtheevaluationofCH4emissions.6.4MANUREACIDIFICATIONTHROUGHDIRECTAMENDMENT6.4.1DescriptionThereductionofmanurepHbydirectlyaddingacidstomanureslurriesorstockpiles.6.4.2ModeofactionMethanogenicbacteriaareinhibitedaspHdecreases.6.4.3EfficacyManureslurryacidificationtoapHof5.5hasbeenreportedtoreduceCH4produc-tionby67to87percentincattlemanureslurries(Petersenetal.,2013a),whereasSokolovetal.(2020)reportedCH4reductionsof77percentindairycattlemanure.6.4.4PotentialtocombinewithothermitigationstrategiesTheacidificationofmanureisnotcompatiblewithanaerobicdigestion.Itmaybecombinedwithothermitigationstrategies.6.4.5EffectsonotheremissionsManureacidificationwillreduceNH3emissions.Acidificationofliquidmanuremayincreasehydrogensulfideemissions.130Mitigationofmethaneemissions6.4.6Productivityandthequalityofmeat,milk,manure,crop,andairNoimpactonmeat,milkormanurequality.TheacidificationofmanuretoapHinthe5.5rangegenerallydoesnotposeproblemsforcropproduction.AcidificationreducesthelossofNintheformofNH3,whichresultsinincreasedNavailabletocrops.ItalsoreducesNH3emissionsduringandafterthelandapplicationofmanure.Surfaceappli-cationofacidifiedslurryisagoodalternativetoslurryinjection(Fangueiroetal.,2017).6.4.7SafetyandhealthaspectsThestorageandhandlingofacidiccompoundsrequireappropriatesafetymeasures.6.4.8AdoptionpotentialThisisahighlydevelopedtechnology,whichisevenlistedasabestavailabletech-nique(BAT)forNH3mitigations.Nevertheless,technicalbarriers(risksrelatedtothestorageandhandlingofacidorthecorrosionofmaterials)andpsychologicalbarriers(consumerdistrust)existinsomecountries.6.4.9ResearchrequiredResearchthatbetterquantifiesN2Oemissionsfromacidifiedmanures,followingtheirincorporationandinjectionintothesoil,isneeded.Long-termeffectsonsoilpropertiesshouldalsobestudiedindifferentpedoclimaticconditions.6.5METHANEINHIBITORS6.5.1DescriptionWhenaddeddirectlytomanurestorage,amendmentssuchastannins(Whitehead,SpenceandCotta,2013),monensin(Clanton,JacobsonandSchmidt,2012)andnarasin(Andersenetal.,2018)havebeenfoundtolimittheformationofCH4instoredmanures.6.5.2ModeofactionAmendmentssuchasmonensinandnarasinareionophores,whicharelipid-solublemolecules.ThesemoleculestransportionsacrosscellmembranesandcausechangestothemicrobialcommunitythatcaninhibitCH4production.Tanninsarepoly-phenoliccompoundsfoundinsomeplantspeciesthathaveaninhibitoryeffectonmethanogenicmicrobes.6.5.3EfficacyNarasinhasbeenshowntostronglyinhibitCH4productionforupto25days,fol-lowingitsadditiontoswinemanureat3.0mgnarasinperkgofmanure.Andersenetal.(2018)reportedthatCH4productionrateswerereducedby9percentforeachmgofnarasinaddedperkgofmanure,andthisreductionwaseffectiveforupto25days.Somelevelofinhibitionwasnotedforupto120days.Quebrachocon-densedtanninsaddedat0.5percentweightpervolumetomanureslurriesreducedCH4productionbyover85percentforupto28days.6.5.4PotentialtocombinewithothermitigationstrategiesTheuseofCH4inhibitorsinfeedormanurecanreducetheefficacyofanaerobicdigestersthatutilizethesemanuresasfeedstocks.Thistechnologycanotherwisebeusedalongsidemostothermitigationstrategies.131Methaneemissionsinlivestockandricesystems6.5.5EffectsonotheremissionsForthefirstweekfollowingtheinitialapplicationoftheseinhibitors,CH4produc-tionmayincreaseandwillthenbeinhibited.6.5.6Productivityandthequalityofmeat,milk,manure,crop,andairWhenaddeddirectlytomanure,noimpactonmeatormilkoccurs.6.5.7SafetyandhealthaspectsNone.6.5.8AdoptionpotentialThistechnologyisfullydevelopedandreadyforuse.However,theadoptionofthisparticularmitigationstrategywilldependontheregistrationstatusindiffer-entjurisdictionsofthesubstancesinvolved.Themainobstacletoadoptionliesintheadditionalcostincurredwhenpurchasingthesecompoundswithnoassociatedincreaseinproduction.6.5.9ResearchrequiredTherearenomajorresearchgaps.6.6DECREASEDMANURESTORAGEINTERVAL6.6.1DescriptionAreductioninCH4emissionsfromstoredmanurecanbeachievedbyreducingthemanurestorageintervalin-house(manurefrequentremoval)andduringoutdoorstorage.6.6.2ModeofactionReducingthelengthoftimemanureisstoredreducestheamountofCH4thatcanbegeneratedduringstorage(Andersenetal.,2015).6.6.3EfficacyThehighestefficacywillbeachievedforanimalproductionsystemsthathavethegreatestCH4yieldfromstoredmanure,suchasdeep-pitswineproductionsystems(Parketal.,2006).Petersenetal.(2013b)reporteda40to50percentreductionofCH4emissionsduetothefrequentmanureremovalforpigs.ForanimalproductionsystemswherethemajorityoftheCH4emissionsarenotgeneratedduringmanurestorage,thisapproachwillhavelimitedeffectiveness.6.6.4PotentialtocombinewithothermitigationstrategiesThismitigationstrategymaybecombinedwithanyoftheothermitigationstrategies.6.6.5EffectsonotheremissionsIftheremovedmanureisland-appliedonamorefrequentbasis,thisstrategycouldresultinincreasedN2OandCO2emissions.Nevertheless,thistechniqueleadstoreducedNH3emissionsandodoursin-house(Santonjaetal.,2017)andduringstorage.6.6.6Productivityandthequalityofmeat,milk,manure,crop,andairNone.132Mitigationofmethaneemissions6.6.7SafetyandhealthaspectsNone.6.6.8AdoptionpotentialThisstrategycanbeadoptedbyproducerswhocanusethemanurethatisavail-ablemorefrequently.Producerswhodonothavemanurelandapplicationorotheropportunitiesforusingmanurewillnotbeabletoemploythisstrategy.Thein-houseimplementationofthistechniquecaneasilybeconsideredinthecaseofnewhouses.Inexistinghouses,acostlymodificationofthemanuremanagementsystemcouldberequired.6.6.9ResearchrequiredAdditionalresearchisneededonthepotentialincreaseinN2OandCO2emissionsfrommorefrequentlandapplication.6.7SOLID–LIQUIDSEPARATION6.7.1DescriptionSolid–liquidseparationhasbecomeacomplementarymanagementoptionformanuremanagementsystems,particularlyforanaerobicsystems.Theseparationprocesscanhelpdivertsolidswithahighphosphorous-to-nitrogenratiotonutrientdeficientareas.ThiscanhelpreducetheGHGemissionsproducedduringmanurestorageandmanureapplication.ReducingCH4emissionsispossiblebecausevolatilesolidsareseparatedalongwiththesolidwastestream.Solidseparationalsoreducescrustformation,whichisusefultolimittheanaerobicconditionsduringmanurestorage.6.7.2ModeofactionRemovingpartoftheOM(i.e.volatilesolids)priortodeliveringmanuretostoragestructuresandlandapplication.6.7.3EfficacyTheCH4reductionrangesfrom7.0to49.0percentdependingonseveralfactors,includingsystemdesign(e.g.screensize),theconcentrationofsolidsinprocessedmanure,manureflowrate,andthetypeandconfigurationofthemanureprocessingpitbeforeitgetstothemechanicalseparator(Zhang,R.etal.,2019).6.7.4PotentialtocombinewithothermitigationstrategiesThismitigationstrategymaybecombinedwithothermitigationstrategies.6.7.5EffectsonotheremissionsEmissionsofN2OandNH3fromlandapplicationofseparatedsolids(Aguirre-Villegasetal.,2019).6.7.6Productivityandthequalityofmeat,milk,manure,crop,andairNone.6.7.7SafetyandhealthaspectsGenerallysafewithpotentialhazardsassociatedwithmovingparts.133Methaneemissionsinlivestockandricesystems6.7.8AdoptionpotentialThereareseveraldesignsthatcouldbeappliedonfarmsofdifferentsizes.Thesedesignscouldalsobeintegratedintocurrentmanuremanagementsystemswithlittlemodifica-tion.Theadoptionwilldependonthecostofretrofittingexistingmanagementsystems.6.7.9ResearchrequiredMeasurementsofemissionsofdifferentgasesafterlandapplicationduringdifferentseasons.6.8MANURECOMPOSTING/AERATION6.8.1DescriptionManurecompostingisthebiologicaloxidationofmanuresinconjunctionwithanadditionalorganiccarbonsource,typicallyatthermophilictemperaturesgeneratedbymicrobialheatproduction.Manurecanbeleftundisturbedduringthecompost-ingprocess(passivecomposting),mechanicallyturned(extensivecomposting)oractivelyaerated(intensivecomposting).6.8.2ModeofactionCompostingisanaerobicprocessthatreducesorpreventsthereleaseofCH4dur-ingOMbreakdown.Iftheprocessisfullyaerobicthencompostingdoesnotpro-duceCH4becauseCH4-producingmicrobesarenotactiveinthepresenceofoxygen.Inpractice,compostingsystemsmaynotachievecompletelyaerobicconditions,andbothaerobicandanaerobicconditionsmayexistwithinthecompostpileorwindrow.6.8.3EfficacyMaureretal.(2016)findcompostingtobe70percenteffectiveatallscalesfordairymanurebutnoteareductionby34percentofCH4emissionsforswinemanurecompostingatallscales.ThislargediscrepancyreflectsthedifferenceinCH4emis-sionsforcompostingsystemswithvaryingaerobicoranaerobicconditions.6.8.4PotentialtocombinewithothermitigationstrategiesCompostingcanbecombinedwithotherCH4mitigationstrategies.Compostingisoftenusedfollowingmanureseparationtopreparetheseparatedsolidsforuseasbeddingmaterialindairycattlesystems.6.8.5EffectsonotheremissionsCompostingisanaerobicprocessthatproducesbothCO2andN2O.NitrogenlossesfromcompostingsystemsintheformofNH3emissionscanalsobesignifi-cant.Maureretal.(2016)reportsN2O-emissioncontrolsof-685and-388percent(wheretheminussignindicatesanincreaseinN2Oemissions)forswineanddairymanurecompostingsystems,respectively.6.8.6Productivityandthequalityofmeat,milk,manure,crop,andairThereisnoimpactonmeatormilkproduction.LossesofNduringcompostingcanbehigh,especiallyviaNH3butalsoN2Oemissions(dependingonthecompostingprocess),andareincreasedbyfrequentturningandmixingofthemanureduringthecompostingprocess.134Mitigationofmethaneemissions6.8.7SafetyandhealthaspectsCompostingcangenerateNH3emissions.Safetyprecautionsshouldbetakenwithwindrowturningandcompostmanagementequipment.6.8.8AdoptionpotentialCompostingandaerationtechnologiesarehighlydevelopedandreadyforuse.Compostingcaneasilybeadoptedforsolidmanuresandslurrywiththeadditionofacarbonsource.6.8.9ResearchrequiredNoadditionalresearchisrequiredtoimplementthisstrategy.6.9BIOFILTERSANDSCRUBBERS6.9.1DescriptionBiofiltersandbiofilter/scrubbercombinationshavebeenfoundtobeeffectiveinreducingCH4emissionsfrombothanimalhousing(mechanicallyventilated)andmanurestoragethroughtheactionofmethanotrophicbacteria(Hilhorstetal.,2002).6.9.2ModeofactionMethanotrophicbacteriagrowninthebiofilteroxidize,therebyreducingorelimi-natingtheemissions.6.9.3EfficacyMaureretal.(2016)reportaCH4mitigationeffectof17to24percentacrossallspeciesandatallscalesintheirsummaryofperformancedatafortechnologiesusedtocontrolgaseousemissionsfromlivestockoperations.6.9.4PotentialtocombinewithothermitigationstrategiesThismitigationstrategymaybecombinedwithanyothers.6.9.5EffectsonotheremissionsBiofiltersandscrubbersareusedtocontrolNH3emissions.WhiletheyareveryeffectiveatreducingNH3emissions,anundesirableeffectisthatN2Oistypicallyformedinthebiofilteraswell.6.9.6Productivityandthequalityofmeat,milk,manure,crop,andairNone.6.9.7SafetyandhealthaspectsNone.6.9.8AdoptionpotentialBiofiltersandscrubbersrequirethereplacementofventilationfanswithunitscor-rectlysizedtoworkagainstthepressuredropdevelopedinthebiofilter.Thecostofthisretrofitcanbeprohibitiveformanyoperations.135Methaneemissionsinlivestockandricesystems6.9.9ResearchrequiredAdditionalresearchonlimitingN2Oproductioninbiofiltersisneeded.6.10MANUREINCORPORATIONANDINJECTION6.10.1DescriptionTheincorporationofmanurefollowinglandapplication,eitherthroughcultivationpracticesorthedirectinjectionofmanure15to20cmbelowthesoilsurface.6.10.2ModeofactionSoilscanserveeitherasasourceorasinkforCH4,dependingontheconditionsandwhetherthemethanogenicormethanotrophicbacteriaareactive(ToppandPattey,1997).Whensoilsserveasasink,methanotrophicbacteriacanoxidizeCH4follow-ingtheincorporationorinjectionofmanurebelowthesoilsurface.Ifsoilcondi-tionsarefavourableformethanogenicbacteriaactivity,CH4emissionscanincreasefollowingtheincorporationorinjectionofmanure.6.10.3EfficacyThemitigationefficacyishighwhensoilconditionsfavourmethanotrophicbac-teriagrowth.Whensoilconditionsfavourmethanogenicbacteriagrowth,soilscanbecomeaCH4source.Methaneemissionsfromthesoilhavebeenshowntospikeimmediatelyfollowingmanureapplication,buttheyquicklyfalltoverylowlevelsfollowingincorporationorinjection(Montesetal.,2013).Lovanh,WarrenandSistani(2008)reportedthattheinjectionofswinemanureresultedinanorderofmagnitudereductionofCH4ascomparedtosurface-appliedswinemanure.ReportsofincreasedCH4emissionsfollowingmanureinjectioncomparedtocom-mercialfertilizercontrolscanalsobefoundintheliterature.Forinstance,Sistanietal.(2010)reportthatCH4emissionsfromcroplandfertilizedwithinjectedswineweresignificantlyhigherthanwhenacommercialfertilizercontrolhadbeenused.6.10.4PotentialtocombinewithothermitigationstrategiesThepotentialtocombineotherstrategies,suchasanaerobicdigestionorsolidsseparation,withmanureincorporationorinjectionisexcellent.Anaerobicdiges-tionofmanureorseparationofsolidspriortoincorporationorinjectionreducestheCavailabletobeconvertedtoCH4andfurtherenhancestheCH4-mitigationpotentialofthisstrategy.6.10.5EffectsonotheremissionsManureincorporationandespeciallyitsinjectionbelowthesoilsurfacecanleadtoincreasedN2Oemissions.Itshouldbenoted,however,thatconflictingresultsarereportedregardingN2Oemissionsfollowingthelandapplicationofmanureviainjection.Vallejoetal.(2005)reportednosignificantdifferencesinN2Oemissionsbetweenthesurfaceapplicationandinjectionofswinemanure.TheinconsistencyinreportedN2Oemissions,calculatedfollowingthelandapplicationofmanure,islikelyduetothediversityofsoilconditionsinwhichtheemissionsweremeasured.6.10.6Productivityandthequalityofmeat,milk,manure,crop,andairNoimpactonmeat,milkormanurequality.Theincorporationorinjectionofmanurehasbeenshowntoconservenutrientsforplantuseandthustoincrease136Mitigationofmethaneemissionsplantnutrientsavailableforcropuptake.IncorporationandinjectionreduceNH3emissionsintotheatmospherebuttheycanincreaseN2Oemissions.6.10.7SafetyandhealthaspectsNone.6.10.8AdoptionpotentialThistechnologyisfullydevelopedandreadyforuse.Itisworthpointingoutthat,inordertomakeuseofthistechnology,producerswillhavetopurchasethepurpose-specificequipmentrequiredtopump,transferandmakeasubsurfaceinjectionofmanure.Thecostofthisequipmentmayproveabarriertoadoptionforsomefarmers.6.10.9ResearchrequiredResearchthatbetterquantifiesCH4andN2Oemissionsfollowingtheincorpora-tionandinjectionofmanureintothesoil.6.11MANUREAPPLICATIONTIMING6.11.1DescriptionManureapplication,atvarioustimesofthedayandindifferentseasons,usingarangeofmethodsthatarecurrentlyemployedforincorporationandsurfaceapplications.6.11.2ModeofactionSoiltemperatureandmoisturecontentaffectmethanogenicbacteriaactivity.6.11.3EfficacyIntheirtableofpossiblemitigationstrategies,Montesetal.(2013)listapplicationtimingashavinganefficacyof≤10percent.6.11.4PotentialtocombinewithothermitigationstrategiesCombiningthisstrategywithothermanuretreatmenttechnologies,suchasmanurestorageandtheproductionofstablemanureproducts(e.g.compostedmanure),couldgiveapplicationtiminggreaterflexibility.6.11.5EffectsonotheremissionsItmayaffecttheemissionsofN2Odependingonweatherconditionsandsoilcon-ditions(i.e.temperature,soilfreeze-thawcycles),andmanuretypeandtreatment(Heetal.,2020).SoilswithhighmoisturecontentmaypromotetheemissionsofN2O(Montesetal.,2013).Ammoniaemissionsincreasedinthefirst10hoursaftermanureapplication(Gordonetal.,2001).Inaddition,whentheavailablepoolofNandCinthesoilisgreater,denitrificationratescanincreaseresultingingreaterN2Oemissions.Assuch,timingmanureapplicationssothatactivelygrowingcropsarepresentcanreduceN2OemissionscomparedtofieldapplicationsduringfallowtimeswhengreaterpoolsofNwouldremainavailable(Chadwick,etal.,2011).Thormanetal.(2007)reportsdirectN2Oemissionsfromfall/wintermanureslurryapplicationswere64percentgreaterthanspringapplicationswhenemissionsareconsideredasapercentageofthetotalNapplied.137Methaneemissionsinlivestockandricesystems6.11.6Productivityandthequalityofmeat,milk,manure,crop,andairNone.6.11.7SafetyandhealthaspectsSafetyprecautionsshouldbetakenwhenusingequipmentformanureapplication.6.11.8AdoptionpotentialItcanbeachievedinpractice,whenstoragevolumeandweatherconditionsallowforit.6.11.9ResearchrequiredThemeasurementsofN2OandNH3emissionsindifferentweatherconditionsandcroppingsystemsneedfurtherresearch.6.12NUTRITIONALSTRATEGIES6.12.1DescriptionReducingtheamountofexcretedOMdecreasestheemissionofCH4frommanure.6.12.2ModeofactionNutritionalmitigationoptionsthatimprovethefeedconversionrateofanimalsthroughimproveddietdigestibility(e.g.feedformulation,feedprocessing,foragemanagement,enzymes,direct-fedmicroorganisms,botanicalextractsandsoforth)decreasetheamountofOMexcreted.Furthermore,thepreparationoffeedintheformofpelletsmayalsoreducefeedlossesonpigfarms.6.12.3EfficacyTheefficacyofthisstrategydependsonthedifferentmitigationoptionsthatareavailableandthestatusofthefarm.Animprovementbetween2and5percentofthefeedconversionratiocanbeachievedundertypicalfarmconditions.6.12.4PotentialtocombinewithothermitigationstrategiesNutritionaloptionscanbecombinedwithotherapproachestomanuremanage-ment(e.g.acidification).Theymaynegativelyimpacttheoperationofanaerobicdigesters.6.12.5EffectsonotheremissionsUsually,animprovedfeedconversionratealsoreducesnitrogenexcretion,leadingtoreducedNH3andN2Oemissions.EntericCH4productionisalsodecreasedinruminants.6.12.6Productivityandthequalityofmeat,milk,manure,crop,andairThefeedconversionratioisanimportantparameterofproductivityforfarmers.Theeffectsonanimalproductivityarecoveredelsewhereinthisdocument(Section5).6.12.7SafetyandhealthaspectsThenutritionalmitigationsolutionsusedtoimprovethefeedconversionratioofanimalsaretypicallyconsideredtobeGRAS(generallyrecognizedassafe)ortheirsafetyisevaluatedbyregulatoryauthorities.138Mitigationofmethaneemissions6.12.8AdoptionpotentialNutritionalsolutionsareeasilyadoptedbyfarmswherecompoundfeedsormixedrationsareused.Insuchcases,theadoptionpotentialishigh.Ingeneral,thecostofthenutritionalmitigationsolutionisoffsetbytheimprovementinfeedconversionrate.However,theimplementationofthesestrategieswilldependontheregulatoryenvironment(e.g.authorizationoffeedingredients)andmaynotbeallowedincer-tainlivestocksystems,suchasorganicfarming.6.12.9ResearchrequiredResearchontheefficacyofnewnutritionalsolutionsthatimprovefeedconversionefficiencyneedstoincludemeasurementsofreducedOMexcretionandassociatedemissions.6.13GRAZINGPRACTICES–PRODUCTIONSYSTEMModifyinggrazingsystemshasrepercussionsontheentireproductionsystem,unlikeapplyingasinglemitigationmethodforstoredorlandappliedmanure.Assuch,theyhavenotbeenincludedinTable5.ModifyinggrazingsystemstodecreaseCH4emissionscanaffecttheamountandcompositionofmanureexcretedbyani-mals.Methaneemissionsfromurineanddungdroppingsofgrazinganimalsareminimalcomparedtothoseemittedthroughanimalconfinementinmanurestoragesystems(Pellerinetal.,2017).ThereductioninCH4emissionsfromanimalexcretaissubstantial,especiallyinwarmclimates.Owingtothehigh-fibreconcentrationoftheherbage,grazinganimalstendtoproducegreaterentericCH4emissionscomparedwithanimalsinconfinementsys-temsthatarefedmixeddiets.Grazingsystems,whenintensivelymanaged,havelargerN2Oemissionsatthefieldlevel.Conversely,NH3emissions(aprecursortoN2O)aregenerallyreducedingrazingvsconfinedsystems.Grazingsystemsreducetheamountofmanureproducedonthefarmbecausethebeddingisnotbeingused,andexcretaaredelivereddirectlytothepasture.Therecanbedifferenceswithingrazingsystemsthataffectthepotentialforsoilorganiccarbonsequestration.139Methaneemissionsinlivestockandricesystems7.MitigationofmethaneemissionfromricepaddiesMethaneisemittedfromricepaddiesduetotheanaerobicdecompositionoforganicmatter,suchassoilorganicmatter,plantresidueandriceroots,underhighlyreducedconditionswhenthelandisflooded.Methaneproducedinanoxicricesoilispartlyoxidizedintheoxicrhizosphereandsurfacesoil.Thus,thebalanceofCH4productionandoxidationcontrolsCH4emission(Figure3).SeveralmanagementpracticesthatinducetheincreasedredoxpotentialofsoilsuppressCH4productionandhencetheemissionsfromricefields.7.1WATERMANAGEMENTModificationsofthewatermanagementhaveaproventrackrecordtoreduceCH4emissionsfromricefieldsandaredeemedthemostpromisingwaytomitigateCH4emissionsfromricepaddies(Wassmann,2019).Thedrainageofricefieldsincreasestheredoxpotential,whichstronglysuppressesthemicrobialprocessesofCH4productionandconcomitantlystimulatesCH4oxidation.However,thedrawbackoffloodwatergeneratesshort-termspikesofgaseousCH4thathasbeenentrappedinthefloodedsoils(Wassmannetal.,1994).Nevertheless,theoverallamountofCH4emittedfromthesoilthroughthecourseofthecroppingseasonissignificantlyreducedasdemon-stratedinnumerousfieldmeasurements(Sander,WassmannandSiopongo,2014).Eithersingleormultipledrainageapproaches,likealternativewetanddry(AWD)management,haveshownconsistentlysignificantmitigationpotential–althoughthemagnitudeofreductiongivenbydifferentstudiesrangeswidely(Yagietal.,2020).Whilethebaselineisdefinedascontinuousflooding,thescalingfactorsforotherwaterregimesintheIPCCguidelinevaryfrom0.41to0.94withanextensiveerrorrangeduetothedifferenceintheextentofdrainageintermsofdurationandfrequency(IPCC,2019).Accordingtoarecentmeta-analysisbasedon201pairedobservations,non-continuousfloodingpracticesreducedCH4emissionsby53percentascomparedtocontinuousflooding(Jiangetal.,2019).IntermsofGWP,thereductioneffecthasaslightlylowerpercentage,namely44percent,whichisattributedtotrade-offsfromhigherN2Oemissions.IncrementsinN2Oemissionsunderunstablewaterregimesarewelldocumented,but–withtheexceptionofindividualrecordsofexcessivelyhighN2Oemissions(Kriteeetal.,2018)–donotreversethetrendofGHGsavingsthroughAWD(Majumdar,2003;Yagietal.,2020).Althoughtheglobalmeta-anal-ysisbyJiangetal.(2019)alsorevealedaslightyieldreductionthroughAWD,theeconomicfeasibilityofthiswatermanagementpracticewilllargelydependonlocalcircumstances,namelythepotentialsavingsinpumpingcosts.IntheVietnameseMekongDelta,theapplicationofAWDimprovedfarmprofitabilitybyupto13per-centcorrespondingtoaboutUSD100perhectare(Frith,WassmannandSander,2021).WatermanagementbeforethecultivationperiodalsoaffectsCH4emissionsduringricecultivation.Aprolongednon-floodedpreseasonoveroneyearhadasignificantlylowerCH4emissionscalingfactor(0.41-0.84),whileafloodedpreseasonover30daysbeforecroppinghadamorethandoubledscalingfactor(2.13-2.73)(IPCC,2019).140MitigationofmethaneemissionsSuchmitigationpracticesareonlyfeasiblewherevercompletecontrolofwatersupplyanddrainageispossible.Inthetropics,watermanagementwillbelesseffectiveinmitigatingCH4emissionsduringrainyseasons(Yagietal.,2020).ThisimpactofprecipitationisalsotakenintoaccountinanewlydevelopedmethodforGIS-mappingofAWDsuitability(Nelsonetal.,2015).However,ifavailable,appropriatewatermanagementpromotesriceproductionwhileeffectivelymitigat-ingCH4emissions(Yagietal.,2020).Landlevellingallowsforaspatiallyhomoge-neouswatermanagement,andwouldcontributetoeffectiveCH4migration.BetterwatermanagementinricepaddiestomitigateCH4emissioncouldalsocontributetosustainablewater,animportantgoalforagriculture(FAO,2020).Ontheotherhand,theprolongedaeratedconditionscouldcauseanenhanceddecompositionofsoilorganicmatter,loweringthecarbonstorageandfertilityofricefieldsoilsinthelongterm.Ameta-analysisbyLivseyetal.(2019)showedthatwhilemildformsofAWDreduceemissionsofCH4byupto52percent,suchmanagementcanincreaseCO2emissionsby45percentwhileincreasingsoil-to-atmospherecarbonfluxby25percentwhencomparedtocontinuousflooding.AWDwasalsofoundtohaveanegativeeffectonbothsoilorganiccarbon–reducingconcentrationsby5.2percent–andsoilorganicnitrogen–potentiallydepletingstocksbymorethan100kgNha-1y-1.WhilesignificantnegativeeffectsofAWDonriceyieldmaynotbevisibleinshort-termexperiments(1-to3-year-longstudies),careshouldbetakenwhenassessingthelong-termbenefitofAWD-likeirrigationpracticesbecausetheycandecreasesoilfertilityandhenceyieldsinthelongterm(Livseyetal.,2019).7.2ORGANICAMENDMENTSMoreCH4isemittedfromsoilsamendedbyorganiccompoundsofeasilydecompos-ablecarbon.Methaneemissionalsoincreasesasafunctionoftheamountoforganicamendmentsthatareappliedtothesoils.Ifricestrawisincorporatedintothesoilafterharvest,thetimingofricestrawapplicationsignificantlyaffectsCH4emissions.AlongintervalbetweenstrawincorporationandfloodinglowersCH4emissionsduringtherice-growingseason,ascomparedtoincorporatingricestrawjustbeforeflooding(IPCC,2019).EitherremovingorburningricestrawdrasticallyreducesCH4emissions,butithasadverseeffectsonthelocalairquality(incaseofburn-ing)andmaydecreasesoilorganiccarbonandsoilfertilityinthelongterm(Yagietal.,2020).Ontheotherhand,long-termexperimentswithfloodedricefieldsshowedhighstabilityofsoilorganicmatter,evenifthestrawhasroutinelybeenremovedinmorethanadecadeofdoublecroppingrice(Pampolinoetal.,2008).Giventheoverallobjectiveofresourcerecycling,theapplicationofcompostedricestrawtothesoil–asopposedtoabaselineofincorporatingfreshstraw–isanotheroptionforreducingCH4emissionsfromricefields(Buendiaetal.,2019;Yagietal.,2020).TheNcontentofricestraw,however,willnotsufficeforrea-sonableyieldlevelsonitsown,sothatadditionalorganicamendments(e.g.ani-malmanure)willberequired.Moreover,CH4productionduringthecompostingprocessshouldbetakenintoaccount(Nguyen-Van-Hungetal.,2020).Farmyardmanureandgreenmanurealsohavealowerscalingfactorthantheincorporationoffreshricestraw(Buendiaetal.,2019),thusofferingtheadditionaloptionofapply-ingorganicamendmentstosustainthefertilityandcarbonstorageinsoil.BiocharhasbeenconsideredasanoptiontoreduceGHGemissionsfromricecul-tivation.Althoughitslong-termeffectremainsunclear,ithasoftenbeenshownthat141MethaneemissionsinlivestockandricesystemsbiocharapplicationisaneffectivewayofreducingCH4emissionsfromfloodedricefields(Jefferyetal.,2016;Mohammadietal.,2020;Yagietal.,2020).Environmentallifecycleassessmentstudiesrevealedthatthecarbonfootprintofriceproducedinbiochar-treatedsoilwasestimatedtorangefrom-1.43to2.79kgCO2eqperkgofricegrain,implyingasignificantreductionrelativetothericeproducedwithoutabiocharsoilamendment(Mohammadietal.,2020).Atthispoint,however,theapplicationofbiocharinriceproductionremainsatthestageofpilotstudies,asthepracticabilityandenvironmentalimpactsofavailablestovesarestilluncertain.AcombinationofAWDwatermanagementwithbiocharapplicationmayfurtherreduceCH4emissions(Sriphirometal.,2020),thoughthereisnotenoughdatatodrawconclusionsabouthowbiocharproductionandapplicationaffectwhole-systemGHGbudgets(Gurwicketal.,2013).7.3FERTILIZERANDOTHERAMENDMENTSTheapplicationofsulfate-containingfertilizer,suchasammoniumsulfateandphosphogypsum,reducesCH4emissions(Yagietal.,2020;Kumaretal.,2020),assulfateionsupportssulfatereductionthatoutcompetesCH4productioninfloodedricefieldsoils(Achtnich,BakandConrad,1995).Biofertilizers,e.g.Azolla(aquaticpteridophytewithsymbioticcyanobacteria)andblue-greenalgae(cyanobacteria),arewidelyusedtoincreasesoilfertilityandriceyieldswiththeirnitrogenfixationactivity.TheycanmitigateCH4emissionsbyoxygenatingthericesoilthroughphotosyntheticactivities(Maylanetal.,2016).Itisreportedthatnitrificationinhibitors,whichslowdowntheconversionofammoniaintonitrate,canreducenotonlynitrousoxidebutalsoCH4emissionsfromricefields(Malyanetal.,2016).Nitrificationinhibitorspromotericeplantgrowththroughincreasednutrientuptake,andtheyincreasetheredoxpotentialintherhizo-sphere,whichreducesCH4emission(Boeckx,XuandVanCleemput,2005).Thereductionofferricironlikewisecompeteswithmethanogenesis(Achtnich,BakandConrad,1995).AddingsteelslagcanmitigateCH4emissionfrompaddyfields(Kumaretal.,2020).SilicaoxideinsteelslagcanalsomitigateCH4emis-sionsfromricebypromotingthedevelopmentofaerenchymainriceroots,whichincreasesoxygentransportationfromtheatmospheretotherootregionandenhancesrhizosphericCH4oxidation(Kumaretal.,2020).7.4PLANTINGMETHODSANDCROPMANAGEMENTPACKAGESComparedtothetraditionaltransplantingofriceseedlings,directseedinghasbeenreportedtoreduceCH4emissions(perm2andday)(Yagietal.,2020;Malyanetal.,2016).Althoughtheyieldofdirect-seededricecouldbelowerthantransplantedrice(Yagietal.,2020),thispracticeisgettingincreasinglypopularduetothelaboursavingsinvolved.Itcouldalsobeoptimizedasamitigationpotentialinmanyrice-growingareas.Inpreseasonconditions,CH4emissionsarereducedbyprolongedperiodswith-outflooding(Yagietal.,2020),causedbyalongfallowseasonoracroprotationwithanuplandcrop.ThiseffectisconsideredintheIPCCguidelinesintheformofapreseasonscalingfactor,namelySFpre=0.59for“non-floodedpreseason>365d”incontrasttoabaseline(SFpre=1)for“non-floodedpreseason>180d”.Thesystemofriceintensification(SRI)isafarmingmethodologycharacterizedbyalow-water,labour-intensivemanagement,whichpresentscertainfeaturesof142Mitigationofmethaneemissionslow-emissionmanagement(Malyanetal.,2016;Yagietal.,2020).Theterm“SRI”,however,hasbeenusedintheliteraturewithreferencetoaverywiderangeofcropmanagementpractices,inparticularthoseregardingtheapplicationoforganicmanure(Lyetal.,2013).TheoriginalSRIconceptencompassesahighamountoforganicinputsthatresultinahighbackgroundlevelofCH4emissions.AstheSRIprescribesintermittentflooding,theactualincrementinemissionswillbelowerthaninthecaseofcontinuousflooding,duetothesuppressedmethanogenesis.TheSRIhasbeenconsideredasamitigationstrategy,whichcanbejustifiedwhencomparedtocontinuousflooding(Lyetal.,2013)oraslongasorganicamendmentsareomitted(Jainetal.,2014).ThecalculatedmitigationeffectbytheSRIwillulti-matelydependonthedefinitionofthebaselinemanagementaswellasontheSRIsubtypeusedforthecomparison.7.5SELECTING/BREEDINGRICEVARIETIESThedifferenceinCH4emissionsfordifferentricevarietieshasbeendocumentedinseveralcasestudies.TheunderlyingmechanismstoreduceCH4emissionsfromricepaddiesthroughvarietyselectionstillremainunclear–exceptforthestraight-forwardapproachofreplacinglong-durationwithshort-durationvarieties,whichwasproposedbackintheyear2000bySetyantoetal.Allotherpossiblechangestoplantmorphologyandphysiologyshowedinconsistentresultsacrossdifferentstudies,duetocomplexgenetics,environmentandmanagementinteractionsthatdirectlyorindirectlyaltertheCH4budget(Wassmann,NeueandLantin,2000).Derivedfromplantmorphology,thelowpermeabilityoftheaerenchymacon-strainstheCH4transferfromthesoiltotheatmosphere(Butterbach-Bahl,PapenandRennenberg,1997;Aulakh,WassmannandRennenberg,2002).Sincethistraitwillalsolimitthetransferofoxygenintotherootsystem,whichhastheoppositeeffectonCH4fluxes,thenetimpactonCH4emissionswillvaryaccordingtospe-cificcircumstances,e.g.waterregimeandfertilizermanagement.Fromaphysiologicalperspective,rootexudationdeterminestheamountofmethanogenicmaterialandisthereforestronglylinkedtoCH4emissions(Luetal.,1999).Theactualamountofrootexudation,however,ischieflyaffectedbythenutrientstatusofthericeplant(Luetal.,2000),andconsequentlyitsimpactonCH4emissionsmaybeconcealedbyotherfactors.Ahighefficiencyofthephysi-ologicalcarbonsink,i.e.theallocationofmetabolitesinthegrain,hasbeenshowntobefavourableforlow-emissionplantsingreenhouseexperiments(DeniervanderGonetal.,2002)aswellasbymeansofgeneticallymodifiedorganisms(Suetal.,2015).Broadlyspeaking,early-maturingricecultivarswithfewunproduc-tivetillers,smallrootsystems,ahighrootoxidativeactivityandharvestindex,andalowrootexudationwereproposedformitigatingCH4emissionsinricefields(Malyanetal.,2016).However,amechanisticunderstandingisstillneededtoselectandbreedvarietiesthatemitlessCH4(Balakrishnanetal.,2018;Yagietal.,2020).7.6REDUCINGMETHANEFROMSTRAWBURNINGAlthoughCH4emissionsfromriceproductionaregenerallyequatedwithbiogenicemissionsfromfloodedfields,commonfarmingpracticesinmanyAsiancoun-triesalsogeneratesizeableamountsofpyrogenicCH4.OpenfieldburningentailsanincompletecombustionofricestrawandthisgeneratesCH4aswellas,toalesserextent,nitrousoxide(Romasantaetal.,2017).Despiteconsiderableeffortsto143Methaneemissionsinlivestockandricesystemseliminatethispractice,strawburningisstillrampantinmanypartsofAsia,causingenormousproblemsrelatedtolocalairpollution(Gadde,MenkeandWassmann,2009).Whilericestrawistypicallykeptinpilesonthefieldsafterharvest,thepro-portionofincompletecombustionisafunctionofthemoisturecontentinthesepilesandthereforeoflocalrainfallevents(Romasantaetal.,2017).Analternativemanagementpracticetostrawburningissoilincorporation,butstrawamendmentsincreasetheCH4emissionsoncethefieldgetsflooded.Basedonthe2019IPCCguidelines,thisincrementinemissionscouldbecurtailedthroughpropertimingofthesoilincorporation,i.e.theconversionfactorof“strawincorporatedlong(>30days)beforecultivation”is0.19asopposedtothebaselineof“strawincorpo-ratedshortly(<30days)beforecultivation”(conversionfactor=1).Theoptionsforexternalstrawuserelyonitsremovalfromthefield,whichconstitutesarelativelylaboriousactivitygiventhetypicallylowlevelsofmecha-nizationinmostrice-producingregionsatpresent.Strawcouldbeusedtomakecompostandthenreturnedtothefield.Whiletheconversionfactorofcompostisfairlylow(0.17)ascomparedtofreshstraw,thelowNcontentofstrawwillrequiresomeadditionalorganicmaterial,suchasanimalmanure,formakingcom-post(Nguyen-Van-Hung,2020).Strawcouldalsobefedtocattle,butitslownutri-tionalvaluewillcausesizeableCH4emissionsfromtheanimals.Inprinciple,strawrepresentsavaluablefeedstockforbioenergyasisshownbytheuseofwheatstrawinmanyindustrializedcountries.Ricestraw,however,hasahighsilicacontentthattendstocausetechnicalproblems(“slagging”)incombustiondevices(ChiengandKuan,2020).Moreover,acommercialusewillrequireforittobeavailableinacompactform,whichfacilitatestransportandstorage.Tothisend,theprevailingtrendtowardsthemechanizationofriceproduction,andthenewbalingmachinesinparticular,maytransformstrawintoareadilyavailablecommoditythatcanbetraded(Nguyen-Van-Hungetal.,2020).7.7CHOICEOFOPTIONSAppropriatewatermanagement,includingmid-seasondrainage,AWDandSRI,isthemostpromisingoptionformitigatingCH4emissionsfromfloodedricefields.Itwouldthusbeourfirstchoiceifpracticable,andprovidedthatsuchwatermanagementdidnotincreaseN2Oemissionsand/ordecreasesoilorganiccarbon.PreventingtheintroductionoffreshorganicmatterlikericestrawintothesoilisaneffectiveremedyagainstexcessiveCH4productionandtheresultinghighemissionsinricefields.SomeofthefertilizationtechniqueshavebeenconsideredforthemitigationofCH4–eitherasanadditionalmeasureorincasethewatermanagementoptionsdidnotprovepracticable.Sulfate-containingfertilizersmayhelptoreduceCH4emissions,buttheyarenotsuitableforsoilswithalowamountofreducibleironthatformsinsolubleironsulfide(FeS),becauseinthatcasethereducedsulphurion(S2-)woulddamagethericeroots.Biofertilizers(Azollaandblue-greenalgae)canoxygenatethesurfacesoil,reducingCH4productionandpromotingCH4oxidation,butwhethertheywillhaveadiscernibleimpactunderfieldconditionsismerelyspeculativeatthispoint.Theapplicationofironandsilica-containingmaterialsthatcanmaintainhigherredoxconditionsinsoilandtherhizospherecanalsoplayapartinreducingCH4emissions.Ameta-analysisofbiochartreatmentsdemonstrated144Mitigationofmethaneemissionsthemitigationpotentialofbiochar(Jeffreyetal.,2016),butthereisnoevidenceastotheapplicabilityofthisoptionatalargerscale.Manyoftheabove-mentionedmethodscouldcontributetobetterplantgrowthandhigheryields,thusreducingGHGemissionperyield,i.e.“GHGintensity”expressedastonnesofCO2eqperha,aswellasacarbonfootprintdefinedasemis-sionsperamountofproduct,andexpressedaskgCO2eqperkgofriceproduct.Asfortheprivatesector,theproduction-basedemissionsaremuchmorerelevantthanthearea-basedemissions.FutureeffortstoreduceCH4emissionsfromricemaybedrivenbyuser-friendlyandtransparentcalculationtoolsandlabelsindicat-ingtheproduct’scarbonfootprint(Wassmann,NeueandLantin,2022).Thiswillthenencompassawiderangeofproduction-enhancingapproaches,e.g.ricehybridtechnologies.Suchefficiencygainsintermsoffoodproductionhaveroutinelybeenconsideredasamitigationoptioninanimalsystems,buttheyhavehardlybeenmentionedinthecontextofriceproduction.7.8NEWLYEMERGINGTECHNOLOGIESInadditiontothecurrentlyavailablemethodsforreducingCH4emissionsfromrice,severalnewtechnologiesunderongoinginvestigationshowahighpotentialasmitigationoptions.Forexample,researchsuggeststhatplantgrowth-promotingrhizobacteria(PGPR)suchasdiazotrophscouldincreaserootmass,therebypro-motingmolecularoxygen(O2)releasetothesoil,andsuppressmethanogenesis(SinghandStrong,2016).ThepotentialoftransgenicstodecreaseCH4emissionsisdemonstratedbyusingabarleytranscriptiongene(Suetal.,2015).Microbialfuelcells(MFCs),whichgenerateelectricityinricefieldsoil,competewithCH4productionandcanthusmitigateCH4emissionfromtherhizosphere(Kouzuma,KakuandWatanabe,2014).Thesenewtechnologiesarestillintheirinfancyandfurtherinvestigationsandverificationsareneededbeforetheycanbeappliedatafieldscale(PrattandTate,2018).145Methaneemissionsinlivestockandricesystems8.Cross-cuttingmethanemitigation8.1GENERALGUIDANCEFORTAKINGANINTEGRATEDAPPROACHTOMETHANEMITIGATIONSTRATEGIESToreliablyassessthepotentialforCH4emissionreductionandensurethatrec-ommendedmitigationstrategiesareappropriateandthattheyminimizepotentialtrade-offs,thewideragriculturalandsystemiccontextandimplicationsmustbeconsidered.Inthissection,wegiveabriefoverviewofwhythesebroadercon-siderationsarenecessary,discussthetoolsdesignedtoensureholisticappraisalsandprovidesomeillustrativeexamplesofCH4reductionstrategiesthatarebeingconsidered.Agriculturalproductioninvolvescomplexinteractionsbetweenbiologicalsys-tems,time-andlocation-specificenvironmentalconditions,andmanagementprac-tices.Thesefactorsresultinconsiderableuncertaintyandvariationinagriculturalemissions(Dudleyetal.,2014).Interventionstargetingoneconcern(i.e.CH4emis-sions)canpromptmultifacetedinteractionswithothercomponentsofthesystem.Theseinteractionscanresultinwiderco-benefits;forexample,generalincreasesinproductionefficiencymayreduceemissionsofGHGsotherthanCH4,includ-ingN2OandCO2,alongsidereductionsinresourceuseandwiderenvironmentalimpacts(Capper,2011).Inothercases,trade-offsmaybeneeded.EffortstoreduceCH4emissions,forinstance,mayincreaseotherGHGs(Cardosoetal.,2016)orraiseconcernswithrespecttoanimalwelfare(Llonchetal.,2017).Similarly,inthecaseofricesystems,onemustconsiderthechangesinnetGHGs,payingattentiontoN2OandCO2emissionsinadditiontoCH4(Kriteeetal.,2018).ComprehensiveassessmentsthatcovermultipleimpactcategoriesaretypicallyprovidedbyanLCA,withanattributionalLCAbeingthemostcommonone.AnattributionalLCAtracksenergy,materialusesandpollutantreleasesoccurringalongthesupplychainandproductionprocessinordertoreportthetotalinven-toryor“footprint”thatcanbeattributedtoagivenoutputorfunctionalunit(ISO,2006).Thefunctionalunitmayeitherbeaproductorcommodityofacertainquali-ty(e.g.milk,fordairyproduction),oramorespecificaspectoftheoutputs(e.g.theproteinorcaloriecontent).Thechoiceofafunctionalunitdependsonthenatureoftheassessmentbeingmadeandtheintendeduseofthelife-cycleinformation.Theboundariesofalife-cyclesystemrelevanttoagivenquestionareextendedasfaraspossible,ideallystartingfromthepointofproductionofallinputs(“cradle”),capturingtheimpactsthathaveoccurredbeforetheagriculturalproductionphase,suchasenergy-useinthecaseofamanufacturingfertilizer.Inmanyagriculturalandfood-relatedLCAs,theproductionprocessistrackeduntiltheendoftheagri-culturalproductionphase(leavingthe“farm-gate”),asthisiswheremostimpactsareaccruedandwherechangingagriculturalpracticeshavethegreatestabilitytoreduceimpacts,butthechaincanalsobefollowedthroughtoprocessing,consump-tionanddisposal(foracomplete“cradle-to-grave”LCA).Disposalfromproduc-tion,suchasinfrastructureormanure,arepartoftheproductionsystem.Inthisway,anLCAprovidesausefulmethodologytoexploreCH4reductionstrategiesinabroadercontext.Whenadoptingalife-cycleperspective,welook146MitigationofmethaneemissionsbeyondthemerereductioninCH4emissionsthatmightbeachievedthroughdiffer-entmeasurestoconsiderthewiderassociatedimpacts,bothpositiveandnegative,suchasthoseassociatedwiththemanufacturingofCH4-reducingfeedadditives.Italsooffersthewholeproduction-systemvantagethatmakesitpossibletoiden-tifywiderco-benefitsorpotentialtrade-offs,asnotedabove.Alifecycleassess-mentgenerallycapturesemissionsperfunctionalunit,whichisoftenaproduct.However,fromaglobalperspective,itistheabsoluteemissionsthatmatterforassessingthemostextremeclimateconsequences.Somemitigationstrategiesmayreduceemissionintensitybyincreasingefficiency,whichmaythenfacilitategreaterproduction,resultinginincreasedabsoluteemissions.Whetheremissionintensityortotalemissionsarethemostrelevantaspectstocharacterizemitigationoutcomesdependsonthewiderpolicyanddevelopmentobjectives.Inadditiontoitsroleinsettingacomprehensiveframeworkforcompilingalife-cycleinventory,anLCAisalsocommonlyusedtoassesstheimpactsresultingfromthisinventory.Thisisdonebytranslatingtheinventorydataintopotentialimpactsofinterestthroughstandardizedreportingindicators.Theclimateimpactassess-mentcomponentofanLCA(oftenreferredtoasthe“carbonfootprint”)takestheinventorydataforindividualGHGemissionsandcombinesthemintoasingleclimateimpactindicator.AmoredetaileddiscussionofvariousmetricsthatcanbeconsideredforLCAisfoundinChapter6.ItmustbenotedthatGWP100isjustonepotentialclimateimpactindicator.Itisa“midpoint”indicator,onlypartofthewayalongthechainoftranslatingGHGemissionsintoaneventualcontributiontoclimatechangeandresultingdamages.Dependingon,forexample,thetimeframeortheaspectofclimatechangethatisofinterest,otherindicatorsmaybeequallyjustifiedyetgiveadifferentanswerastowhetheraspecificinterventionhasanoverallpositiveornegativeimpact.RecentguidancerecommendsconsideringmultiplemetricchoicesinLifeCycleImpactAssessments(Levasseuretal.,2016),includingtheGTP.TherelativevaluationofCH4,ashort-livedGHG,isparticularlysensitivetothemetricchoiceandtimehorizon.Part4ofthisreportdiscussestheusagetowhichdifferentGHGmetricsareputandhowtointerpretcontributionstoclimatechange.ThemetricssectionofthisdocumentexploresalternativewaysofquantifyingtheimpactofCH4mitigation.ClimateimpactismerelyonecomponentofatotalimpactassessmentinanLCA;waterscarcity,landuse,biodiversityloss,airandwaterpollutionaresomeoftheothercommonoutcomes.InterventionsaimingtoreduceCH4emissionscanbeweighedagainsttheseotherimpacts,notunlikewhenexploringtheinfluencetheymayhaveonotherGHGemissions,asdescribedinSection5.Thesewiderimpactcategoriescomewithstandard,simplifiedindicatorsdesignedtoreportresultsandprovideasimpleappraisalofrelativeperformance.Aswiththeassessmentofclimatechangeimpacts,theremaybedifferentindica-torsandmodellingapproachessuitablefordifferentpurposes,alongwithguidanceonhowtoexplorethesensitivitytodifferentmetricsandensurethemethodusedcanbesensiblyappliedtothequestionathand(Frischknechtetal.,2016).Whilewerecommendascomprehensiveanassessmentaspossible,whethertoexploreothercategoriesandwhichonestochooseinadditiontotheGHGemissionsisultimatelyatthediscretionoftheuser/investigator.Separateimpactcategoriescanalsobeconfrontedandcombinedintoaggregatedindicators,asinthe“disability-adjustedlifeyears”estimatingthetotalburdenonhumanhealth,thefinancial147Methaneemissionsinlivestockandricesystemsvaluationprovidingacommoncurrencyforallimpactsandoutputs,orabstractscoresactingasasimplecommunicationdevicefortotalimpacts.Thereis,however,nouniversallyagreeduponmethodofindicatorweighingoraggregating,anddoingsocanobscureindividualresults;itisthereforestandardpracticetoretainseparatereportingcategoriesinadditiontofullyaggregatedindicatorresults.Thesechallengesresultinlimitationsandpotentiallysubjectiveappraisalsinagri-culturalLCAs,asiswidelyacknowledged.Forexample,vanderWerf,KnudsenandCederberg(2020)arguedthatanLCAiscurrentlyill-equippedtoreliablyassesstheimpactsoforganicorlower-intensityagriculturebecausesomeimpactindicatorsremainweakwiththefocusonproduct-levelassessmentbeingtoonarrow.InthebroadercontextofassessingCH4reductions,itisimportanttonotethatanLCAmaygiveussomeinsightintoaswellasameansofquantifyingwiderbenefitsand/ortrade-offs,buttheLCAresultsdependonmethodologicalchoicesthatmaketheoutcomehighlyuncertain.Theremaybeotherconsiderationsthatdeterminehowpolicymakersandsocietyatlargecometoviewcertainsystemtransitionsaspositive.SomeofthesebroaderissuesmaybeaddressedthroughaconsequentialLCA–amethodthatlinksLCAdataandmethodologiestoconsequential(largelyeco-nomic)modelsofwhatmighthappeninresponsetochanges(e.g.changesintheproductionmethodorthetypeorquantityoffunctionalunitproduced),ratherthanjustcomparingindividualsystemimpacts.WhereanattributionalLCAallo-cateselementaryflowstoindividualproducts,whichmaythenbecompared,acon-sequentialLCAestimatesthedeviationsintheelementaryflowsresultingfromasystemchange(Rebitzeretal.,2004;EkvallandWeidema,2004).ConsequentialLCAmaybeparticularlyrelevantwhereproposedCH4-reductionmeasuresentailmajor,systemicimpacts,suchasaglobalshifttowardsmoreintensiveruminantproduction,orreductionsintotalruminantproduction.AfullreviewoftheconsequentialLCA,whichhasitsownchallengesandlimitations(YangandHeijungs,2018),isbeyondthescopeofthisreport.Itmay,however,provideanotherusefulapproachwhichhasnotbeenhithertosufficientlyexploredwhenitcomestoassessingspecificagriculturalinterventions.Concernsaroundlargerscaleassessmentmustalsobekeptinmindwhenconsi-deringthescalabilityofCH4mitigationmethods.Somepotentialmethodsthatareapplicabletointensivesystems,suchasfeedadditivesorCH4-inhibitoryvaccination,maynotbepossibleorappropriateformoreextensivesystems.Thiswilllimitthetotalmitigationpotentialassociatedwithagiventechnologyormanagementpractice.Insummary,thecomplexandinterlinkednatureofagriculturalproductionmeansthatwemustconsiderCH4reductionsinawidercontext,asfurtherexploredintheexamplespresentedbelow.AnLCAremainsavaluablemethodtoensurecomprehensivenessandhelpcompileinventoriesofactivitiesthatmaybeassociatedwithclimateandenvironmental(orother)impacts.Itcanalsoprovideguidanceandusefulframeworksforcarryingoutanenvironmentalimpactassessment.However,anexhaustiveanalysisoftheimpactsofagriculturalproductionsystems,theextenttowhichtheymaybedeemed“sustainable”,andthefullrangeofconsiderationsrequiredfordecision-making,mayneedadeeperappraisalandinterpretationthanwhattheLCAimpactindicatorsalonecanprovide.Thismayincludeanassessmentofwhetherreductionsinemissionintensityorabsoluteemissionsaretherelevantmeasureofsuccess.Thereisagrowingbodyofliteraturethatcontinuestodevelop148MitigationofmethaneemissionstheLife-CycleImpactAssessmentmethodologies,andsuggestsrefinementsastohowtheyareapplied.GiventhefocusofthisreportonCH4reduction,weprovideanextendeddiscussionofhowCH4emissionsinparticulararereported,butthewidercontextoutlinedhereremainsimportant.8.2LCASCENARIOANALYSISFORINTENSIVESYSTEMSLivestockfarmingsystemscontributetoGHGemissionsarisingdirectlyfromentericandmanureCH4,manureN2Oemissions,andindirectlyfromcropproduc-tion,soilemissions,andfossilfueluseforoperatingmachineryandmanufacturingofinputs(fertilizer,importedfeeds).Somemitigationoptions,particularlyfeedandmanureadditives,haveassociatedCO2andN2Oemissionsduringtheirproductionandtransportation.ItisthereforeimportanttoconsiderthenetreductionsintotalCO2eqemissionswhenpromotingaCH4mitigationstrategy.Methane-reducingdietaryformulations,feedadditivesandsupplementscanbeeffectiveinreducingentericCH4emissionsinbeeffeedlotsanddairies(Nguyen,2012;Beaucheminetal.,2020);however,thenetbenefits/burdensonCO2eqemis-sionsshouldbequantifiedbyincludingtherelatedlifecycleimpactofproducingsuchdiets,feedadditivesandsupplements.IntheCalifornianintensivedairysystem,FengandKebreab(2020)evaluatedthenetmitigatingeffectoftwofeedadditives,3-NOPandnitrate.Inthecaseof3-NOPthedietwasnotchanged,andsoonlytheadditionalemissioninproducing3-NOPwastakenintoaccountinthecalculations.Theauthorsreportedthattheemissionsassociatedwith3-NOPproductionwere35to52kgCO2eq/kg3-NOPproduced,dependingonhowandwheretheadditivewasproduced.Thetransportationofadditivestothefarmwasalsoincluded.Fornitrate,theemissionsassociatedwiththeproductionoftheadditiveaswellastheimpactofchangingthedietcompositionwereconsidered,asnitratesupplementa-tionreplacesothernitrogensourcesinthediet.Inameta-analysis,Dijkstraetal.(2018)reportedthat3-NOPonaveragereducedCH4productionandCH4yieldby32.5percentand29.3percent,respectively.Anotherrecentmeta-analysisbyFengetal.(2020)indicatedthatnitratereducedCH4productionandCH4yieldby14.4percentand11.4percent,respectively,inadose-responsemanner.Inthefinalanalysis,FengandKebreab(2020),usingacradletofarmgatesystemboundary(Figure5),reportedthattheaveragenetreductionrateswithsupplementationof3-NOPandnitrateintheCaliforniandairyfarmingsystemwere11.7percentand3.95percent,respectively,whenupstreamanddownstreamemissionswereincludedintheLCA.Animalproductionwasassumednottobeaffectedbytheinclusionoffeedadditive.Theimplementationofmitigationoptionsreviewedintheprevioussectionshasassociatedeffectsonthesystem,includingchangesindietcomposition,transporta-tion,manurecompositionandmanureapplicationtosoil.WhileOwensetal.(2020)reportedthatsupplementingbeefcattlewith3-NOPdidnotsignificantlyaffectmanureCH4emissionsduringstorage,othermitigationoptions,particularlythosethatchangethechemicalcompositionofthediet,shouldbeanalysedfordown-streamemissioneffects.Toanalysetheeffectofmorecomplexmitigationstrategies,combiningmitigationmeasuresormeasuresthathaveeffectsatdifferentlevelsofthefarm(e.g.animalvsmanuremanagement),anLCAapproachthatusesfixedemissionfactorsmaynothavesufficientcapacitytocapturetheinteractionswithinfarmcomponents,andthereforewouldbeunabletoevaluatepotentialtrade-offs149MethaneemissionsinlivestockandricesystemsofGHGmitigation.Forthesesituations,frameworksthatcaptureinternalfeed-backsandloopsbetweenfarmcomponentsarerequired(delPradoetal.,2013;Rawnsleyetal.,2016).Integratingwhole-farmmodellingwithLCA,forexample,canbeusedasaframeworktostudyclimatechangemitigationandadaptationinruminant-basedfarmingsystems(delPradoetal.,2013).ThistypeofframeworkhasbeenshowntoidentifyinparticularhoweffectiveGHGmitigationmeth-odsmay,insomecases,altertheemissionsofotherformsofpollutionandhaveverydifferentimpactsonbroaderaspectsofsustainability,includingprofitability(delPradoetal.,2010).Thedownsideofthistypeofapproachisitslackofavail-abilitybeyondacademiaandalevelofcomplexitywhichisgreaterthanthatofemissionfactor-basedframeworks.Formanuremanagement,themanureNappliedtosoilinfluencesfeedproduc-tionandcomposition,andthereforeaffectsanimalproductivity.Ahigh-fatdietfordairycattle,forexample,canreduceentericCH4emissionsbutitmayalsoincreasetheCH4productionpotentialoftheslurry(ifOMdigestibilityisdecreasedduetofatsupplementation)andthusleadtogreaterCH4emissionsfrommanureduringstorage(Petersenetal.,2013a).Hence,unlessanaerobicdigestionisusedtocapturethisadditionalCH4fromslurry,fat-richdietscouldresultinanegativeinteractionwithrespecttoGHGmitigation.Moreover,farmmodelshavebeenusedtoidentifypotentiallynon-additiveeffectsofcombinedmitigationmeasures,i.e.theeffective-nessofthecombinedmitigationmethodsmaynotbeequaltothesumoftheindi-vidualmethodswhenappliedontheirown(delPradoetal.,2010).8.3LCASCENARIOANALYSISFORLESSINTENSIVESYSTEMSLessintensivelivestockproductionsystemstendtohaveagreatershareoftheirtotalcarbonfootprintasCH4,especiallyfromentericfermentation.Incontrasttointen-siveruminantproductionsystemswhereentericCH4typicallycompriseslessthan40percentofthetotalCO2eq/kgofproductbasedonaGWP100(e.g.cattle:24per-cent,delPradoetal.,2013;sheep:25percent,Batallaetal.,2015;andgoats:39per-cent,Pardoetal.,2016),extensivesystemshavegreaterproportionsofentericCH4duetotheuseofforagesandlimiteduseofconcentrates,combinedwithfeweremis-sionsfromuseoffossilfuel.Insomeextensivesystems,entericCH4cancomprisemorethan70percentofthecarbonfootprintofmeatandmilkduetotheuseoflessdigestible,fibrousfeedandareducedlevelofanimalproductivity(Flysjöetal.,2011;Chobtangetal.,2016;SánchezZubietaetal.,2021).Typically,forpasture-basedsystems,themostdesirableproductionsystemisonethatefficientlyutilizeshighlevelsofgrazedpastureintheanimals’feedbudget,exploitsexistingfacilitiesonthefarmandreturnsthegreatestprofit(Crossonetal.,2011).OneentericCH4mitigationmeasureforgrassland-basedlivestocksystemsispasturequalityimprovement.BetterpasturerenewalpracticesordietimprovementshavebeenidentifiedaspromisingmeasurestoreduceentericCH4fromlowinputsystems(Goopy,2019).IncreasingthedigestibilityofforagehasbeenidentifiedasastrategytodecreaseentericCH4emissionintensity.However,themitigationeffectneedstobeanalysedcasebycase.Forexample,ashiftfromfeedinglessgrasstomorewhole-plantmaizesilagewasshowntoreduceNexcretionandentericCH4intensityby6percentand14percent,respectively,assimulatedbyafarmmodel(delPradoetal.,2011),althoughinsomesystemssilagemaynotbeanoption.However,suchachangeinfeedingstrategyrequiredlandusechangefrompasturetomaize150Mitigationofmethaneemissions(whichisnotpossibleformarginallands),leadingtosoilCandNlossesthatcouldbemuchgreaterthanemissionreductionsatanimallevel(VellingaandHoving,2011).Yan,HumphriesandHolden(2013)conductedanLCAtoassessGHGemissionsfrompasture-basedmilkproductionrelyingmainlyon(i)fertilizerNor(ii)whiteclover,andtheresultsindicatedthatthecarbonfootprintforwhitecloverwas11to23percentless(perkgofenergy-correctedmilk)thanthatoffertilizerN,suggestingclovercouldbeusedtoreducethecarbonfootprintofmilkfromgrazingdairycows.Similarly,Schilsetal.(2005)foundthatGHGintensityfromagrass-cloversystemwas10percentlowerthanthatfromagrass-fertilizerNsystem.Lahartetal.(2021)comparedtheeffectofgeneticmeritinHolstein-Friesiandairycowsacrossthreecontrastingfeedingpasture-basedproductionsystems(extensivetointensive).TheauthorsreportedthatimprovedgeneticmeritcombinedwithreducingconcentratesupplementationledtoageneralimprovementinGHGintensityaswellasanimprovedNuseefficiencywithinthecontextofpasture-baseddairyproductionsystems.Inagreement,vanderWeerdenetal.(2018)compared“improved”dairyproductionsystemsdesignedtoreduceNleachingwithexistingpasture-baseddairyproductionsystemsinNewZealand,andreportedthatlowerfeedsuppliesandassociatedlowerstockingratesofthe“improved”systemswerethekeydriversoflowertotalGHGemissions.ResearchalsoshowedthathighconcentratedietsleadingtoanincreasedaveragedailygainandshorterfinishingperiodsreducedCH4emissionsperunitofprod-uct(Lovettetal.,2005).BothPelletier,PirogandRasmussen(2010)andMurphyetal.(2017)reportedthatGHGemissionintensitiesweregreaterforbeeffinishedFigure5SystemboundaryofthelifecycleassessmentfortheCalifornianmilkproductionCO2,N2OCH4,CO2AdditiveproductionFarmgateOutputsEnergyTransportationDairycowsMilkproductionWater1kgECMFertilizerPesticideExcessmanureCropproductionLandapplicationManurestorageFixedCO2CO2,N2OCH4,N2OECM=energy-correctedmilk.ItshouldbenotedthatCO2fromanimaloriginisconsideredanetzerocarbonbalancewithplantsequestration(seeSection5).Source:AdaptedfromFeng,X.Y.&Kebreab,E.2020.NetreductionsingreenhousegasemissionsfromfeedadditiveuseinCaliforniadairycattle.PLoSONE,15(9).https://doi.org/10.1371/journal.pone.0234289151Methaneemissionsinlivestockandricesystemsatpasturethanonahigh-concentratediet.Althoughtheproportionsofentericfermentationweresimilarforpasture-basedandhighconcentrate-basedfinishingsystems,thequantitiesweresignificantlygreaterforpasture-basedfinishingsys-temsasemissionswereaccumulatedoveralongerproductionsystemcomparedtotheshorterconcentrate-intensiveproductionsystem.AnumberofstudieshaveshownthatslaughteringanimalsatayoungeragereducesGHGemissionsperanimalfinishedandperkgofcarcass.However,Tayloretal.(2020)contendedthatearlierageatslaughterdidnotnecessarilyleadtothegreatestprofitabilityowingtothelowergrossoutputvalueachieved.Inimprovedpasture-basedsystems,theabilitytoslaughteranimalsatayoungerageoftenleadstoagreaterstockingdensity,thusresultinginincreasedGHGemissionsperhectarecomparedwithmoreextensiveproductionsystems,althoughtherewouldbefeweremissionsperkgofbeef.Crossonetal.(2011)andMurphyetal.(2018)reportedthatincreasingoutputperhectareisoftenconsistentwithlowerGHGemissionintensity.Moregenerally,strategiesthatincreasedrymatterproductionperhectaretendtoreducetheemissionintensityoffoodproductionbutincreasetotalemis-sionsperhectare.Whetherreducingemissionintensityorreducingabsoluteemis-sionsperhectareistherelevantmeasureofsuccessdependsonoverallmitigationobjectivesthatmaydifferbetweencountriesandevenwithincountries,dependingondomesticpolicyframeworks.Higherintensitycanleadtolandsparingwithcon-stantoutputorenhancedoutputwithconstantlanduse,andthereforethesystemneedstoanalysetheseconsequentiallandusestoo.Grasslandscanbeacarbonsourceorsinkdependingonclimate,sitecharac-teristicssuchassoiltype,andmanagementpracticeslikegrazingmanagement,leveloffertilizerandlimeapplication,inclusionoflegumesandhistoricallanduse(Bellarbyetal.,2013).Inclusionofcarbonsequestrationfrompermanentgrass-landwouldsignificantlyimprovetheperformanceofpasturerelativetograin-basedproductionsystemsfromanetGHGemissionperspective(Soussana,TallecandBlanfort,2010).However,duetothetemporalandspatialuncertaintiesincalculat-ingthepotentialofsoilcarbonsequestration,carbonsequestrationisoftenomittedfrommodellingstudiesofpasture-basedruminantsystems(Crossonetal.,2011).ThesameappliestoGHGemissionsduetolandusechangethatmightresultfromlowerproduction,dependingontheeconomyandpolicies.152PART4Metricsforquantifyingtheimpactofmethaneemissions9.IntroductionThedistinctchemicalandphysicalpropertiesofdifferentGHGs,andtheirultimateeffectsonglobalwarming,bothintermsofthestrengthanddurationofanyclimateimpacts,aregenerallywell-understoodandscientificallyuncontested.Forthepur-posesofmostclimatescience,wecanworkdirectlyfromourphysicalunderstand-ingofindividualgases,usingclimatemodelsofvaryingcomplexitytoexplorethecontributionofdifferentGHGstoglobalwarmingandotherclimateimpacts,ortoquantifythebenefitsofpotentialemissionreductions.Emissionmetricscanprovideameansofcomparingdifferentgreenhousegasemissionsbyputtingthemontoonescale,typicallybyquantifyingaspecifiedclimateimpactofanon-CO2gasrelativetothatofaCO2emission,reportedas“CO2-equivalents”.Emissionmetricsareusedforavarietyofpurposes,inparticularforreportingandmonitoringemissionsattheglobal,national,regionalorinstitutionallevels;tradingemissionsofdifferentGHGsagainsteachother;aidingmitigationdecision-making,especiallyintrade-offsituationswhenreducingonegasisverycostlybutreducinganotheroneismuchlessso,orwhendecreasingtheemissionsofoneGHGcontributestoincreasingtheemissionsofanother.Inprinciple,emissionmetricscanalsobeusedtocomparetheeffectofnon-gaseousclimateforcers(e.g.aerosoloralbedochange;Collinsetal.,2013;BrightandLund,2021)withthatofgreenhousegasemissions.However,therearesomeimportantdifferences,inthattheclimateimpactofaerosolemissionsdependsstronglyonthelocationofemissionsandcanhavevariableimpactsonprecipitation.Inthisreport,ourfocusisonmetricsforgreenhousegasemissionsonly,primarilyCH4,andtoalesserextentN2O.Wewillthereforeusethe“GHGemissionmetrics”terminology.ThefollowingdefinitioncomesfromtheglossaryoftheIPCC’sSixthAssessmentReport:Greenhousegasemissionmetric:Asimplifiedrelationshipusedtoquantifytheeffectofemittingaunitmassofagivengreenhousegasonaspecifiedkeymeasureofclimatechange.ArelativeGHGemissionmetricexpressestheeffectfromonegasrelativetotheeffectofemittingaunitmassofareferenceGHGonthesamemeasureofclimatechange.Therearemultipleemissionmetricsandthemostappropriatemetricdependsontheapplication.GHGemissionmetricsmaydifferwithrespectto(i)thekeymeasureofclimatechangetheyconsider,(ii)whethertheyconsiderclimateoutcomesforaspecifiedpointintimeorintegratedoveraspeci-fiedtimehorizon,(iii)thetimehorizonoverwhichthemetricisapplied,(iv)whethertheyapplytoasingleemissionpulse,emissionssustainedoveraperiodoftime,oracombinationofboth,and(v)whethertheyconsidertheclimateeffectfromanemissioncomparedtotheabsenceofthatemission,orcomparedtoareferenceemissionslevelorclimatestate.Notes:MostrelativeGHGemissionmetrics(suchastheGlobalWarmingPotential(GWP),GlobalTemperaturechangePotential(GTP),Global155MethaneemissionsinlivestockandricesystemsDamagePotential,andGWP),useCO2asthereferencegas.Emissionsofnon-CO2gases,whenexpressedusingsuchmetrics,areoftenreferredtoas“CO2equivalent”emissions.Ametricthatestablishesequivalenceregardingonekeymeasureoftheclimatesystemresponsetoemissionsdoesnotimplyequivalenceregardingotherkeymeasures.Thechoiceofametric,includingitstimehorizon,shouldreflectthepolicyobjectivesforwhichthemetricisapplied(IPCC,2021b,p.2232).Awiderangeofemissionmetricshavebeenproposed.Asdifferentgreenhousegasesarenotdirectanaloguesofeachother,withdifferencesinhoweachemissionaffectstheclimateovertime,anydefinitionof“equivalence”reliesonajudgmentaboutwhataspectisbeingcompared.Consequently,differentemissionmetricssome-timesprovidestrikinglydivergentresults,despitebeingbaseduponthesamephysi-calunderstandingoftheeffectsofGHGemissionsontheclimate.Thedifferencesbetweenmetricsrestonparticularaspectsofthephysicalresponsethatareusedasproxiestorepresentclimatechangeoveratimehorizonthatneedstobedetermined.Forshort-livedspeciessuchasCH4,theCO2-equivalencecandiffersubstantiallybetweendifferentmetrics,whereasforlonger-livedspeciessuchasN2O,thevaluesprovidedarerelativelyconsistentacrossdifferentmetricsontimescalesofuptoacentury.Afundamentalconclusionfromthescientificliteratureonmetricsisthatthemostappropriatemetricdependsontheobjective(i.e.onthespecificenvironmentalorclimaticinformationbeingsought,orthepolicyquestiontobeaddressed,andoverwhichtimehorizon).Forsomeapplications,theremaybeexternalrequirementstouseaspecificemissionmetric.Forexample,theParisRulebookstatesthatcountriesmustreporttheiremissionsusingthe100-yearGlobalWarmingPotential(GWP100),andtheGWP100isthedefactostandardmetricforarangeofotherpurposes.ThisisdespitethecautiousnoteaddedbytheIPCCwhenitintroducedtheGWPinitsFirstAssessmentReportin1990.Specifically,theauthorsnoted,“Itmustbestressedthatthereisnouniversallyacceptedmethodologyforcombiningalltherelevantfactorsintoasingle[metric]…Asimpleapproach[i.e.theGWP]hasbeenadoptedheretoillustratethedifficultiesinherentintheconcept”(IPCC,1990;bracketsaddedbyShine,2009).Apartfromtheconceptualconsistencybetweenmetricsandpolicyobjectives,relevantconsiderationscanalsoincludethescientificuncertaintyofmetricvalues,theeaseofcommunicationandthetangiblerelevanceofametricforavarietyofstakeholdersanduses(e.g.thelinkbetweenphysics-basedmetricsandtheirinter-pretationinaneconomicorbroaderpolicycontext),andtheconsistencyorcompat-ibilityofanygivenmetricwithexistingclimatechangetargetsandobligations(e.g.Balcombeetal.,2018).Giventhiswidesetofcriteria,mostmetricsarereasonablywellsuitedforsomeapplicationsandlesswellsuitedforothers.Forsomeapplica-tions,emissionmetricsmaynotbenecessaryatall.Theultimatechoicemustbebetweenusingalargenumberofdifferentmetricsforthesakeofscientificorpolicycompletenessandoptingforasmallsetofmetricsthatmaybeimperfectbutcouldbeconsideredgoodenoughforarangeofapplicationsorapragmaticpolicychoice.Inthischapter,weexpandonthesepoints,describingandexplainingsomeofthekeyemissionmetrics,anddiscusshowtheymightrelatetodifferentscientificorpolicyconcerns.Wewillguidethereaderthroughthemeaningsandimplicationsofsomekeymetrics,withsimplifiedillustrationsoftheiruses.Thisdescriptionis156Metricsforquantifyingtheimpactofmethaneemissionsprimarilyaimedataidingthoseinvolvedinmakingbaselineassessmentsandgreen-housegasmitigationchoiceswithinagriculturalsupplychains.9.1CONTEXTANDDEFINITIONS9.1.1KeyprinciplesofGHGemissionmetricsTheprimaryroleofgreenhousegasemissionmetricsistohelpprovideinformationonhowdifferentgreenhousegasemissions(oractivitiesemittingthem)contributetocli-matechangeandassociatedimpacts(e.g.Fuglestvedtetal.,2010)or,conversely,onthebenefitsthatavoidinganygivenemission(s)wouldbringbynotcontributingtoclimatechangeanditsimpacts.Thismaytaketheformofdescribinghowdifferentactivitiesorsectorscontributetooverallclimatechangeorclimatechangeimpacts,assessingtheprioritiesandtrade-offsassociatedwithemittingormitigatingdifferentGHGs,oraid-ingdecision-makingandidentifyingthemostefficientwaystomeetoverarchingcli-matetargets.Emissionmetricsprovideashortcutinthecause–effectchainandtranslateemissionstoimpacts,asshowninthefigurebelow.Emittingagreenhousegasincreasestheatmosphericconcentrationofthatgasforacharacteristiclengthoftime,dependingonhowlongittakesforthatgastoFigure6Thecause–effectchainfromemissionstoclimatechangeimpactsEmissionsAtmosphericconcentrationsMetricsRadiativeforcingDevelopmentofMeasurestoqualifyClimatechangemitigationstrategies,impactofemissionsImpactsincludingIncreasingmitigationcosts,policyrelevancedamagecosts,discountratesIncreasinguncertaintyItillustratestheroleofmetricsindefiningtheestimatedresponsestoemissions(left)andinthedevelopmentofmulticomponentmitigationstrategies(right).Therelevanceofthevariouseffectsincreasesfromemissionstoimpactsbuttheuncertaintyincreasesaswell.Thedottedlineontheleftshowsthateffectsandimpactscanbeestimateddirectlyfromemissions,whilethedottedlineontherightsideindicatestheuseoftheseestimatesinthedevelopmentofstrategiestoreduceemissions.(AdaptedfromFuglestvedt,J.S.,Berntsen,T.K.,Godal,O.,Sausen,R.,Shine,K.P.&Skodvin,T.2003.Metricsofclimatechange:Assessingradiativeforcingandemissionindices.ClimaticChange,58:267–331.https://doi.org/10.1023/A:1023905326842andPlattnerG.-K.,Stocker,T.,Midgley,P.&Tigno,M.2009.IPCCExpertmeetingonthescienceofalternativemetrics,Oslo,Norway,18-20March2009.www.ipcc.ch/site/assets/uploads/2018/05/expert-meeting-metrics-oslo.pdf).Source:FigurereproducedfromMyhre,G.,Shindell,D.,Bréon,F.-M.,Collins,W.,Fuglestvedt,J.,Huang,J.,Koch,D.,Lamarque,J.-F.,Lee,D.,Mendoza,B.,Nakajima,T.,Robock,A.,Stephens,G.,Takemura,T.&Zhang,H.2013.Anthropogenicandnaturalradiativeforcing.In:T.F.Stocker,D.Qin,G.-K.Plattner,M.Tignor,S.K.Allen,J.Boschung,A.Nauels,Y.Xia,V.Bex&P.M.Midgley,eds.Climatechange2013:Thephysicalsciencebasis.ContributionofWorkingGroupItotheFifthAssessmentReportoftheIntergovernmentalPanelonClimateChange.Cambridge,UK&NewYork,USA,CambridgeUniversityPress.www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_Chapter08_FINAL.pdf157Methaneemissionsinlivestockandricesystemsbreakdownordissipateintheatmosphere.3One-off(pulse)emissionsofshort-livedgasessuchasCH4(withanaverageatmosphericlifetimeofaroundadecade)willraiseatmosphericconcentrationsforacoupleofdecades,whileemissionsoflong-livedgasessuchasN2O(withanaverageatmosphericlifetimeofaroundacentury)willresultinmoreprolongedconcentrationincreases.Carbondioxidehasacomplexatmosphericlifetimeasitisremovedfromtheatmospherebyvariousprocesseswithdifferentrates,butitcanlargelybeconsideredanextremelylong-livedgas,withasignificantfractionofemissionsremainingintheatmosphereformillennia(Archeretal.,2009;Joosetal.,2013).Figure7showsthedifferenteffectsonradiativeforcingandtemperaturechangeforonegigatonne(Gt)ofCO2,CH4andN2Opulseemissions.Theremainderofthissectionwillexplainthekeyprin-cipleswhichunderlieGHGemissionmetrics.Figure7Differenteffectsonradiativeforcingandtemperaturechangeforonegigatonne(Gt)ofCO2,CH4andN2Opulseemissions(a)Radiativeforcing(b)Integratedradiativeforcing0.2040.15Wm-2Gt-1Wm-2yrGt-130.1020.0510.000050100150050100150(c)0.06Temperature0.050.04CGt-1CO2×200.03CH40.0250100N2O÷100.010.001500(a)solidlinesaretheglobalmeanradiativeforcingchangefollowingapulseemissionofeachgas.TheabsoluteGWPforeachgasisdefinedastheareaundereachcurve(hatched)uptothechosentimehorizon.(b)linesrepresenttheareasunderthecurvesintheleft-handpanel.TheabsoluteGWPisthevalueofthecurveatthechosentimehorizon.(c)linesrepresenttheglobalmeantemperaturechangefollowingapulseemissionofeachgas.TheabsoluteGTPforeachgasisdefinedasthevalueofthecurveatachosentimehorizon.Contributionsfromeachgashavebeenscaledbydifferentmultiplierstomakeiteasiertocomparedifferentgasesonthesamegraph.Source:Authors’ownelaboration.3Thelifetimeofagreenhousegasisthetimeittakesfortheincreasedconcentrationarisingfromaninstantaneouspulseemissiontodecayintheatmosphere.Forgasesfollowinganexponentialdecay,thelifetimeischaracterizedbyitsexponentialdecayconstant.158MetricsforquantifyingtheimpactofmethaneemissionsChangesingreenhousegasconcentrationsimpacttheclimatebychangingtheatmosphericenergybalance(radiativeforcing).Theextenttowhichagivenchangeintheconcentrationofagasleadstoradiativeforcingisknownasits“radiativeeffi-ciency”,andcanberegardedasameasureofthe“greenhousestrength”ofdifferentgases(Forsteretal.,2021).Anygasthatwarmsthesurfaceperturbstheterrestrialandoceaniccarbonfluxes(Aroraetal.,2020),typicallycausinganetfluxofCO2intotheatmosphereandhencefurtherwarming.ThisaspectisalreadyincludedinthecarboncyclemodelsthatareusedtogeneratetheclimateeffectsofapulseofCO2(Joosetal.,2013),soforconsistencythisalsoneedstobeincludedfornon-CO2gases(GillettandMatthews,2010;Gasseretal.,2017).ThemetricvaluesprovidedbyIPCC’sAR6(Forsteretal.,2021)thereforenowincludethecarboncycleresponsebydefault.Theimpactoftheemissionsofchemically-reactivegasesonothergreenhousegasesalsoneedstobeaccountedfor.Forexample,asCH4breaksdownintheatmo-sphere,itleadstotheformationoftropospheric(loweratmosphere)ozoneandstratospheric(thelayeroftheatmosphereabovethetroposphere)watervapour.Increasedconcentrationsoftroposphericozoneandstratosphericwatervapouralsoresultinradiativeforcing,andCH4emissionmetricsgenerallyincludetheseindirecteffectsintheirassessmentoftheeffectofCH4emissionsontheclimate(seeForsteretal.,2021).Asthephysicaldriverbywhichclimateisaffected,radiativeforcingpresentsapotentialproxymeasureof“climaticimpacts”tocompareemissionsofdifferentgasesandisusedasthepointofcomparisoninthemostcommonGHGemissionmetric,theGlobalWarmingPotential(GWP,Section9.1.2.1).Itisalsopossibletocontinuealongthecause–effectchain(seeFigure6),andbasecomparisonsontheexpectedclimatechange(e.g.increaseinglobaltemperature)thatwillresultfromthisradiativeforcing.Anotherrelativelycommonemissionmetric,theGlobalTemperaturechangePotential(GTP,Section9.1.2.2),takesthisapproach,com-paringemissionsonthebasisoftheirrelativecontributiontoglobaltemperaturechangeataspecificpointintimefollowingtheemission.Metricscanprogressfurtherstilltoquantifyimpactsasthedamagesresultingfromclimatechange,forexampleeconomicdamages(Hammittetal.,1996)orindividualenvironmentalimpactssuchasprecipitationandsea-levelrise(Shineetal.,2015;Sterner,JohanssonandAzar,2014;Kirschbaum,2014).AshighlightedinMyhreetal.(2013),usingapointofcomparisonfurtheralongthecause–effectchaincanprovidemoredirectinformationneededforcommunicatingimpactsandinformingdecision-making,butitalsoaddstogreateruncertaintyasmoreprocessesmustbemodelledateachstepalongthecause–effectchain.SomeoftherelativelysimplephysicalmetricssuchasGWPandGTPcanalsobelinkedtocost-benefitandcost-effectivenessapproachestoclimatepolicyinspecificcontexts(seeSection9.2.2andSection9.2.3fordetails).9.1.2Pulse-emissionmetricsMostgreenhousegasemissionmetricsarebasedonthecomparisonofapulseofemissionsof1kgofonegastoanother,andprovidearelativevaluationor“exchangerate”forcomparingtheimpactsofthoseemissions.Thisvalua-tionistypicallymadeinrelativeterms,withCO2takenasthereferencegastoprovideasingleweightingfactortoconvertemissionsofnon-CO2gasestoa159MethaneemissionsinlivestockandricesystemsCO2-equivalent(CO2eq)quantity;thevaluesinTable6andTable7showhowmanykgofCO2a1kgemissionofCH4isequivalentto.Differentgasesdifferbothintheirclimaticimpactsandatmosphericlifespan.Thequantificationandcomparisonbetweendifferentgasesthereforerequiresapriordefinitionoftheassessedclimateimpactandrelevanttimehorizon.EventhoughGWPisarela-tivelysimplemetricbasedonphysicalsciencealone,itcanserveasaproxyformet-ricsthatevaluatethedamageduetoemissionsfromaneconomicperspective(Toletal.,2012).GlobaldamagepotentialsarediscussedinSection9.2.3.9.1.2.1GWPThemostcommonGHGemissionmetric,theGlobalWarmingPotential(GWP),comparestheradiativeforcingaccumulatedoverauser-definedtimehorizonresult-ingfromapulseemissionofaspecificGHGcomparedtoapulseemissionofanequalmassofCO2.Themostfrequentlyused,andeffectively“standard”,versionofthismetricisthe100-yearGlobalWarmingPotential(GWP100).Itisdefinedasthetotalradiativeforcingoccurringoverthesubsequent100-yearperiodafteraGHGemission,relativetothatofapulseemissionofCO2ofequalmass.Myhreetal.(2013,p.711)putitasfollows:“AdirectinterpretationisthattheGWPisanindexofthetotalenergyaddedtotheclimatesystembyacomponentinquestionrelativetothataddedbyCO2”.Forshort-livedgreenhousegases,suchasCH4,GWPvaluesvarysignificantlydependingonthetimehorizonused.Withincreasingtimehorizons,therelativevalu-ationofshort-livedvslong-livedgasesdeclines,asthereisanextendedperiodoverwhichthelong-livedgascontinuestoexertaradiativeforcingeffectontheclimatewhiletheshort-livedgasisnolongerintheatmosphereandcannolongerexertadirectradiativeeffect.ThisisshowninTable6below(GWPvaluesfromtheIPCC’sAR6,Forsteretal.,2021),wherethe20-yearGWPforCH4ismuchgreaterthanits100-yearGWP.Nitrousoxidehasalifetimeofoveracentury,soitsGWPvaluesarelesssensitivetothechoiceoftimehorizon(upto100years,atleast)thaninthecaseofCH4(Table6).Therearelargeuncertaintiesinallmetrics(30-40percent)duetouncertaintiesintheradiativeefficiencyofdifferentgasesaswellasindirecteffects,anduncertaintyastotheatmosphericlongevityofCO2andanygasesthatCO2iscomparedwith.ThecategorizationofCH4aseither“fossil”or“non-fossil”dependsonwhetherthecarbonintroducedintotheatmosphereisconsiderednewornot(oralreadyincludedinbudgets)(seeSection9.2.7).Table6.GWPvaluesfromtheIPCC’sSixthAssessmentReport(AR6)GWP20GWP100FossilCH482.5+/-25.829.8+/-11Non-fossilCH479.7+/-25.827.0+/-11N2O273+/-118273+/-130Source:Forster,P.,Storelvmo,T.,Armour,K.,Collins,W.,Dufresne,J.-L.,Frame,D.,Lunt,D.J.,Mauritsen,T.,Palmer,M.D.,Watanabe,M.,Wild,M.&Zhang,H.2021.TheEarth’senergybudget,climatefeedbacks,andclimatesensitivity.In:V.Masson-Delmotte,P.Zhai,A.Pirani,S.L.Connors,C.Péan,S.Berger,N.Caud,Y.Chen,L.Goldfarb,M.I.Gomis,M.Huang,K.Leitzell,E.Lonnoy,J.B.R.Matthews,T.K.Maycock,T.Waterfield,O.Yelekçi,R.Yu&B.Zhou,eds.Climatechange2021:Thephysicalsciencebasis.ContributionofWorkingGroupItotheSixthAssessmentReportoftheIntergovernmentalPanelonClimateChange,pp.923–1054.Cambridge,UK&NewYork,USA,CambridgeUniversityPress.https://doi.org/10.1017/9781009157896.001160MetricsforquantifyingtheimpactofmethaneemissionsTable7.GTPvaluesbasedonformulaefromtheIPCC’sSixthAssessmentReport(AR6)GTP20GTP100FossilCH454+/-217.5+/-2.9Non-fossilCH452+/-214.7+/-2.9N2O297+/-134233+/-110Source:Forster,P.,Storelvmo,T.,Armour,K.,Collins,W.,Dufresne,J.-L.,Frame,D.,Lunt,D.J.,Mauritsen,T.,Palmer,M.D.,Watanabe,M.,Wild,M.&Zhang,H.2021.TheEarth’senergybudget,climatefeedbacks,andclimatesensitivity.In:V.Masson-Delmotte,P.Zhai,A.Pirani,S.L.Connors,C.Péan,S.Berger,N.Caud,Y.Chen,L.Goldfarb,M.I.Gomis,M.Huang,K.Leitzell,E.Lonnoy,J.B.R.Matthews,T.K.Maycock,T.Waterfield,O.Yelekçi,R.Yu&B.Zhou,eds.Climatechange2021:Thephysicalsciencebasis.ContributionofWorkingGroupItotheSixthAssessmentReportoftheIntergovernmentalPanelonClimateChange,pp.923–1054.Cambridge,UK&NewYork,USA,CambridgeUniversityPress.https://doi.org/10.1017/9781009157896.0019.1.2.2GTPAnotherrelativelycommonmetricistheGlobalTemperaturechangePotential(GTP).ItcomparesthetemperatureincreaseresultingfromapulseemissionofaspecificGHGcomparedtotheeffectofapulseemissionofCO2ofequalmass,ataspecificuser-definedpointintimeaftertheemission(Shineetal.,2005).Forexam-ple,the20-yearGTPofCH4representstheincreaseinglobalaveragetemperatureresultingfromapulseCH4emissioncomparedtothatofapulseCO2emissionofthesamemass20yearsaftertheseemissions.The100-yearGTPprovidesthesamecomparison100yearsaftertheemission(i.e.foremissionsoccurringintheyear2023,itcomparesgasesbasedonthetemperatureincreaseresultingfromtheseemissionsintheyear2123).AsshowninTable7,theGTPforshort-livedgasesishighlysensitivetothechoiceoftimehorizon.TheglobaltemperaturechangepotentialismoresensitivetothechoiceoftimehorizonthantheGWPbecauseitisanend-pointmetricthatcomparesimpactsonlyattheend-pointofthespecifiedtimehorizon,whereastheGWPintegratesimpactsoverallindividualyearswithinthetimehorizon.Asanintegratedmetric,theGWPprovidesinsightsintototalimpacts(withradiativeforcingastheproxyimpactmeasure)thatresultfromagivenemissionoverthewholetimehorizon.Thiscanbeappropriatefortryingtoreducetheoverallpotentialdamageswhentheeffectdependsonhowlongthechangeoccursfor,notjusthowlargethechangeisatasinglefuturepointintime.Incontrast,theGTPasanend-pointmetricprovidesinformationaboutimpacts(withtemperaturechangeastheproxyimpactmeasure)onlyfortheindividualyearspecified.AkeyapplicationofGTPwouldbeforthequantificationofthecontributionstotheemissionofdifferentgaseswiththegoalofnotexceedinganysettemperaturetargetataspecificfuturepointintime.Theglobaltemperaturechangepotentialcanalsobeappliedtoasustainedconstantchangeinemissions(i.e.anemissionof1kgofgasperyear,insteadofasingleemis-sion)andisthenknownasthesustainedGTPorGTPs(Shineetal.,2005).AnotherrelatedmetricistheintegratedGTP,e.g.iGTP100integratesGTPover100years,andhasvalueswhicharesimilartoGWP100(Petersetal.,2011).WehavenotshownvaluesforthesustainedorintegratedGTPhere,astheydonotappearintheAR6.9.1.3Step-pulsemetricsDuetothestronginfluenceofthechosentimehorizononthepulse-emissionmet-ricsfortheshorter-livedspeciesdescribedabove,alternativesforcalculatingclimateequivalencehavebeendeveloped.“Step-pulse”equivalencehasbeenproposedasanalternativemeansofcomparingtheemissionsoflong-andshort-livedgreenhouse161Methaneemissionsinlivestockandricesystemsgases.Thistypeof“equivalence”ispossiblebecauseasinglepulseemissionofCO2andasustainedstep-changeincreaseinCH4emissionshavesimilarimpactsonglobalmeantemperatureincreases(Allenetal.,2022a).Thisapproachcanbethoughtofasdefiningequivalencebyworkingbackwardsfromtherespectivetemperatureout-comes.IfanindividualCO2emissionhasacertainimpactonglobaltemperature,isitpossibletodefineequivalentCH4emissionsthatwouldresultinapproximatelythesametemperatureimpact?Anumberofpaperspublishedoverthepastdecade(e.g.Smithetal.,2012;Lauderetal.,2013;Allenetal.,2016;Collinsetal.,2020)havesuggestedthatthiscanbeachievedbyequatingapermanentstep-changeintherateofCH4emissionstoanindividualpulseofCO2emissions,asbothwouldresultinasimilarincrementalincreaseinlong-termglobalmeantemperature.Analternativeperspectiveresultingfromthistypeofequivalenceisthattheglobalmeantempera-tureeffectovertimeofanindividualCH4emissionismoreakintoalargeCO2releasefollowedbyasubsequentremovalofaslightlysmalleramountofCO2,ratherthantoasingleindividualpulseofCO2emissions(Allenetal.,2021).Asthistypeofequivalenceisbasedonmatchingeventualwarmingoutcomesoftheemissionsbeingdescribed,ithasbeensuggestedthatstep-pulsemetricscanreportaCO2-warmingequivalence(CO2-we),incontrasttoCO2eqfrompulse-emissioncomparisons(Cainetal.,2019).Therewereearlierattemptstomatchwarmingorforcingoutcomesunderascenariobasedonpulse-orstep-basedmet-rics(Wigley,1998;Tanakaetal.,2009a,2013).Understep-pulsemetrics,introducinganewsustainedCH4emissionfromasource(i.e.astep-changefromnoemissiontoaconstantemission)couldbeconsideredasequivalenttoalargeone-offpulseofCO2emission,bothresultinginsignificantaddi-tionalwarming.ThisnewsustainedCH4sourcewilldriveanincreaseintemperatureoverthefirstfewdecadesafteritsintroduction.Afterthis,thetemperaturewillgradu-allystabilize,butatahighertemperaturethanbefore,asongoingemissionswillbebal-ancedbythechemicalreactionswhichwilldestroyatmosphericCH4followingafewdecadesofstableCH4emissionrates.This,inturn,willcausestableatmosphericcon-centrationsofCH4andastablecontributiontoradiativeforcing.Additionalwarmingwillcontinueatamuchlowerrateforseveralcenturies,astheclimatefullyadjuststotheelevatedradiativeforcing(Cainetal.,2019;Smith,CainandAllen,2021).ThisscenarioisshownbythemiddleofthethreepanelsinFigure8,whichillustratesconstantemis-sionsofCO2andCH4andthelevelofwarmingtheyeachgenerate.IfthesustainedCH4emissionsarereducedatanypoint,CH4concentrationswilldeclineasnaturalremovalscontinuewithouttheremovedCH4beingreplaced.Thiswillthenleadtolowertemperatures(rightcolumninFigure8).TosimilarlyreducethelevelofwarmingfromanearlierCO2emission,itwouldhavetobeactivelyremovedfromtheatmosphere.Assumptionsaboutpastemissionsandtheclimaticimpactstheymaystillbeexerting,andhowtodefineexistingornewsources,thushavealargeimpactonthecalculatedequivalentCO2emissionsthatwouldresultinthesametemperaturechange(i.e.CO2-warmingequivalence).“Step-pulse”equivalencehasbeendefinedviaasmallnumberofapproaches.Oneapproach(referredtoasGWP,denotingamodifiedGWPapproach)estimatestheequivalenceintermsofaglobalmeansurfacetemperatureincreasebetweenasus-tainedflowofCH4emissionsandanindividualpulseemissionofCO2(Allenetal.,2016).Thisapproachhasbeenupdatedtoimprovetheaccuracyoftherelation-shipbetweentheCO2-warmingequivalentemissionscalculatedusingGWPand162MetricsforquantifyingtheimpactofmethaneemissionsFigure8Anillustrationofhowrising(left),constant(middle)andfalling(right)emissionsofCO2(red)andCH4(blue)affectlevelsofglobalwarmingRisingemissionsConstantemissionsFallingemissionsEmissionsCO2EmissionsCO2EmissionsCH4CH4TimeCH4CO2TimeTimeCO2CO2CO2WarmingCH4WarmingCH4WarmingTimeTimeCH4TimeForbothCO2andCH4,risingemissionsdrivetemperaturesup.Forconstantemissions,CO2drivestemperaturesupataslowerratethanforrisingemissions,butforCH4thelevelofwarmingisonlyveryslightlyrising.Forfallingemissions,CO2continuestodrivetemperaturesupuntilemissionsareeliminated.ForCH4,fallingemissionsleadtofallingtemperatures.ThisfundamentaldifferencebetweenCO2andCH4iswhypulse-emissionmetricsdonotreflecttemperaturechangesarisingfromshort-livedpollutantsaccurately,andwhystep-pulsemetricsweredevelopedtoassesstemperatureoutcomes.Source:FigurereproducedfromAllen,M.R.,Lynch,J.,Cain,M.&Frame,D.2022b.Climatemetricsforruminantlivestock.Oxford,UK,OxfordMartinProgrammeonClimatePollutants.https://www.oxfordmartin.ox.ac.uk/downloads/reports/ClimateMetricsforRuminentLivestock_Brief_July2022_FINAL.pdfmodelledtemperature(Cainetal.,2019;Smith,CainandAllen,2021).Lynchetal.(2020)demonstratedthevalidityofGWPinawiderrangeofscenarios,exploringitsusetoestimatetemperatureresponsestonon-globalemissiontrajectories,whileCainetal.(2021)usedGWPtoevaluatescenarioswhichaimtoachievetheParisAgreementtemperaturegoals.TheequationtoconvertaCH4emission–CH4(t)–toaCO2-warmingequivalent(CO2-we(t))–emission,usingGWP,is:CO2-we(t)=GWP100×(4.53×CH4(t)-4.25×CH4(t-20))whichsimplifiesto:CO2-we(t)=8×CH4(t)+120×∆CH4(t)whereGWP100isthenormalGWPforpulseemissionsofCH4andCO2fromAR5(followingSmith,CainandAllen,2021andForsteretal.,2021);CH4(t)andCH4(t-20)arethecurrentCH4-emissionratesandthose20yearsearlier;and∆CH4(t)=CH4(t)-CH4(t-20)isthedifferenceinCH4-emissionratebetweentimetand20yearsprior(Smithetal.,2021).163MethaneemissionsinlivestockandricesystemsFigure9CumulativeCO2-equivalentemissionsofmethaneareshown,calculatedusingdifferentmetrics,fortwomitigationscenariosnamedSSP4-6.0(panela)andSSP1-2.6(panelb)Warmingequivalenceofcumulativeemissions(b)SSP1-2.61.2Annualmathaneemissions(MtCH4yr-1)600(a)SSP4-6.02500Temperaturechange(°C)20005001.01500CumulativeCO2-equivalentemissions(GtCO2)10004000.83000.62000.4100Warming0.25000190020002100180019002000001800CGTP1002100AnnualemissionsGSATGWP100GWP20GTP100GWPThetemperatureresponsefromtheseemissions,calculatedusinganemulator,isshownwiththeblackline(labelledGSATforglobalsurfaceairtemperature).Source:Forster,P.,Storelvmo,T.,Armour,K.,Collins,W.,Dufresne,J.-L.,Frame,D.,Lunt,D.J.,Mauritsen,T.,Palmer,M.D.,Watanabe,M.,Wild,M.&Zhang,H.2021.TheEarth’senergybudget,climatefeedbacks,andclimatesensitivity.In:V.Masson-Delmotte,P.Zhai,A.Pirani,S.L.Connors,C.Péan,S.Berger,N.Caud,Y.Chen,L.Goldfarb,M.I.Gomis,M.Huang,K.Leitzell,E.Lonnoy,J.B.R.Matthews,T.K.Maycock,T.Waterfield,O.Yelekçi,R.Yu&B.Zhou,eds.Climatechange2021:Thephysicalsciencebasis.ContributionofWorkingGroupItotheSixthAssessmentReportoftheIntergovernmentalPanelonClimateChange,pp.923–1054.Cambridge,UK&NewYork,USA,CambridgeUniversityPress.https://doi.org/10.1017/9781009157896.001ThisformulaforGWPallowsustocalculateCO2-warmingequivalentemis-sionsforanytimeseriesofCH4emissions,i.e.notjustasingleandpermanentstep-change.TheresultingCO2-weemissionswillthenresultinapproximatelythesamechangeintemperatureasthetimeseriesofCH4emissions.ThisisshownfortwofuturescenariosinFigure9.Thefirstpanel(a)showsalowerambitionscenarioforCH4emissions,andpanelbahigherambitionscenario.Themodelledwarmingfromtheemissionsisshownbytheheavyblackline.CumulativeCO2-weemissionscalculatedusingGWPareshowningreen,andtheyareagoodapproximationofthemodelledwarmingforbothscenarios.TheGWPisatwo-termapproximationintendedtofindtheCO2-equivalentemissionsthatwouldgeneratethesameradia-tiveforcingtimeseriesasthatgeneratedbytheCH4emissions(Allenetal.,2021).Asecondapproach,developedbyCollinsetal.(2020),providesanalternativemethodwheretheforcingortemperatureofapulseofCO2emissionsiscomparedwithastep-changeintherateofemissionsofshort-livedgasesoveraspecifiedperiodtoreportthecombinedglobalwarmingpotential(CGWP)andthecombinedglobaltemperaturechangepotential(CGTP),respectively.TheCGTPmetricissimilartoGWP,inthatitcomparesthewarmingresultingfromastep-changeintherateofCH4emissionswiththewarmingthatresultsfromapulseemissionofCO2.TheapproximationmadeinCGTPisthatthetimeevolutionoftheCH4emissionsisunimportant,andthatonlythedifferencebetweentheinitialandfinalemission164Metricsforquantifyingtheimpactofmethaneemissionsratesarerelevant(providedthatmostofthechangeinemissionratesisachievedafewdecadesbeforetheendofthetimehorizonofinterest).ThismakesitusefulforaddressingtheeffectsofpermanentchangesinCH4emissionratesonlong-termwarming,thoughislessaccuratewhentheCH4emissionratesvaryclosetothetimeframeofinterest.CumulativeCO2-weemissionscalculatedusingCGTP100forthetwoscenariosaremarkedwithanorangelineinFigure9,andshowgoodagreementwiththemodelledwarming(heavyblackline)forboth.Thetwostep-pulsemetrics(CGTP100andGWP)areabletocapturethereductioninwarmingresultingfromCH4emissioncuts,whichcannotbecapturedwithGWP100(darkblue)orGWP20(lightblue).GWPalsorepresentsthehistoricalperiodmoreclosely.Furtherdis-cussionofbothcanbefoundinForsteretal.(2021).9.1.4Keydifferencesbetweenstep-pulseandpulse-metricsAshighlightedabove,thereisafundamentaldistinctionbetweenpulse-emissionmetricsandstep-pulsemetrics.Onewaytoconsiderthedifferentmetricconceptsistoexplorehowtheymightbeused.Inthissection,weusetheterm“marginal”torefertotheeffectoffutureemissionscomparedtothosefutureemissionsnotoccur-ring.Marginalemissionscapturetheeffectfromthoseemissionsandthereforethebenefitofavoidingthoseemissions,whichisrelevantforchoicesabouttheeffortandcoststhatmightbejustified(fromacost-benefitorcost-effectivenessperspective)tomitigatefutureemissions(Dhakal,MinxandToth,2022,supplementarymaterial).Weusetheterm“additionalwarming”tomeantheeffectontemperatureofemis-sionsafteraspecificyear,relativetothelevelofwarminginthatspecificyear.ThemarginalwarmingfromfutureCH4emissionsisalwayspositiveandcanbecom-paredtothemarginalwarmingfromCO2(seeFigure10).TheadditionalwarmingfromfutureCH4emissionscanbenegativeiftheyarereducedyearonyear.Climatechangeimpactscouldbeassessedbyusingmodellingofradiativeforcingortemperaturechangeasproxies,orbygoingintogreaterdetailindescribingtheconnectionbetweentemperaturechangesandresultantimpacts(Kirschbaum,2014,2017).Impactscanbecalculatedforjustonepointintime,ortheycanbeintegratedoverthewholetimehorizon.Pulseandstep-pulsemetricscanbothbeusedtounderstandmarginalandadditionalclimatechangeoutcomes,buttheyachievethisthroughdifferenttypesofapplications.Pulse-emissionmetricsprimarilyprovideinformationaboutmarginalimpacts.Eachpulse-emissionmetricgivesanaccountofthefutureclimateimpacts(asdefinedbythespecificmetric)thatwouldbecausedbyanextraunitofemissionofagivengas.Forexample,GWP100quantifiestheradiativeforcingoverthenext100yearsthatwouldresultfromemitting1tonneofCH4,comparedtonotemittingthistonne,andexpressesthisintermsofemittingaspecifiednumberoftonnesofCO2thatwouldresultinthesametotalradiativeforcingoverthenext100years.Bycontrast,step-pulsemetricshaveprimarilybeenusedtoshowthechangeintemperatureovertimecausedbyaparticularemissionspathway,relativetowarm-ingatareferencedatecausedbypreviousemissions(“additional”warmingsincethereferencedate).Forexample,GWPapproximatesthetemperaturechangethatwouldresultfromachangeinCH4emissionsrelativetoemissions20yearsprior.ThisisthenexpressedintermsoftheeffectofemittingorremovingaspecifiednumberoftonnesofCO2withthesameeffectsonglobaltemperatures.Thesedif-ferentperspectivesareillustratedinFigure10.165MethaneemissionsinlivestockandricesystemsFigure10ContributionstoglobalwarmingfromglobalnetCO2emissionsandglobalCH4emissionsfromlivestock,inapathwaythatlimitsglobalwarmingto1.5degreeswithlimitedovershoot(c)Temperatureanomaly(°C)40Temperatureanomaly(°C)1.40.25301.2201.00.20Emissions(GtCO2/yr)100.80.15020002025205020752100-10Year0.102025MarginalcontributiontofuturewarmingfromCO2200020252050207521000.30.05emissionsfrom2020Yearonwards0.002000205020752100Year(f)Temperatureanomaly(°C)0.10Emissions(MtCH4/yr)120Temperatureanomaly(°C)1000.08800.06600.2-400.04Marginalcontributionto200.1futurewarmingfromCH40.022025emissionsfrom2020020252050207521000.02025205020752100onwards2000Year2000Year0.002000205020752100YearStripedarrows:Solidarrows:Modelledtemperature(pastandfutureemissions)warmingrelativetowarmingwarmingrelativetoabsenceofModelledtemperature(pastemissionsupto2020)inareferenceyearfutureemissionsModelledtemperaturein2020Stripedarrowsillustratethewarming/coolingfromfutureemissionsofCO2andCH4relativetowarmingin2020(“additional”warming),whilesolidarrowsandshadedareasindicatethewarmingfromfutureemissionsofCO2andCH4relativetotheabsenceofthosefutureemissions(“marginal”warming).Marginalwarmingisshownintheright-handcolumnofthepanels.Notethatthescales,showingglobalnetCO2emissionsandlivestockCH4emissions,aredifferentintheverticalaxes.Source:AdaptedfromReisinger,A.,Clark,H.,Cowie,A.L.,Emmet-Booth,J.,GonzalezFischer,C.,Herrero,M.,Howden,M.&Leahy,S.2021.Hownecessaryandfeasiblearereductionsofmethaneemissionsfromlivestocktosupportstringenttemperaturegoals?PhilosophicalTransactionsoftheRoyalSociety.SeriesA–Mathematical,PhysicalandEngineeringSciences,379(2210):20200452.https://doi.org/10.1098/rsta.2020.0452Figure10providesanillustrationofthesedifferentperspectives,withthestripedarrowsshowingwarmingrelativetoareference(orbaseline)year(“additionalwarming”),andsolidarrowsshowingwarmingrelativetoanabsenceoffutureemissions(“marginalwarming”).Itshowsthatthechoiceofdefin-ingimpactsofemissionsrelativetoabaseline/referenceyearorrelativetotheabsenceofongoingemissionshassignificantimplicationsforthedifferentgases.TheleftpanelsshowglobalCO2(upperplot)andCH4(lowerplot)trajectoriesinanambitiousmitigationscenario.Therightpanelsshowthecorrespondingcon-tributiontoglobaltemperatureincrease(abovepreindustrialtemperatures)fromeithergas,withthethickerlineindicatingtemperature-changecontributionifthegasesfollowtheirrespectiveemissionpathways,whilethethinnerlineshowsthetemperature-changecontributionifemissionsofthegasceasedentirelyin2020.Theshadedareasshowthemarginalwarmingfromthetwogases(i.e.thecontri-butiontoglobalwarmingfromfutureemissionsofthosegasesand,conversely,theamountofglobalwarmingthatcouldbepreventediffutureemissionsofthosegaseswereavoided).166MetricsforquantifyingtheimpactofmethaneemissionsTherelativetemperaturechangeresultingfromtheseemissionscenarioscanbeconsideredfromtwodifferentperspectives:theeffectonglobaltemperaturesrelativeto2020(whichmightbeusefultoassesshowdifferenttrajectorieswouldcontributetooverallglobaltemperaturechange,forexample),asillustratedbythestripedarrows;ortheeffectonglobaltemperatureoftheseemissionscom-paredtonotemittingthem(whichmightbeusefultoassessthewarmingcausedbyfutureemissions,andthebenefitsofavoidingdifferentemissions,forexample),asillustratedbythesolidarrows.Thesolidarrowsarewhatpulsemetrics,suchastheGWPorGTParetypicallyusedtoexpress(termeda“marginal”approachinReisingeretal.,2021;seeChapter2ofIPCC,2022andsupplementarymaterialformoredetails),whilethestripedarrowscorrespondtothewaystep-pulsemetricssuchasGWPhavebeenusedtodate–whatwerefertoasa“baselined”approachbelow–whichrepresents“additionalwarming”relativetothatbaselineyear.Duetothedifferentatmosphericlifetimesofthetwogases,theconsequencesofa“no-emission”pathwaydiffergreatlyforCO2andCH4,forthereasonsdescribedabove.Whilethemarginal(solidarrow)oradditional(stripedarrow)approachesareverysimilarforCO2,theyprovideverydifferentperspectivesonhowtoconsidertheimpacts(oravoidedimpacts)ofCH4emissions(solidandstripedarrowsinthelowerrightpanelofFigure10).ThesedifferenceshaveimportantconsequencesfortheinterpretationandunderstandingofCO2-equivalentemissionscalculatedundereithertypeofmetric.Whichperspectiveisdeemedmostappropriatemaydependonpracticalconcerns(e.g.thecost-effectivenessofmitigatingdifferentemissions)orequityconsiderations(e.g.acknowledgingtheroleofdifferentsectorsoractivi-tiesinoverallglobalwarming),ashighlightedinsubsequentsections.Inthecaseofstep-pulsemetrics,theCO2emissionsthataredescribedasequiva-lenttoagivenchangeintherateofCH4emissionsarethosethatwouldresultinthesamechangeintemperature,relativetothebaselineyear.Inotherwords,inapply-ingstep-pulsemetricsonemustdeterminethereferenceconditionsagainstwhichtojudgechanges,andthestep-pulsemetriccanonlydescribetemperaturechangesrelativetotheseconditions.ForCO2,thereis(broadly)nofurtherchangefromareferencetemperaturewhentherearenofurtheremissions(ornet-zeroCO2emissions).Forshort-livedgases,however,iftherewereprioremissionscontributingtothereferencetemperature,thenascenarioofongoingemissionsisalsoeffectivelyembeddedinthereferencecondi-tionstomaintainthistemperature(anditresultsinaCO2-equivalenttemperatureoutcome).Decisionsoverwhatreferencestatetouseforstep-pulsemetricscanthere-forehavesignificantimplicationsontherelativevaluationofemissionsofshort-livedgases.Forexample,areferenceyearof2020,1990,1900or1750wouldleadtoverydifferentvaluations,butalltheseyearscouldbeappliedtostep-pulsemetrics.Italsoleadstopotentialequityimpactsthatneedtobeconsidered,particularlywhentheapproachisappliedtoemissionassessmentsatsubglobalscale(seeSection9.3.4).Step-pulsemetricscandirectlyillustratetheanticipatedtemperaturechangesresultingfromdifferentemissionpathwaysandincorporatetheminto“cumulativeemissionbudgets”.Bycontrast,pulsemetricsansweradifferentquestion.Theyshowtherelativeclimateeffectatonetimehorizonthatwouldresultfromanemis-sionwithoutneedingacomparisonwithpastemissions.Hence,thereisnoincon-sistencybetweenthedifferentmetrics,solongasitisrecognizedthattheyprovidedifferentinformation.167MethaneemissionsinlivestockandricesystemsInprinciple,bothpulseandstep-pulsemetricscanyieldmarginaloradditionalinformation.GWPcanbeappliedtoatimeseriesofemissions,withemissionsatthebeginningofthetimeseriessettozero(e.g.RogeljandSchleussner,2019).Thiswouldprovideinformationontheamountofwarmingcausedbysubsequentemissions,comparedtotheabsenceofthoseemissions,andhencethewarmingthatwouldbeavoidedifthosefutureemissionsdidnotoccur.Forexample,ifonewishedtoknowthemarginalwarmingcausedbyCH4emissionssince1990(asopposedtotheadditionalwarming),onewouldsetCH4emissionspriorto1990tozerowhenapplyingGWP.“Warmingsince1990”,and“warmingcausedbyemissionssince1990”arenotthesameforCH4(unlikeforCO2,asshowninFigure10),hencethepolicyquestionseek-ingananswerneedstobeclear,inparticularforshort-livedgases.Conversely,pulsemetricslikeGWPandGTPcanbeappliedtothedifferencebetweenagivenemissionandabaselineemissionslevel,andcouldthusbeusedinanadditionalapproach.9.1.5Timehorizon/endpointformetricsThepulsemetricsdiscussedinsection9.1.3dependverystronglyonthechosentimehorizon.Thechoiceoftimehorizondependsonpolicypriorities.Whilegivenpolicygoalsmaynotdirectlyspecifyaparticulartimehorizon,somepossibletimehorizonscouldbearguedtomakemoresensethanothers(Shineetal.,2005;AbernethyandJackson,2022).Forinstance,ifthegoalisspecificallytolimitwarmingto1.5degreeswithnoorlim-itedovershoot,peakwarmingwilloccurroughlyaround2050(determinedbyclimate-economicmodellingsuggestingplausibleemissionreductionscenariosthatwilllimitwarminginaccordancewiththistarget).Fromthatperspective,andifthepurposeofametricistodesignaclimatechangemitigationstrategybasedonarelativevaluationofpresent-dayemissionsaccordingtotheirmarginalcontributiontothistemperaturegoal,itcouldthusmakesensetovalueeachemissionbasedonthecontributionitmakestoglobalwarmingintheyear2050;i.e.tousetheGTPwithatimehorizonof30yearsforemissionsoccurringin2020.Applyingthislogicconsistentlywouldmeanthatemis-sionsoccurringintheyear2030wouldbevaluedwithGTP20(althoughthetimeframeswouldlikelyneedtobere-evaluatedasthetargetisapproached).ThisapproachisalsoreferredtoasthedynamicGTP(Shineetal.,2007).ItwouldbeinconsistentwiththisstatedpolicygoaltouseGTP100,becausethewarmingintheyear2120(whichiswhatGTP100describes,foremissionsoccurringintheyear2020)hasnodirectsignificancerelativetoapolicygoaloflimitingwarmingto1.5degreeswithnoorlimitedovershoot.Inpractice,theremaybemultiplepolicygoals,andnotallpolicygoalscanbetrans-latedintotimehorizonsandrelevantmetricchoices.Forexample,ifthegoalistolimitwarmingto1.5orwellbelow2degrees,peakwarmingcouldoccurasearlyas2050oraslateasperhaps2080,whichmeansthereisnosingleGTPvaluethatsatisfiesthesegoals.Inaddition,stakeholdersmaynothaveaclearglobalpolicygoalinmindandonlywanttodotheirpartinlimitingtheirimpactontheglobalclimate.Inthatcase,usingametricthatismoreakintotheglobaldamagepotential(GDamP)maybemorerelevant,althoughthisispath-dependent(seeSection9.2.3).9.1.6DiscountratesconsiderationAsclimateimpactsareexperiencedatdifferenttimesinthefuture,decisionsmustbemadeabouthowtovalueimpactsaccordingtohowfarintothefuturetheyoccurifametricisintendedtoreflectthefuturedamagescausedbyeachemission.168MetricsforquantifyingtheimpactofmethaneemissionsDiscountratesarecommonlyusedtoquantifyfutureimpactsinpresentvalueterms.Thehigherthediscountrate,themoreimpactsaredevaluedthefurtherintothefuturetheyoccur.Thiswouldshiftthemitigationemphasistowardsshort-livedclimateforcers,likeCH4,whilereducingthefocusonlong-livedclimateforc-ers,suchasCO2andN2O(vandenBergetal.,2015).Incontrast,alowdiscountratewillplacetheemphasisrelativelymorestronglyonlong-termclimateforcers.ThechoiceofdiscountratesishenceoneofthemostcriticalcomponentsofanyimpactanalysisandcanberelatedtothetimehorizonofGWPorGTPasdiscussedbelow.Aswithtimehorizons,thechoiceofdiscountratescannotrelysolelyonanobjectivescientificbasis.Furthermore,someauthorsargueformultiplediscountratesdependingonthepurposeoradeclining-in-timediscountrate(Arrowetal.,2014).DifferenttimehorizonsusedforexampleinGWPandGTPcanbeusedasproxiesfordiscountrates.BycomparingtheGWPtoGDamP(seesection9.2.2),itbecomespossibletoestimatetheeffectivediscountrate.Usingthatapproach,theGWP100wasestimatedtocorrespondtodiscountratesbetweenabout3percent(MallapragadaandMignone,2020)and3.3percent(withaninterquartilerangeof2.7to4.1percentinasensitivityanalysis;SarofimandGiordano,2018).GWP20correspondedtoadis-countrateof7percentorgreater(MallapragadaandMignone,2020)and12.6percent(interquartilerangeof11.1to14.6percent;SarofimandGiordano,2018).Itshould,however,benotedthatsuchrelationshipsaresensitivetounderlyingfuturescenarios,amongotherassumptions(MallapragadaandMignone,2020).9.1.7Non-radiativeforcingimpactsMethanehasotherimportantsocialcosts,besidesitsradiativeforcingeffects,primarilythroughincreasingground-levelozoneconcentrationsthatworsenairquality.Thisisamajorhazardtohumanhealthaswellasbeingtoxictoplants,withimpactsoncarbonuptakeandcropyields(Shindell,FuglestvedtandCollins,2017).ReducingCH4emissionswouldthereforealsoreducehumanmortalityduetolowerozoneconcentrations,andSarofim,WaldhoffandAnenberg(2017)cal-culatedthatthishealthbenefitwouldexceedtheclimatechangemitigationbenefitofthoseemissionreductionsiftheywerevaluedatUSD46pertonneCO2eq.TheUNEPCH4assessment(UNEPandCCAC,2021)foundthateveryMtreductioninCH4emissionspreventsapproximately1430annualprematuredeathsinadditiontoannuallossesof145000tonnesofwheat,soybeans,maizeandrice.Nitrousoxideemissionsdepletestratosphericozone.Thishasbeenestimatedtoincreaseitssocialcostby20percentabovethepureclimateimpact(Kanteretal.,2021).Carbondioxideemissionsalsoleadtooceanacidificationandallforcingagentswillcontributetosealevelrise,whichcarriesonformanydecadesaftertheemis-sionoccurs(Sterner,JohanssonandAzar,2014).SummarypointsAmetricthatestablishestheequivalenceregardingonekeymeasureoftheclimatesystem’sresponsetoemissionsdoesnotimplyequivalenceregardingotherkeymeasures.Thechoiceofametric,includingitstimehorizon,shouldreflectthepol-icyobjectivesforwhichthemetricisapplied.Themostappropriatemetricdependsontheobjective(i.e.whataspectofclimatechangedoesthepolicyfocuson,andoverwhichtimehorizon){Section9.1.1}.169ThelargedifferenceinlifetimesforCO2andCH4meansthatthepulse-emissionmetricsstronglyvarywiththechosentimehorizon{Section9.1.2}.Step-pulsemet-ricsforforcingandtemperature(comparingachangeintherateofCH4emissionswithaone-offemissionofCO2)showmuchlessvariationwiththetimehorizon{Section9.1.3}.Astep-pulsemetric(GWP)canbeusedtocalculateanequivalentCO2emis-sionstimeserieswhichgivesagoodapproximationofthetemperaturetimeseriesthatwouldresultfromtheoriginalCH4emissionstimeseries{Section9.1.3andshowninFigure9}.Thereisnosolelyscientificbasistodeterminethechoiceofmetricoritstimehorizon.However,certainpolicygoalssuchascost-effectivelydeployingemissionreductioneffortstokeepwithintemperaturelimitsmayimplicitlysuggestthatpar-ticularmetricsandtime-horizonrangesaremorerelevantthanothers{Section9.1.6}.ClimatemetricsforCH4includetheradiativeeffectsoftheresultingincreasesinozone(andstratosphericwatervapour)butnottheeffectsonhumanhealthandcropyields.ThesecoulddoublethesocialcostofCH4{Section9.1.7}.9.2THEUSEOFGHGMETRICSINIMPACTANDMITIGATIONAPPLICATIONSEmissionmetricsallowaquantificationofthecontributionofspecificactivitiesandrelatedGHGemissionsourcestoclimate-changeimpacts,oraquantificationofthebenefitsoftheclimate-changeimpactspreventedbyreducingtheiremissions.TheessenceofthedefinitionofGHGemissionmetricsistoallowsuchquantificationtoprovideobjectiveinformationaboutthebenefitsortrade-offsinvolvedinspecificdeci-sions.Decision-makersmayhavetodecidebetweendifferentmitigationoptionswithdifferentcostsandbenefits,whichmayinvolveevaluatingtheimpactofreducingCO2andCH4emissions.Tomakeanobjectivechoicebetweentheseoptions,decision-mak-ersneedtobeabletoquantifytheeffectofinterestforbothemissiontypes.However,metricsarenotalwaysneeded.Relativemetricsonlyneedtobeusedwhenthereisaneedtocomparebetweentheeffectsorcontributionofdifferentgasestoclimate-changeimpactsorotherclimate-changeeffectsofinterest,suchasradiativeforcingortemperaturechanges.Atonelevel,theassessmentofallgasesisclear.AllCH4(orotherGHG)emissionscontributetoglobalwarming.Allreduc-tionsofCH4emissions,therefore,helptoreduceglobalwarming.CH4andCO2differintheiratmosphericlifetimesandconsequentradiativepropertiesinthatCO2hasanongoingwarmingeffect,centuriesafteritsinitialemission,whereasthewarmingfromCH4halvesafterafewdecades(Solomonetal.,2010).Thisimpliesthatglobalnet-zeroCO2emissionsareneededtohaltglobalwarming.ForCH4,however,net-zeroemissionsarenotnecessarilyneededtostabilizetheclimateinthelong-termduetothedecayofCH4intheatmosphere.Nonetheless,ongoingCH4emissionscontinuetoalsocontributetohighertemperaturesthanwouldbethecaseintheabsenceoftheseemissions,andscenariostargetingbothCH4andCO2emis-sioncutsleadtolowertemperatureoutcomes(Sunetal.,2021).StakeholdersmaywishtosetanindividualreductiontargetforCH4emissions,inwhichcasethereisnoneedtouseanymetrictotrackprogresstowardsthatspecificemissionsreduc-tiontarget.Nonetheless,stakeholdersmaystillwishtousemetricstohelpjustifythelevelofambitionforaspecificgastargetcomparedwiththelevelofambitionforothergases,byexpressingtheirtargetsintermsofCO2equivalents.170Metricsforquantifyingtheimpactofmethaneemissions9.2.1LifecycleassessmentandcarbonfootprintingLifecycleassessment(LCA)isascience-basedmethodologytoquantifytheenvi-ronmentalimpactoverthelifetimeofaproductorservice,coveringabroadrangeofenvironmentalimpactcategoriessuchasglobalwarming,ecotoxicity,waterscar-cityandhumanhealth.Itcaninformusersabouttheclimateimpactofusingorthebenefitofavoidingagivenproductorservice,orabouttheconsequencesofsubsti-tutingoneproductorserviceforanother.TheISO14044standard(ISO,2006)notonlyspecifiestherequirementsandprovidesguidelinesforLCAsoverall,butalsoforlifecycleinventory(LCI)studies,whichisthedatacollectionportionofLCA.AnLCIisaccountingforallprocessinputsandoutputs(includingresourceinputsandemissionstotheenvironment)involvedinthesystemofinterest.Atthelifecycleimpactassessment(LCIA)stage,LCAsusecharacterizationfactorstoaggregatetheattributedemissionsandresourceusesofdifferentpartsofthesystem’slifecycleintoasinglevalueforvariousimpactcategories,suchasglobalwarming,ortofullyaggregatethemintoasinglescorefortypically10to20mid-pointimpactcategories.Themetricsshouldbechosentomatchtheuser’simpactobjectives.Forcharacterizingtheiraggregateclimatechangeimpact,LCAsinevitablyrequiretheaggregationorremovalofdifferentgreenhousegasemissionsintoacommonclimatechangeimpact,hencenecessitatingtheuseofGHGmetrics.BesidesspecificchoicesintheLCIA(i.e.howtomeasureandallocateemissionsintoprocesses/products),anyLCAneedstochooseappropriateimpactassessmentmodels.AvailableLCIAmethods,includingReCiPe2016(Huijbregtsetal.,2017)orLC-IMPACT(Veronesetal.,2020),bringtogetheranumberofenvironmentalimpactcategories(e.g.carbonfootprintorclimate-changeimpacts,eutrophication,ecotoxicityandothers)andproposecharacterizationfactors(CFs)soastoquanti-tativelylinktheelementaryflowstotheselectedimpactcategories.Toprovideguidanceandstandardizeprocedures,aUnitedNationsEnvironmentProgramme(UNEP)workinggroupgaverecommendationsforspecificimpactcategories.Thechoicesofimpactcategoriesandimpactassessmentmethodsneedtobedefinedaspartofastudy’sgoalsandthedefinitionofitsscope.Thisinvolvesdeterminingthetemporalscopeandselectinganappropriatemetricforclimate-changeimpactassessmentorasimpleclimatemodelasusedinLIME(InabaandItsubo,2018;Tang,TokimatsuandItsubo,2018).ThetemporalaspectsincludeboththetimeofaGHGemission(inventory)andthetimehorizonoftheimpactassessment(throughthechosenmetric).Thesechoicesneedtobejustifiedforanystudy.ISO(2006)alsorecommendsthattheselectionoftheimpactcategoriesshouldbebasedonthespecificrequire-mentsoftheLCApractitionerformeetingtheobjectivesofagivenstudy(EuropeanCommission,2010),whichleavesthechoiceofmetricsopentopractitioners.SpecificallyaddressingGHGemissions,ISO14067describestheprinciples,require-mentsandguidelinesforquantifyingthecarbonfootprintaccordingtoISO14040.Allnetfossilfuelemissionsshouldbeincludedinthequantificationofthecarbonfootprint,whilenetbiogenicemissionsshouldbeassignedalowerweightingthanfossilfuel-basedCO2emissionswhenapplyingISO14067toanassessment.EarlierLCAguidancereportsissuedbyFAO(FAO2016a,2016b,2016c,2016d,2018a,2018b)wereallbasedonusingGWP100butdiscussedpossiblereasonsforusingdifferentclimate-changeimpactmetricstoestimatetheoverallimpactsofdif-ferentGHGsemittedwithinlivestockproductionsystems.Morerecently,theglobal171Methaneemissionsinlivestockandricesystemslifecycleimpactassessmentmethod(GLAM)oftheLifeCycleInitiativehostedbytheUnitedNationsEnvironmentProgramme(UNEP,2021)hasrecommendedthatLCAsshouldreportclimateimpactassessmentswithboththeGWP100(torepresentshorter-termimpacts)andGTP100(torepresentlonger-termimpacts),withconsider-ationgiventoGWP20andGTP20forsensitivityanalysesexploringveryshort-termimpacts(Cherubinietal.,2016;Levasseuretal.,2016;Jollietetal.,2018).TheserecommendationsusedmetricvaluesfromIPCC(2013)thathavesubsequentlybeenappliedinvariousimpactassessments(e.g.Reisinger,LedgardandFalconer,2017;Iordan,VeronesandCherubini,2018;Tanakaetal.,2019;TibrewalandVenkataraman,2021).Itshouldbenotedthatthedefinitionofveryshort,short,andlongtermissub-jective.TheseconsiderationsandwiderpointsarealsodiscussedinanotherrecentreportontheLCAforfooditemspublishedbyFAO(McLarenetal.,2021).WeighingupCH4reductionsvsotherfactorsisevenmoredifficult.AnLCAcanprovidetheframeworktoensuretheanalysisiscomprehensiveandthatdifferentwaysofvaluingCH4emissions(orreductions)canbeusedtoassessthebenefitsofreducingemissionsagainstpotentialnegativetrade-offs.Thesemaybeduetoincreasedemis-sionsofothergreenhousegasesorotherecosystemservices,suchasfoodproductionorotherenvironmentalbenefits.Somestudieshavealsoattemptedcomparingandaggre-gatingvariousLCAimpact-indicatorcategoriestodirectlyquantifythecombinedoverallimpactacrossallthedifferentindividualimpactsconsidered.Theyincludetheso-called“endpoint”methods(e.g.ReCiPeandLC-IMPACT),whichdividealltheimpact-categoryresults(suchasGHGemissions,landuseandwaterconsumption)intoimpactsonhumanhealth,ecosystemqualityandresourcedepletion.Theyarethenfollowedbyanoptionalnormalizationandweightingsteptoarriveatasinglescoreresult.Existingmethodsusedifferentmetricsforassess-ingclimate-changeimpacts,butmostmethodsrelyonGWP100.However,thereisnoobviouswaytoquantitativelycomparetheimpactofgreenhousegasemissionswithunrelated,butequallyimportant,impactssuchaswateryield,erosioncontrolorbiodiversityconservation.Ultimately,somejudgmentsmustbemadeinthesecomparisonswhenitisnotpossibletocompareimpactsonapurelyobjectivesci-entificbasis.Thefinalweighingupshouldreflectdifferentvaluesthatneedtobeagreedoninanopendiscussion.Theunderlyingissuesarealsofurtherhighlightedanddiscussedinthecross-cuttingsectionofthisreport.Suchanaggregationintoasingle-scoreLCAresultneedstobecriticallyexaminedsinceitrestsonmanynormativechoicesandcandisguisethecomplexityandtrade-offsinvolvedinLCAassessments.Modellingfromimpact-categoryresults(e.g.CO2equivalents)toend-pointresults(e.g.impactsonhumanhealth)leadstomoreuncertainty,astheeffectsofclimatechangeonhumanhealthinvolveadditionalandhighlyuncertainmodels.Insummary,forLCAstudiestobeinlinewithISOstandards,therearestatedrequirementsintermsofmethodologyandreportingmetrics.ThegoalandscopeofanLCAneedstoclearlydefinetheobjectiveofthestudy,andthismightleadtodifferentmetricchoices.Needlesstosay,itisimportanttoreflectonthechoiceofmetricsasthesecangreatlyaffecttheoutcomeofanyassessment,butnogeneralguidancecanbegivenonthemetricstouseasthiswilldependonthegoalsandobjectivesofthestudy.9.2.2Cost-benefitassessmentofclimatechangemitigationAcost-benefitanalysisrequiresthebenefitsofreducingclimatechange-relateddamagesbyavoidingfutureemissionstobequantified.Thiswouldallowusto172Metricsforquantifyingtheimpactofmethaneemissionseva-luatethetrade-offsbetweengreenhousegasmitigationchoicesandanydet-rimentaleffectsfromclimatechange(forexample,ifemissionsofonegasincreasewhilethoseofanotherdecrease),orbetweenseveralmitigationoptionsthattar-getdifferentgases.Damagemetricsaretypicallybasedonthecostofdamagesasafunctionofchangesinradiativeforcingorglobalsurfacetemperature(Deuber,LudererandEdenhofer,2013)and,conventionally,cumulativedamagesovertimeareusedtoassessthelossesorcostsofclimatechange.Aweaknessofmanyassessmentmodelsisthattheymaynotadequatelyaccountforthefulleffectsofthecatastrophicimpactsofclimatechange(Weitzman,2012,2013;Pindyck,2013).Whereincluded,theimpactofcatastrophicphenomena(forexample,thedangerousriseinsealeveloruncontrollablepositiveclimate-forcerfeedbackssuchasalargeandrapidreleaseofCH4frompermafrost)candrasticallyincreasetheestimateddamagevalues(Weyant,2017).Oneemissionmetricthatisconsistentwiththecost-benefitframeworkistheGDamP(ReillyandRichards,1993;Schmalensee,1993;Fankhauser,1994;Kandlikar,1995;Hammittetal.,1996;Toletal.,2012;Kolstad,2014).ItcanbeinterpretedasamoregeneralformoftheGWP(Toletal.,2012;Deuber,LudererandEdenhofer,2013).TheGDamPhasbeenderivedfromanoptimalpathwayindicatedbyanintegratedassessmentmodel(IAM)withinacost-benefitframe-work.Underanoptimalpathway,theGDamPisdefinedastheratioofincrementaldamagesavoidedbyreducingtheemissionsoftwogases(forexample,CO2andCH4).Itisthustime-dependentbecauseavoideddamagesgenerallyvaryovertimeandwiththepathwayofemissionreductions.Ontheonehand,theGDamPisthemostcomprehensiveavailablemetricinthecontextofacost-benefitappraisalofemissionsasitusesasingleframeworktocon-sidermitigationanddamagesaswellastheunderlyingclimatephysics.Ontheotherhand,theGDamPishighlyuncertainbecauseoftheuncertaintyinthemanyassump-tionsthatarerequiredtotranslateemissionsintodamages,includingthechoiceofdis-countrateandthequantificationofclimatedamagesassumedinanIAM.Forexample,Boucher(2012)estimatedtheGDamPforCH4at24.3(mean)butwithalargerangeofuncertaintiesfrom12.5to38.0(5-95percentinterval).AsnotedinKolstad(2014),thedifficultiesinestimatingtheGDamParecloselyrelatedtothelargeuncertaintiesinthesocialcostofCO2andnon-CO2gasesintheatmosphere(MartenandNewbold,2012;Waldhoffetal.,2014;Shindell,FuglestvedtandCollins,2017;Erricksonetal.,2021).Sincedamagefunctionsareuncertain,asensitivityanalysisfordifferentdamagefunctionscanprovidegreaterinsightsintothedependenceofultimateoutcomesontheassumeddamagefunctions(Kirschbaum,2014;Kumarietal.,2019).Kirschbaum(2014)putforwardtheclimatechangeimpactpotential(CCIP),ametricbuiltfromdamagefunctions.TheCCIPgivesequalweighttothreecatego-riesofdamagesparameterizedthroughelevatedtemperature,therateofwarmingandcumulativewarming.Backgroundconditionsarecalculatedundertherepre-sentativeconcentrationpathway(RCP)witharadiativeforcingof6.0W/m2bytheendofthiscentury(RCP6.0),withCCIPcalculatingmarginalimpactsforextraemissionunitsofdifferentgases.AnotabledifferencewiththeGDamPisthattheCCIPdoesnotrequireanIAM,whichmeansthattheCCIPconsiderssolelydam-agesunderthespecificpathwaywithoutconsideringthecostofabatinggreenhousegasemissions.DamagefunctionsusedintheCCIPalsopartlydependonthefuturepathofbackgroundconditions(Kirschbaum,2014).173MethaneemissionsinlivestockandricesystemsThecost-benefitordamagemetrics,suchastheGDamPandCCIP,havenotyetbeenappliedinthedevelopmentorassessmentofreal-worldclimatepolicies,althoughCCIPshavebeenusedforimpactassessments(Kirschbaum,2017;Brandãoetal.,2019).TheGDamPhasnotbeenusedmuchinrecentwork,butitwasdis-cussedaspartofthedebateonthesocialcostofCO2andnon-CO2gases(MartenandNewbold,2012;Waldhoffetal.,2014;Rennertetal.,2022).ThesemetricsarealsousefulforevaluatingandinterpretingothermoreoftenappliedmetricssuchasGWP100fromacost-benefitperspective.9.2.3Cost-effectivenessofdifferentmitigationoptionsAcost-effectivenessanalysisisaspecialcaseofamoregeneralcost-benefitanalysis,withthedamagecostfunctionsettozerouptotheleveloftheclimatetargetandtoinfinitythereafter(Toletal.,2012).Itconsidersonlythecostofmitigationtoachieveaspecifiedclimatetarget,suchasthelong-termtemperaturetargetoftheParisAgreement.Itdoesnotconsiderthecostassociatedwithclimatedamagesandadaptation,whicharegenerallyregardedasbeinghighlyuncertain.Anotherdifferencebetweenthetwoframeworksisthat,whileacost-benefitanalysissimul-taneouslycalculatesatargetandapathway,acost-effectivenessanalysisrequiresatargetspecificationfirst,andthenacost-effectivepathwayiscalculatedtoachievethetarget.Thecost-effectivenessprincipleisoneofthekeyprinciplesoftheUnitedNationsFrameworkConventiononClimateChange(UNFCCC)(Article3ofUnitedNations[1992])andaguidingprincipleforclimatemitigationpathwayspresentedinpreviousIntergovernmentalPanelonClimateChange(IPCC)reports.Ametricthatisconsistentwiththecost-effectivenessframeworkistheglobalcostpotential(GCP)(ManneandRichels,2001;Johansson,2012;Toletal.,2012;Tanakaetal.,2013,2021).TheGCP,whichcanbeseenasamoregeneralformoftheGTP(Toletal.,2012),isdefinedastheratioofthecostforsavingtheemissionofanadditionalunitofagasofinteresttothatofCO2ateachpointintimeunderacost-effectivepathway.SimilartotheGDamP(seeSection9.2.2),acalculationoftheGCPrequiresanIAM(butonethatisrununderacost-effectivenessframework),whichmakestheGCPpath-andtime-dependent.TakingCH4asanexample,theGCPforCH4istheratiooftheanticipatedfuturepricesofCH4andCO2onacost-effectivepathway(alsocalledthe“priceratio”[ManneandRichels,2001])asderivedfromanIAMforagivenclimatetarget(forexample,a2°Cwarmingtarget).TheGCPdependsontheclimatetarget,thechosenpathwaytowardsthetemperaturegoalandarangeofsocio-economicassumptions.TheGCPistime-dependentbecausethepricesofCO2andCH4changeovertimeunderacost-effectivepathway.TheGCPincreasesovertimeuptothepointwhenatemperaturetargetisreached,andstaysatapproximatelythesamelevelthereafter(ManneandRichels,2001;Johansson,2012;Tanakaetal.,2013).Tanakaetal.(2021)showedthattheGCPforCH4isrelativelyclosetoGWP100upuntilmid-centuryunderavarietyofpathways,butbeyondmid-century,GCPstartstosignificantlydeviatefromGWP100,dependingstronglyonthefuturepathwaythatwillunfold.ThisanalysissupportstheuseofGWP100fortheParisAgreementatleasttillthemid-century,withmetricsthathaveshortertimehorizonsbecomingmoreappropriatethereafter.ThetemporalchangeoftheGCPvaluecanbeapproximatedbythecost-effectivetemperaturepotential(CETP)(Johansson,2012).TherisingtrendofGCPupto174MetricsforquantifyingtheimpactofmethaneemissionsthepointofstabilizationcanbecapturedbyadynamicGTP(Shineetal.,2007)andotherdynamicmetricssuchastheTEMperatureProxyindex(TEMP)(Tanakaetal.,2009a,2013).Adynamicmetricusesatimehorizonwiththeendpointtypicallybeingtiedtotheyearofmeetingaclimatetarget(Berntsen,TanakaandFuglestvedt,2010;AbernethyandJackson,2022;McKeough,2022).Inotherwords,adynamictimehorizonwillbeshortenedasitmovesforwardtothefuture,andthemetricwouldhavetobeadjustedastheemissionpathwayunfolds.TheproximityofthedynamicGTPtoGCPjustifiestheuseofthedynamicGTPforanalysesofcost-effectiveness,butithasrarelybeenappliedoutsideofacademicresearch,pos-siblybecausethereisnocommonlyagreedyearofmeetingatemperaturetarget.Thepath-andtime-dependenceofGCPshowsthattherearelimitstotheopti-malityofstaticmetricssuchasGWP100.Thatis,thereisaneconomiccostassociatedwiththeongoinguseofGWP100insteadofthatoftheGCPorothertime-varyingmetrics.Previousstudiesshowed,however,thattheuseofGWP100increasesglobaltotalabatementcostsunderstabilizationpathwaysbyonlyafewpercent(O’Neill,2003;Aaheim,FuglestvedtandGodal,2006;Johansson,PerssonandAzar,2006;vandenBergetal.,2015;Tanakaetal.,2021).Despiterelativelysmallglobalimpacts,therearelikelytobemoresubstantialregionalandsectoralimpacts,includingfortheagriculturalsector,fromthechoiceofmetrics(Reisingeretal.,2013;Strefleretal.,2014;Harmsenetal.,2016).Thenon-optimalityofGWP100neverthelessincreasesinthecaseofovershootscenarios(Tanakaetal.,2021),underwhichthetemperaturetargetoftheParisAgreementistemporarilyexceededbeforeeventu-allybeingachieved.SimilartotheGDamP,theGCPhasnotbeenusedinclimatepoliciesinreal-worldapplications.WhiletheGCPisvaluableforquantifyingthecost-effectivenessofdifferentmetrics,thereareconceptualdifficultiesinmakingtheGCPoperationalbecausethevalueoftheGCPitselfrequiresanassumptiononalong-termfutureemissionpathwaytowardsatemperaturegoal.Asacompromise,theuseoftheGCPhasbeenrecommendedtoguidethechoiceofemissionmetricsatcertainpointsinthefutureastheemissionmitigationpathwayevolves(Tanakaetal.,2021).WhilethesestudieshaveshownthattheuseofGWP100doesnotguideperfectemissionpathwaystowardsselectedmitigationgoals,theintroducednon-optimali-tiesarenonethelesssurprisinglysmall.Inotherwords,ifonewantstomitigateCH4emissionstocost-effectivelyachievesomefuturetemperaturetarget,orsimplyquan-tifythemarginaldamagescausedbyCH4emissions,onearrivesatCO2-equivalentmetricsforCH4somewherebetweenabout20and40.ThisisroughlyconsistentwithGWP100,butcontrastswithvaluesgeneratedusingothermetricssuchasGTP100orGWP20.Whenmorecomplexnetemissionpatternsareinvolved,however,thentheuseofdifferentmetricsappliedtothesamenetemissionpatternscanresultinverydifferentlyassessedmitigationoutcomes(Brandãoetal.,2019).Therefore,eventhoughGWP100wasnotdevelopedtoderivecost-benefitorcost-effectiveoutcomes,itmaybeadequateforthosepurposesandisnotnecessarilyincompatiblewithcost-benefitandcost-effectivenessapproachestoclimatepolicy(e.g.cross-chapterBox2ofIPCC,2022).9.2.4OverallemissionreductionpolicyandtheroleofagricultureAnyoverallemissionreductionortemperaturetargetscanbeachievedmostcost-effectivelyifallsectorscontributetowardstheemissionreductioneffort,including175Methaneemissionsinlivestockandricesystemstheagriculturalsector.Agriculturehasanunusualemissionsprofileas,unlikemostothersectors,emissionsaredominatedbyCH4andN2OinsteadofCO2.Emissionreductionpoliciesusuallyinvolvetrade-offs.Tosatisfytheongoingdemandforfood,reductionsinagriculturalproductionfromonesectororregioncanincreasethedemandforalter-nativetypesoffood,orsupplyfromotherregions,whichmayultimatelyleadtohigherorloweremissionsthantheoriginalfoodproduction.Anyconsequentchangesinemis-sionsneedtobefactoredinwhenassessingtheoveralleffectofanymitigationpolicy(e.g.byconductingconsequentialanalysessuchasSmithetal.,2019).Itisimportanttoenablecross-sectoralcomparisonsaswell.Thiscanbedonebycomparingthecontributionofdifferentsectorsorcountriestopastandanticipatedfuturetemperaturechanges.Comparisonsalsoneedtobecarriedoutoverdifferenttimescales,andthisiswherethedifferentatmosphericlifetimesofCH4andCO2createparticularchallenges.Inthiscontext,theroleofmetricsbecomescriticallyimportanttobeabletoassesstherelativecontributionsofagriculturalCH4andothersectors’CO2inameaningfulway.Theuseofmetricsbecomesnecessarya)ifonewantstocomparethecontribu-tionsofdifferentemitters,sectorsandsoonthatmaybeemittingdifferentgases,orb)wheretherearepotentialpolicies’trade-offsthatmaydifferentiallyaffecttheemissionsofdifferentgasessothatthevalueofCH4reductionsmustbeweighedagainstpossibleincreasesintheemissionofothergases.Fora),itcomesbacktotheissuesaddressedearlierinthischapter,andmaybefactoredintoacost-benefitanalysis.Forb),wecanuseanLCAtoassesshowanyCH4reductioneffortswillaffectotherGHGemissions,orhaveotherenvironmentalimpacts,suchasleadingtotheemissionofspecificpollutants.Theymayalsohaveimpactsforlanduse,asdiscussedinanearlierchapter(Section8).9.2.4.1AssessmentboundariesDefiningtheboundariesforGHGemissionsisasimportantaschoosingtherightGHGmetricforanassessment.MostagriculturalproductionsystemsarecomplexandasaresultmanyactionsaimedatreducingagriculturalCH4emissionswillhaveimpactsonotherGHGs;forexample,somefeedadditiveswillleadtohigherCO2emissionsbutreduceCH4emissions(asdiscussedinthesubsectionsofSection5addressingmitigation).Therefore,policiesdevelopedtoreduceemissionsshouldconsidertheknock-oneffectsofmitigationactionsondifferentGHGs,trade,foodsecurity,landuse,waterconsumption,waterandairpollution,amongotherfactors.DirectorindirectleakageofCH4emissionsacrossnationalboundariesbecomesanissuewhenevaluatinganynationalmitigationstrategies.Clearly,areductioninrumi-nantsinonecountryachievedthroughimportingproductsderivedfromanincreasedruminantpopulationinanothercountry(withapotentiallyhigheremissionintensity)wouldbeaninstanceofdirectleakagethatcouldbereportedasmitigatingemissionsintheimportingcountrywhileeffectivelyincreasingthemintheexportingcountry,withnobeneficialeffectfortheworldasawhole(dependingontherespectiveemissioninten-sitiesofthetwocountries).Anexampleofindirectleakage,ontheotherhand,wouldbeifonecountryreduceditsCH4emissionsbyreducingtheexportsofruminant-derivedproductsandifthatactionresultedinincreasedruminantnumbersinthecountriesthatpreviouslyimportedruminantproductstoreplacethelackofavailabilityofimports.Thisisanissueforallfood-productionsystemssinceanychangeinfoodproductioninoneregionmayhaveaneffecteitherontheproductionlevelinotherregionsoron176MetricsforquantifyingtheimpactofmethaneemissionsalternativefoodproductsandtheirlikelyemissionratesofallGHGs.Theseleakagescanbecheckedcrudelybyassessingwhetherthebalancebetweendomesticproductionandconsumptionisbeingachievedthrougheitherincreasedimportsordecreasedexportsoflivestockproducts.Amorecooperativeapproach,onethatisdeemedacceptableundertheParisAgreement,wouldbetojointlyreportonthemitigationactivitiesofmultiplecountriesthatproducelivestockproductsaswellastradingalargenumberofothergoodsbetweenthemselves.Thiswouldbeonepossiblewayofensuringthatassessmentsfactorinthetranslocationoflivestockemissionsourcesacrossnationalboundaries.Withinthesystemboundaries,theemissionofeachGHGneedstobereported.Wherelifecycleinventorydataisusedasthereferenceforemissionsproducedbycomponentswithinthesystem,orinputsintoandoutputsfromthesystem,itisessentialtodescribeemissionsofeachGHGratherthanusinganavailableaggre-gateGHG-equivalentemissioncalculatedwithasinglemetric.Becausetherehasbeennoscientificconsensusonhowtoseparateoutdirecthuman-inducedfromindirecthuman-inducedandnaturaleffects(e.g.Canadelletal.,2007),allnaturalemissionsthatoccuronmanagedlandareconsideredanthropogenicundertheUNFCCC(IPCC,2003;IPCC,2006).Nevertheless,manycountriesdonotreportemissionsthattheyconsidernaturalbutthatoccuronmanagedland.Thatsaid,naturalCH4emissionsfromwetlands,inlandwatersandwildlife(includ-inginsects)thatoccuronmanagedlandcouldbeincludedinscenariosofCH4fluxes.Manynaturalemissionsareexpectedtoincreasewithglobalwarming(Deanetal.,2018),makingitimportanttofullydeveloppathwaystoreachclimatechangetargets.Somenaturalemissionscouldalsobeaffectedbylivestockmanagementsuchaschangeinnaturalpasturemanagementthatcouldinturnaffectwildruminantpopulationsaswellastermiteabundance,whicharebothsourcesofCH4(ManzanoandWhite,2019).Forexample,acomprehensiveregionalmitigationevaluationconductedinSweden(Skytt,NielsenandJonsson,2020)thatincludednaturalemissions,foundthatreducingCH4emissionsfromwaterbodiesratherthanCH4emissionsfromlivestockwasthepreferredmitigationaction.Togaingreaterclarityandallowbet-terinterpretationoftheresults,itcouldbeusefultoseparatelyreportindirectanddirectemissions,wherepossible.9.2.4.2DesigningaholisticemissionreductionstrategyThesuccessfulmitigationofoverallclimatechangeimpactsrequiresaholisticemis-sionreductionstrategyspecificallydesignedtoachievethedesiredemissionreduc-tiongoals.Thestrategyshouldassessanytrade-offsandco-benefitsofmitigationchoices,givingdueconsiderationtotheappropriatetimehorizonsinorderfortheresultstobeachievedcost-effectivelywhileminimizinganyunintendedadverseconsequences.SomepolicydecisionscouldenduptemporarilydelayingCO2reductioneffortsinfavourofactiononothergasesorviceversa,ifthiscanachievespecificgoalsinamorecost-effectiveway,especiallyatsubgloballevels.Whatisdeemedthebestapproachcallsforapublicpolicydebate,addressingdifferentsocio-economicdevelopmentsandhowalternativemitigationstrategiesmightcontributetotheoverallgoalofsus-tainability.Thedonosignificantharm(DNSH)principleinvokedbytheEuropeanCommissionmayalsobehelpfulinthiscontext.Itstatesthatameasureshouldnotleadtosignificantharmtotheenvironmentorgetinthewayofanyoneofthesixmajorenvironmentalobjectives.177MethaneemissionsinlivestockandricesystemsAssessingavarietyofpotentialscenariosandapplyingdifferentmetricsisnecessarytohaveabetterunderstandingoftheoutcomesofdifferentpolicydesigns.AnyGHGemissionmetricsimplifiesthecomplexityoftheclimatesystemresponsetogreenhousegasemissions.InsteadofdescribingfutureemissionmitigationtargetsintermsofCO2equivalents,whichareambiguous,itwouldbeclearerifthesetargetswerealsospecifiedforindividualgases–evenifonlyindicativedistinctionscouldbemade,oriftheemissionsofthelong-livedandshort-livedGHGswereatleasttreatedseparately(Denison,ForsterandSmith,2019;Allenetal.,2021).TheGWPormodellingofwarmingeffectsovertime(i.e.aclimatemodel)canrevealtemporaldetailsandtrade-offsthatarenotnecessarilyapparentwiththeGWP100orothersingle-pulsemetrics.Thismattersespeciallyifclimatetargetsarenotjustaddressingimpactsatacertainpointintime,butalsoassessingtemporaldevelopmentsandanytrade-offsofwarmingimpactsofdifferentGHGpoliciesbeforeandafterthispointintime.Forintegratedpolicies,itisimportantthatthegoalsnotbeindependentlydefinedforthedifferentsectorsoftheeconomy,butthattheyalsoaddressthepolicies’effectivenessandanytrade-offsbetweensectorsthatmayberequiredtocost-effectivelylimitoverallGHGemissions.Additionally,reducingGHGemissionsmightinvolveagriculturalintensifica-tionthatcouldhavenegativeimpactsonanimalwelfareandbiodiversity.Insomesectors,GHGreductionsmightentailfewertrade-offsthaninothers,andevensynergies.InordertoassesstheimpactofchangesonwholeeconomiesandtofindthemosteffectiveGHGmitigationsolutions,integratedassessmentmodelsshouldbeused.Becausethesecontainconsiderableuncertainties,itisbesttoapplyseveralmodelsandscenarios,whichforthetimebeingputsagreatstrainonresources.Improvedscientifictoolsmightberequiredforbroaderapplications.Thefollowingsectionsprovidesomeinsightsbasedonavailablestudies.9.2.5Cross-sectorcomparisonsCurrently,mostsectoralcomparisonsarebasedonemissionsinagivenyear,aggre-gatedusingapulse-emissionmetric.So,forexample,withGWP100,thisissimplydefinedasthemarginalradiativeforcingintegratedoverthefollowing100years.Anyaggregationofthesectoralcontributiontooverallgreenhousegasemissionsisthushighlydependentonthespecificmetricsusedfortheintegration.Forinstance,theIPCCAR5synthesisreport(IPCC,2014)comparedthesectoralcontribu-tiontooverallemissionsin2010usingthreeofthemostcommonlyusedmetrics,GWP100,GWP20andGTP100(Figure11).Thecalculatedcontributionofagriculturetototalgreenhousegasemissionsrangesfrom7.2percentforcalculationsbasedonGTP100to22percentforcalculationsbasedonGWP20.ThesedifferencesarelargelyattributabletothedifferingweightassignedtoCH4emissions.Theclimateimpactsofdifferentsectorscanalsobecomparedbyexploringtheircontributiontoglobaltemperatureincreasesfrompastemissions.Thisprovidesanalternativeperspectiveandovercomestheproblemofrelyingondifferentgreen-housegasmetricstomakecomparisonsbetweentheemissionsofdifferentgreen-housegases.ReisingerandClark(2018)demonstratedthisapproachforthewarm-ingcontributionfromlivestockfarming,usingasimpleclimatemodeltocalculatetheactualcontributionofdirectgloballivestock-basedemissionstoglobaltem-peratureincreasesupto2015(Figure12).178MetricsforquantifyingtheimpactofmethaneemissionsFigure11Sectoralcontributiontoannualtotalgreenhousegasemissionsin2010weightedbythreedifferentgreenhousegasmetrics,GWP100,GWP20andGTP100GWP100GWP20GTP100AgricultureForestryand8.2%7.2%13%14%otherlanduse6.7%11%20%Buildings17%16%6.3%22%17%Electricity30%Transportandheat14%production24%5.7%9.8%IndustryOtherenergy21%6.2%21%11%Source:ReproducedfromIPCC.2014.Climatechange2014:Synthesisreport.ContributionofWorkingGroupsI,IIandIIItotheFifthAssessmentReportoftheIntergovernmentalPanelonClimateChange(R.K.Pachauri&L.A.Meyer,eds.),151pp.Geneva,Switzerland.Figure12Modelledglobaltemperatureanomaliesfrom1850to2015forallanthropogenicemissions(a)1.00.8ModelledtemperatureContributionfromdirectlivestockCH4Modelledtemperature(°C)ContributionfromdirectlivestockN2O(relativetopreindustrial)ContributionfrompastureconversionsCO20.60.40.20-0.2188019001920194019601980200020201860YearThecontributionfromdirectlivestockemissionsofCH4(blue),N2O(red)andCO2frompastureconversions(green)andotheranthropogenicemissions(grey).Source:ReproducedfromReisinger,A.&Clark,H.2018.Howmuchdodirectlivestockemissionsactuallycontributetoglobalwarming?GlobalChangeBiology,24(4):1749–1761.https://doi.org/10.1111/gcb.13975179MethaneemissionsinlivestockandricesystemsAcomparisonofdifferentsectorsbasedontheircontributionstopast,presentorfutureglobaltemperaturechangesprovidesinformationthatisfundamentallydifferentfromtheassessedmarginalclimate-changeimpactofanindividualyear’semissions(asdefinedbywhatevermetricisusedtoaggregateorcomparedifferentgases)orthemarginalimpactofallfutureemissions(Reisingeretal.,2021).Allsuchapproachesmaybeofinterestandpotentiallyrelevanttopolicydevelopment,butthecalculationmethodandanyemissionmetricusedmustberelevanttotheques-tionbeingposed.9.2.6AggregationofdifferentGHGsforreportingandaccountingAsreflectedintheexamplesabove,howandwhethertoaggregatethecontribu-tionsfromdifferentGHGsiscontext-specificanddependsontheinformationauserwantstogain.Forsomepurposes,officialguidancespecifiestheprotocoltobeusedforemissionaggregations.Fornationalemissioninventorysubmissions,forexample,theemissionsofindividualgreenhousegaseshavetobereportedwithoutconversion.Inaddition,theUNFCCChasmandatedthatnationsshouldusetheGWP100toalsoreportaggregatedvaluesoftheseemissions.For“productcarbonfootprints”andsimilarassessments,wereferbacktothediscussionregardingthelifecycleassessment(Section9.2.1).Thesensitivitytomultipleemissionmetrics,suchas(butnotlimitedto)GWP20,GWP100andGTP100,couldbeexploredtohigh-lighthowdifferentimpactsvaryoverarangeoftimeframes.Itshouldbeemphasizedthat,irrespectiveofthemethodusedfordataaggrega-tion,itisusefultoalsoreportdisaggregateddataofindividualGHGemissions.Thisensuresfulltransparencyandenableswideranalysesbeyondtheindividualmetricsand/oraggregationmethodprovided.Thetimeofemissioncanalsobecriti-callyimportant,especiallyforCH4emissionsbecauseoftheirshortatmosphericlifetime.Methaneemissionsinlaterpartsofthiscenturycanthereforehavegreaterclimate-changeimpactsthanemissionsduringthepreviouscenturybecausethewarmingcontributionofCH4inthelatetwenty-firstcenturywouldbefeltatatimeofhigherglobalbackgroundtemperatures.Thiswideranalysiscouldincludeare-analysisunderdifferentmetricsoraggregationmethods,oremploymorerobustclimatemodellingapproaches.9.2.7Biogenicmethane–ImplicationsformetricsAnyCH4releasedintotheatmospheremusthavebeeneitherproducedfromcar-bonlaiddowninthepastfromfossilsourcesorstoredinpeat,permafrostorsimilardeposits,orfromcarbonrecentlyfixedinricepaddiesorbyentericfermentation(Wilosoetal.,2016).Inthecontextofclimatechange,itisimportanttodistinguishbetweenCH4indirectlygeneratedfromcarboninrecentlygrownbiomassandthatderivedfromoldcarbonsources,suchasfossildeposits.IfCH4isgeneratedfromnewlygrownbiomass,forexamplebyentericfermentation,carbonisconvertedtoCH4whereasitwouldotherwiseberespiredasCO2.ThismeansthatthegenerationofbiogenicCH4slightlylowerstheatmosphericCO2concentration,whichdoesnothappenwhenCH4isreleasedfromafossilorigin.WhenCH4iseventuallyoxidized,thecarbonwilltransformbackintoCO2,aprocessthatiscommontobothbiogenicandfossilCH4.InthecaseofCH4fromfossilorigin,thisoxidationleadstoanetincreaseinatmosphericCO2concentrationsbyaddingmoreCtotheatmosphere180Metricsforquantifyingtheimpactofmethaneemissionsthatwassequesteredinfossildepositsmillenniaago(whichisthesameasdirectCO2emissionsfromfossilresources).BiogenicCH4,incontrast,doesnotleadtoaneventualnetincreaseinatmosphericCO2.ForbiogenicCH4frombiomassCthathadonlyrecentlybeenproducedfromCO2,VarshneyandAttri(1999)proposedthatGWP100shouldbereducedby5percentfromtheIPCCvaluesforfossilCH4.Kirschbaum(2014)andMuñozandSchmidt(2016)proposedthatwarmingpotentialsofCH4shouldbereducedby2.75kgCO2eqforbiogenicCH4relativetothevalueforfossilCH4toaccountfortheassociatedreductionofCO2bytheformationofCH4.Thisconformstotheassump-tionthatonemoleculeofCO2isremovedforeachmoleculeofCH4generated,witha1:1molarratioconvertingtoaweightratioofCO2toCH4of2.75.ThedefaultvaluesformetricsinIPCCAR5wereforbiogenicCH4.However,thisdoesnotaccountforthedifferenceintemporaldevelopments.IPCCAR6assumesthatonly75percentofCH4oxidationleadstoCO2,while25percentofcarbonisremovedbydepositionofreactiveintermediates.AccountingforthetimetakenforCH4oxidation,thisslightlydecreasestherequiredchangeoftheGWP100ofbiogenicCH4to1.9units(Forsteretal.,2021).Similarly,Boucheretal.(2009)suggestedthatthewarmingpotentialsofCH4fromfossilCshouldbeincreasedby0.7to2.7unitstoaccountfortheconversionofoxidizedCH4toCO2,whilethewarmingpotentialofbiogenicCH4generatedfromrecentbiomassshouldbereducedby1.4to0units.ThelatestIPCCreportlistsvaluesforGWP100as29.8forfossilCH4and27.0forbiogenicCH4(IPCC,2021).Thesearethemostrecentlycalculatedglobalwarmingpotentialsunderthelateststateofthescienceandatmosphericgasconcentrations.RecommendationsItisrecommendedtoreportgreenhousegasemissionsforindividualgases,wherepossible,inadditiontoanyemissions’aggregationthroughtheuseofchosenmetrics.Climatemetricscanonlyprovideinformationaboutthedirectclimateconse-quencesofemissionsandmitigationactions.Ultimatepolicychoicesneedtocon-sidernotonlythedirectclimateconsequencesofanymitigationeffortsbutalsootherrelevantclimateandnon-climatefactors.Applyingarangeofmetricscanhelptestthesensitivityofclimate-changeimpactassessmentstothechoiceofmetrics.Thiscanbeparticularlyusefulifthereisnosingleclearlydefinedpolicyobjective.InapplyinganymetrictoCH4emissions,itisnecessarytodistinguishbetweenCH4derivedfromfossilversusofrecentbiogenicorigin,withglobalwarmingpotentialsofCH4offossiloriginassignedawarmingpotentialhigherby2.75units.9.3CLIMATETARGETSANDRELATEDISSUESTheagricultureandlivestocksector’suseofclimatemetricsoccurswithinalargerpolicycontextrelatingtoclimateactionandsustainabledevelopment.ThissectiondiscussestheParisAgreement,theterm“climateneutrality”,sustainableagricul-tureandequity.Itsaimistoprovideadditionalinformation,sothatuserscanmakedecisionsinformedbythewidercontext,includingtheglobalgoalstowhicheverysectorcontributes.Thesectiondoesnotformulateanyspecificrecommendations.181Methaneemissionsinlivestockandricesystems9.3.1TheParisAgreement9.3.1.1ThegoalsoftheParisAgreementTheParisAgreementprovidesthecurrentbasisforinternationalclimatepol-icyundertheUnitedNationsFrameworkConventiononClimateChange(UNFCCC).Itsetsoutaframeworktostrengthentheglobalresponsetothethreatofclimatechangeby“holdingtheincreaseintheglobalaveragetemperaturetowellbelow2°Cabovepreindustriallevelsandpursuingeffortstolimitthetemperatureincreaseto1.5°Cabovepreindustriallevels”(Article2.1).Inordertoachievethislong-termtemperaturegoal,theParisAgreementfurtherdescribesthe“aimtoreachglobalpeakingofgreenhousegasemissionsassoonaspossible,…toundertakerapidreductionsthereafter,…soastoachieveabalancebetweenanthropogenicemissionsbysourcesandremovalsbysinksofgreenhousegasesinthesecondhalfofthiscentury.”Furthermore,theAgreementdescribestheneedtopursuethesegoals“onthebasisofequity,andinthecontextofsustainabledevelopmentandeffortstoeradicatepoverty”(Article4.1).9.3.1.2TheParisAgreementandmethaneemissionsItisimportanttonotethattheParisAgreementdoesnotspecificallydiscussCH4.TheParisAgreementdoesnotprescribehowmuchandhowquicklytheemissionsofindividualgasesmustbereduced.StrategiestoachievetheParisAgreementaretobeworkedoutwithinnationalcontexts.Thatsaid,theIPCC’sSixthAssessmentReport(IPCC,2021a;IPCC,2022)underscorestheneedfordeep,rapidandsustainedreductionsingreenhousegasemissionsandreachingatleastnet-zeroCO2emissions.Thesereportsalsohighlighttheimportanceofstronglyreducingtheemissionsofothergreenhousegasesandairpollutants,especiallyCH4,asthiswillhavebenefitsforbothhumanhealthandtheclimate.Opportunitiestoreduceemissionsvaryacrossdifferentsourcesandsectors.Itisimportanttobemindfulofthetrade-offsbetweenreductionsofdifferentgaseswhileworkingtowardthegoalsoftheParisAgreement.9.3.1.3UseofclimatemetricsTheParisAgreementdoesnotspecifymetrics.Nordoesitusetermssuchas“netzero”or“carbonneutral”,althoughArticle4.1doesrefertothegoalof“achievingabalanceofanthropogenicsourcesandsinks”.Thatsaid,thecommonmetricusedundertheUNFCCCsincetheKyotoProtocolhasbeenGWP100(UNFCCC,1997).TheuseofGWP100followingtheParisAgreementhasbeenassessedintheacademicliterature,withvaryingconclusions.Initssummaryforpolicymakers,theIPCC(2021)statedthatemissionpathwayswhichreachandsustainthenet-zeroGHGemissionsdefinedbyGWP100areprojectedtoresultinadeclineinsurfacetemperatureafteranearlierpeak.Schleussneretal.(2019)concludedthatinterpretingtheParisAgreementclimateobjectivesbyusingGWP100isinternallyconsistent.Incontrast,Wigley(2021)concludedthattemperatureoutcomesinascenariousingGWP100-scalingformethanewereerroneous,andthereforeitsusewasnotrecommended.WithsubsequentIPCCreports,theassessedvaluesofGWP100haveundergonechanges(Table8).Atthe24thsessionoftheConferenceoftheParties(COP24)oftheUNFCCC,GWP100wasadoptedasthecommonmetricfortheimplementationofthetransparencyframeworkoftheParisAgreement(paragraph37oftheAnnextoDecision18/CMA.1).Later,partiesdecidedtouseGWP100valueswithoutcli-mate–carbonfeedbacktoreportaggregateemissionsandremovalsasprovidedby182MetricsforquantifyingtheimpactofmethaneemissionstheIPCC’sFifthAssessmentReport(AR5)(orasubsequentIPCCreportuponfutureagreement).BesidesthemandatoryreportingbasedonGWP100,theCOP24decisionalsoallowscountriestoreportadditionalinformationonaggregatedCO2-equivalentemissionsbyusingothermetricsassessedinIPCCreports,suchastheGTP,forexample.Inaddition,partiestotheParisAgreementagreedtousethesameemissionsreportingframeworktoaccountfortheirnationallydeterminedcontributionsbeyond2030(Decision4/CMA.1).Table8.GWPvaluesformethaneacrossthedifferenthistoricalIPCCreportsSAR(IPCC,1995)TAR(IPCC,2001)AR4(2007)AR5(2014)AR6(2021)100-yeartimeperiod2827.03029.8CH4non-fossilorigin2123258479.7CH4fossilorigin8582.520-yeartimeperiodCH4non-fossilorigin566272CH4fossiloriginSource:Authors’ownelaboration.9.3.1.4Discussionoflong-andshort-livedgreenhousegasesinrecentIPCCreportsFollowingtheadoptionoftheParisAgreement,theUNFCCCinvitedtheIPCCtoproduceaSpecialReportonglobalwarmingof1.5°C(IPCC,2018).Thereportobservesthat“reachingandsustainingnet-zeroglobalanthropogenicCO2emis-sionsanddecliningnetnon-CO2radiativeforcingwouldhaltanthropogenicglobalwarmingonmulti-decadaltimescales(highconfidence).”Itpositsadistinctionbetweennet-zeroCO2emissionsandnet-zeroGHGemissions,whichisfurtheremphasizedintheSixthAssessmentReport’sSummaryforPolicymakers:D.1.8AchievingglobalnetzeroCO2emissions,withanthropogenicCO2emissionsbalancedbyanthropogenicremovalsofCO2,isarequirementforstabilizingCO2-inducedglobalsurfacetemperatureincrease.ThisisdifferentfromachievingnetzeroGHGemissions,wheremetric-weightedanthropogenicGHGemissionsequalmetric-weightedanthropogenicGHGremovals.ForagivenGHGemissionspathway,thepathwaysofindividualGHGsdeterminetheresultingclimateresponse,whereasthechoiceofemissionsmetricusedtocal-culateaggregatedemissionsandremovalsofdifferentGHGsaffectswhatpointintimetheaggregatedGHGsarecalculatedtobenetzero.EmissionspathwaysthatreachandsustainnetzeroGHGemissionsdefinedbythe100-yearglobalwarmingpotentialareprojectedtoresultinadeclineinsurfacetemperatureafteranearlierpeak(highconfidence)(IPCC,2021c,p.30).Howindividualgasescontributetoglobaltemperatureincreasesisthusdifferen-tiatedbasedonthedistinctdynamicsbetweenlong-andshort-livedgases.ForCH4,arelativelyshort-livedgreenhousegas,decliningradiativeforcingcanbeachievedwithasteadygradualdecreasecomparedtocurrentemissionrates.183MethaneemissionsinlivestockandricesystemsMethane’satmosphericlifetimeissufficientlyshortforatmosphericconcentrationstobelargelydrivenbyemissionsoccurringonlyinrecentdecades.Thus,bring-ingemissionratesdownbelowlevelsexperiencedafewdecadesagowillleadtoreducedanthropogenicCH4concentrations,impliedforcingandacontributiontotemperaturechange.Thetime-independentrelationshipbetweenwarmingandtotalcumulativeemissions,observedforCO2,thereforedoesnotapplytoCH4.Tolimitfurthertemperatureincreases,therequirementfor“net-zero”emissions–whereemissionsmusteitherbecompletelyeliminatedoroffsetwithadditionalCO2removals–isonlystrictlynecessaryfromaphysicalscienceperspectiveforCO2,giventhatitscumulativeimpactsextendintotheverylongterm.Forshort-livedgases,aclimateimpactequivalentto“net-zeroCO2”canbeachievedwithsomeongoingemissions.Ithasbeendemonstratedthatnet-zeroGHGemissionsarenotnecessarilyrequiredfortemperaturestoremainbelow1.5°Cor2°C(IPCC,2022),anditwouldintheorybepossibletoachievethistemperaturegoalwithoutentirelyeliminatingoroffsettingCH4emissions(TanakaandO’Neill,2018).Itshouldbenoted,however,thattheParisAgreementdoesnotrefertostabilizedtemperatures,butrathersetsupperlimitsfortemperatureincrease(Mace,2016).Schleussneretal.(2019)showthatusingastep-pulsemetricsuchasGWPinthecontextoftheParisAgreementgoalscouldunderminetheintegrityoftheAgreement’smitigationtargetbyfailingtodelivernet-zeroCO2emissionsandensuringthatwarmingishalted.TherationalefortreatingCH4differentlytoCO2ismademoreevidentstillintheIPCC’sSpecialReportonglobalwarmingof1.5°C(IPCC,2018).Itstatesthattheinterquartilerangeofmethaneemissionsfromagricultureacrosspathwaysassessed,whichlimitglobalwarmingto1.5°Cwithnoorlimitedovershoot,shouldfallgloballybyapproximately11to30percentby2030,and24to47percentby2050,relativeto2010levels.Thephysicaldynamicsofhowlong-andshort-livedgasescontributetoover-alltemperaturechangearewellunderstood,andthedifferentgas-specificoptionsforreachinganygivenclimatetargetarewidelyrecognized(Allenetal.,2021).However,thefundamentalphysicalrequirementsoutlinedabovearenottheonlythingsthatamulti-gasclimatepolicymustconsider.Cost-effectiveness,equityandtechnicalfeasibilityareotherimportantconsiderations.Indeed,allmodelledglobalpathwaysassessedbytheIPCCthatlimitwarmingto1.5orbelow2degreesshowstrongandsustainedreductionsofglobalCH4emissions,inadditiontoreachingatleastnet-zeroCO2emissions(IPCC,2022).9.3.2Climateneutrality9.3.2.1Differentusesoftheterm“Climateneutral”isatermthatisbeingusedwithincreasingfrequency.However,sincetheconceptofclimateneutralityisnotuniquelydefined,itisusedinavari-etyofwaysandwithavarietyofmeanings.Itisthusimportantforthetermtobeclearlydefined,wheneveritisused,soastoavoidmisunderstandings.Inmanycases,totalkofclimateneutralityissynonymouswithachievingnet-zeroGHGemissions(consider,forinstance,theUnitedNations’ClimateNeutralNowinitiative,https://unfccc.int/climate-action/climate-neutral-now).FortheaggregationofdifferentGHGemissionsandremovals,theGWP100climatemetricistypicallyused.Climateneutralityissometimesusedmoreorlesssynonymously184Metricsforquantifyingtheimpactofmethaneemissionswiththeterm“carbonneutral”,forexampleinthedraftISOstandardISO/CD14068,wherecarbonneutralincludesallGHGs.However,initsSixthAssessmentReport,theIPCCdefinescarbonneutralityinrelationtoCO2aloneasa“conditioninwhichanthropogenicCO2emissionsassociatedwithasubjectarebalancedbyanthropogenicCO2removals”(IPCC,2021b,p.2221).TheIPCCusesthetermGHGneutralitywhenothernon-CO2greenhousegasesareincluded.ClimateneutralityisalsosometimesthoughtofasgoingbeyondbalancingemissionsandremovalsofGHGstoalsoincludeotherradiativeforcingmechanismssuchasaerosols,orchangesinalbedothataffectthelocalclimate.TheIPCC’sSpecialReportonglobalwarmingof1.5°Cdescribesclimateneutralityinthefollowingterms:Conceptofastateinwhichhumanactivitiesresultinnoneteffectontheclimatesystem.Achievingsuchastatewouldrequirebalancingofresidualemissionswithemission(CO2)removalaswellasaccountingforregionalorlocalbiogeophysicaleffectsofhumanactivitiesthat,forexample,affectsurfacealbedoorlocalclimate(IPCC,2018,p.545).Thepracticalimplementationofthisconceptiscomplexasitincludeswell-mixedgreenhousegasesthatcontributetoglobalclimatechange,aswellasclimateforcersthathaveonlyalocalclimateeffect.Recently,thetermclimateneutralhasalsobeenusedtodescribeasystemthatismakingeithernonetcontributiontochangesinradiativeforcing(Ridoutt,2021a)ornonetcontributiontoadditionaltemperatureincreases(Costaetal.,2021;PlaceandMitloehner,2021;Allenetal.,2022b).ThesedistinctionsaremadebyanalogywiththeCO2-specificoutcomesofachievingcarbonneutralityasdefinedabove,andbasedontheunderstandingthattostabilizetheclimateatanylevelemissionsneedtobemanagedinsuchawaythatradiativeforcingandtemperaturesarenotbeingdrivenhigherandhigher.Thisapproachimpliesaverydifferentroleofshort-livedgaseslikeCH4,sinceanongoingemissionofCH4ataslowlyreducingrateresultsinacontributiontowarmingthatremainsstableovertime.Theevaluationoftargetssuchasnonetadditionalchangeintemperatureatthenationalorcor-poratelevelcannotbeundertakenfromaphysicalscienceperspectivealone;italsodependsoneconomic,social,equityandpoliticalconsiderations,includingrespon-sibilityforpastwarming(Allenetal.2022a).Thisconceptofclimateneutralityhasbeenappliedtoradiativeforcingfootprints(RidouttandHuang,2019)oranyofthevariousstep-pulsemetrics(Allenetal.,2016,2018;Collinsetal.,2020;Smith,CainandAllen,2021).Itisimportanttonotethatthestabilizationoftemperaturecontributionsfromeachindividualgasdoesnottelluswhetherthisiscost-effectiveorequitable,orwhethertheimpliedemissionreductionsaretechnicallyfeasible.Eachoftheabovedefinitionsofclimateneutrality–net-zeroGHGemissionsusingGWP100,havingnoneteffectonclimate,ornonetadditionalchangeintem-peratureorradiativeforcingrelativetoareferencedate–canhaveverydifferentimplicationsfortheemissionreductionsthatwouldneedtobeachievedifshort-livedgasessuchasCH4playasignificantroleintheoverallemissionsofasector,andforthetotalamountofglobalwarmingcontributedbysectorsthatadopt“cli-mateneutral”targets.Forthisreason,claimsandtargetsofclimateneutrality,andthemitigationambitionsimpliedbythosetargets,areeasilymisunderstoodandmisinterpretedunlesstheirspecificmeaningandimplicationsareclarified.185Methaneemissionsinlivestockandricesystems9.3.2.2ClimatemetricsandclimateneutralityIfacarbonneutralcommitmentismade,followingthedefinitionintheIPCC’sSixthAssessmentReport,climatemetricsarenotneededasonlyCO2emissionsareconsidered.Themainissuesthatariserelatetothelevelofoffsettingrelativetoemissionsreductioninthesystemitself.However,ifaGHGneutralcommitmentismade,themetric-weightedanthro-pogenicGHGemissionsassociatedwithasubjectarebalancedbymetric-weightedanthropogenicGHGremovals.NeutralityoftenincludesScope3emissions(whichareindirectemissionseitherupstreamordownstreamofabusiness,andnotdirectlywithinthebusiness’scontrol).Net-zeroGHGemissionsarealsometric-weightednetanthropogenicGHGemissions,butoftendonotincludeScope3emissions.Differentorganizationsworkwithdifferentexactdefinitions.ThequantificationofGHGemissionsandremovalsdependsontheGHG-emissionmetricchosentocompareemissionsandremovalsofdifferentgases,aswellasthetimehorizoncho-senforthatmetric.Consequently,thechoiceofemissionmetricstoreachandsus-tainnet-zeroGHGlevelswillaffecttheirresultingtemperatureoutcome(IPCC,2021;Fuglestvedtetal.,2018).Inpracticeandbyconvention,theGWP100climatemetricisusedinmostprogrammes.Reachingandsustainingnet-zeroGHGemis-sionstypicallyleadstoapeakanddeclineintemperatureswhenquantifiedwiththeGWP100(IPCC,2021).However,itisimportanttobemindfulthatwhenorganiza-tionsmakecommitmentstoreduceand/oroffsetaggregatedGHGemissionsusingtheGWP100climatemetric,itisnotimmediatelyclearhowmuchthiswillchangefutureradiativeforcingandtemperatures,asthiswillvaryovertimedependingontheparticularbasketofGHGemissionsinvolved(Fuglestvedtetal.,2018;TanakaandO’Neill,2018;Allenetal.,2021,2022c).Net-zeroGHGemissionsdefinedbyCGTPorGWPimplynet-zeroCO2andotherlong-livedGHGemissions,andgraduallydecliningemissionsofshort-livedgases.Theglobalwarmingevolutionresultingfromglobalnet-zeroGHGemissionsdefinedwithastep-pulsemetriccor-responds(intermsofradiativeforcingandtemperature)approximatelytoreachingnet-zeroCO2emissions,andwouldthusnotleadtodecliningtemperaturesafternet-zeroGHGemissionsareachievedbuttoanapproximatetemperaturestabiliza-tion(IPCC,2021).Thetemperaturelevelsatstabilizationwilldependoncumu-lativeCO2emissionsovertheentirehistoricalperiodandtheongoingemissionratesofshort-livedgases.Whilethisisarobustphysicalconcept,theassessmentofwhatconstitutesanappropriatetargetforanysubglobalentityalsodependsoneconomic,social,equityandpoliticalconsiderations,includingresponsibilityforpastwarming(Allenetal.,2022a).Thevastdifferencesinthefutureclimateimpactofshort-andlong-livedclimateforcersareakeyissue.Forlong-livedclimateforcers,suchasCO2andN2O,net-zeroemissionsleadtoclimatestabilization.TheclimateimpactofaCO2emissionpotentiallylastsformillennia.Therefore,ongoingnetemissionsofCO2wouldbeinconsistentwithclimatestabilizationwithinanyhumantimeframe.EvenN2Ohasalifetimeandclimateimpactthatexceedsthetimeframebywhichclimatestabili-zationneedstooccuriftheParisAgreementtemperaturetargetsaretobemet,ifnotfarexceeded.Additionally,theemissionoftheseGHGswillnotonlyhampertheachievementofclimatetargetsby2100,butalsoleadtoincreasedtemperatureoverlongertimeframes.However,short-livedclimateforcerslikeCH4,withanatmosphericlifetimeintheorderofadecadeorless,donotneedtobereduced186MetricsforquantifyingtheimpactofmethaneemissionstonetzerotoachievetheParisAgreementgoals.WithCH4,amodestlyreducingemissionsprofileovertime,wherebynewemissionsarebalancedbythedecayofCH4fromrecenthistoricalemissions,leadstoCH4-causedclimatestabilization,ataleveldeterminedlargelybytheongoingrateofCH4emissions.However,green-housegasemission-reductionpathwaysthatlimitglobalwarmingtobelow2°C,assessedbytheIPCC’sSixthAssessmentReport(IPCC,2022),requirestringentCH4emissionreductionsofbetween45and50percentby2050relativeto2019levels,giventheplausibleratesofenergydecarbonizationandtheanticipatedevo-lutionofotherclimateforcers.MoresubstantialCH4emissionreductionsprovideamechanismtolowerthetemperature,andthusmaybeanimportantcontributortowardsachievingtheParisAgreementtemperaturegoal,wheretheunmaskingofaerosolwarmingwillleadtoadditionalchallenges(ShindellandSmith,2019),e.g.limitingpeaktemperatures(e.g.Smithetal.,2012)orcontributingtoadeclineintemperatureafter1.5°Cisexceeded.Usingstep-pulsemetricstodefineglobalGHGneutrality(ornet-zeroGHGemissions),bothshort-andlong-termclimateforcerscanbeaggregatedonthebasisoffuturechangeinwarming.Whilethiscouldbeusedtoachieveclimatestabiliza-tionglobally,itdoesnotfollowthatGHGneutralitydefinedinthiswayisconsis-tentwiththeParisAgreementgoals(Mace,2016).Inaddition,weknowwithhighconfidencethatmerelystabilizingglobalwarmingduetoCH4wouldmakeachiev-ingtemperaturegoalsmuchmoredifficultorimpossibletoachievebecausestabiliz-ingradiativeforcingfromCH4wouldaddapproximately0.2°Ctoglobalwarmingin2100comparedtoatypical1.5°C-compliantscenario(Cainetal.,2022).CurrentCH4emissionscontributeabouthalfadegreetoglobalwarming(IPCC,2021),whichcouldbereducedbyfutureemissionreductions.Theremainingcarbonbud-getforlimitingwarmingto1.5°Crangesbetween600and300GtCO2dependingonwhethernon-CO2climateforcingsarestronglyorweaklymitigated,furtherdemonstratingthemagnitudeoftheirrole(IPCC,2023).AnapproachusingGWPatasubgloballevelhasbeenusedincasestudiesinthelivestocksector(Ridoutt,2021b;delPrado,ManzanoandPardo,2021).Akeychallengeinapplyingthemethodhastodowithestablishingwhatisanindefinitechangeintherateofshort-livedclimateforceremissions.Thisrequiresdefiningabaseline,andinmanypasture-andrangeland-basedlivestockproductionsystems,emissionscanfluctuatequitestronglyfromyeartoyear.Itcanthereforebedifficulttoascertainapermanentchangeinemissionrates.Anotherapproachistheradiativeforcingclimatefootprint,wherethecontri-butiontoradiativeforcingofcurrentyearemissionsissummedwiththeradia-tiveforcingfromhistoricalemissionsthatremainintheatmosphere(RidouttandHuang,2019;ISO,2021).Bytrackingprogressovertime,anorganizationorsectorcanassesswhethertheirtotalcontributiontoradiativeforcingisincreasingandtakemanagementactiontostabilizeorreduceit.Asituationwhereanorganiza-tionorindustryismakingnoadditionalcontributiontoradiativeforcingcouldberegardedasconsistentwithclimatestabilizationanddescribedasclimateneutralforthisparticulardefinitionoftheterm,notingthatsuchaninterpretationcanbecontestedonthegroundsofeconomic,social,equityandpoliticalconsiderations,includingresponsibilityforpastwarming,especiallygiventhelackofaccepteddefi-nitionsofclimateneutrality,asoutlinedabove.ThisapproachhasbeenappliedtothemainGHGsassociatedwithlivestockproduction,i.e.CO2,CH4andN2O,187MethaneemissionsinlivestockandricesystemsasdemonstratedforsheepproductionformeatinAustralia(Ridoutt,2021a).Itcanalsobeextendedtoincludenotwell-mixedGHGsandotherdriversofradiativeforcing(suchaschangeinalbedo)inaregionalcontext.Thiscouldberelevantwheretheburningofbiomassoccurs,wherelandtransformationandmanagementprac-ticesleadtochangesinsurfacealbedo,andinsensitiveorhigh-riskenvironments.Neitheroftheseapproachestoneutralityalonecanresolvethequestionofwhatanacceptablelevelofradiativeforcingfromanorganizationorindustrycouldbe.Ifanacceptablelevelofradiativeforcingorglobalwarmingcouldbeidentifiedonthebasisofeconomic,social,equityandpoliticalconsiderations,thenanyoftheabovedefini-tionscouldthenbeappliedinsuchawayastoremainwithinthatacceptablelimit.9.3.3MethaneabatementandsustainableagricultureClimatemetricscanhelptodefineandreportclimategoalsandactionsfromamultigasperspective.Thishelpstoassesstheimpactsofemissionsandremovalsofdifferentgreenhousegasesandfostersanunderstandingofthetrade-offsbetweennear-andlonger-termclimateeffects.However,inpursuingclimateaction,itisimportanttoalsoconsiderwidersustainabilitygoals,suchasthoseoutlinedintheUnitedNations’SustainableDevelopmentGoals,aswellassustainabilityprioritiesrelevanttoeachlocalcontext.Forexample,differentcommunitiesandcountrieshavedifferentlevelsoffoodsecurity.Thesocio-economicaspectsareespeciallyimportantforthesmall-scalelivestocksectorindevelopingandemergingecono-mies,sinceitcanprovideadditionalincomeandsupportsocio-economicdevel-opments.Sustainabilityisabroadconceptwithsocial,environmental,economicandculturaldimensions.Sustainableagriculturehasbeenvariouslydescribed.Onerecentdefinitionofsustainablelivestockproductionstates:Livestocksustainabilityreferstoproductionapproachesthatsimulta-neouslymeetlong-termconditionstoensuresociety’sfoodandnutri-tionsecurity,livelihoodsandeconomicgrowth,animalhealthandanimalwelfare,andstableclimateandefficientresourceuse(thefourlivestocksustainabilitydomains)inordertocontributetosustainablefoodsystems(GASLSecretariat,http://www.livestockdialogue.org/).Foreachdimensionofsustainability,avarietyofindicatorsexist.Onlybyassessingimpactsbroadlycantrade-offsbeevaluatedandmanaged.9.3.4EquityconsiderationsAconcernforequityisreflectedintheParisAgreement,andthisisaconsiderationwhenusingclimatemetricstodefineandreportclimategoalsandactions.Thatsaid,equityconsiderationsgobeyondscienceandultimatelyrestuponvaluejudgementsandethics(Stavinsetal.,2014;RobiouduPontetal.,2016;KlinskyandWinkler,2018).Equityisnotanattributeofclimatemetricsthemselves,butequityconsider-ationscanhelptodeterminewhatmetricsareused,howmetricsareappliedandforwhatpurposes.Therearedifferencesacrosscountriesforbothresidualemissionsandtheremovalpotential,leadingtoscientific,politicalandequityissuesrelatedtoglobalnet-zeroGHGemissions(Fuglestvedtetal.,2018).Certainapplicationsofemissionmetricsmayraiseequityconcernsifrelevantissuesarenotconsideredupfront,andtheremayalsobecaseswheremetricscanhelpillustrateclimateequity188Metricsforquantifyingtheimpactofmethaneemissionsconsiderations.AfullanalysisofrelevantclimateequitytopicsisbeyondthescopeofthisLEAPreport,andthereisrelativelylittleliteratureexploringtheintersectionbetweenequityandGHGemissionmetricsspecifically(RogeljandSchleussner,2019;Harrisonetal.,2021).Modellingstudieshaveshownthattheuseofdiffer-entmethodstoattributehistoricalresponsibilitytodifferentnationsgivedifferentresults,duetonon-linearitiesintheclimatesystem(TrudingerandEnting,2005;HöhneandBlok,2005).Pulse-emissionmetrics(suchasGWP100)cannotdirectlyreflecttheoverallcon-tributiontoglobalwarmingmadebydifferentemittersfromaseriesofemissionsoveranextendedperiodoftime.Lynchetal.(2020)suggestthatuseofGWPcouldallowemitterstobeheldaccountablefortheirfullhistoricalcontributionstoglobalwarming,inawaythatisnotpossibleusingGWP100,butthisrequirestracingthewholeemissiontrajectoryfromasufficientlyearlybaseline(forexample,prein-dustrial).Asstep-pulsemetricspresentanaccurateweightingintermsoftempera-tureoutcome,theymaythusprovideanalternativemethodtoincludeshort-livedgasesincumulativeemissionbudgets.Therefore,theycouldbeusedtoexplorenationalorsectoral“fairshares”oftotalwarmingcontributions.Suchapproaches,iftheyarenotappliedtothefullhistoricalemissions,needtobemindfulofequityconsiderations.Inthissense,RogeljandSchleussner(2019)arguedthat,giventheinequalityinhistoricemissions,usingGWPwithapresent-daybaselinecouldresultinhighlyunequalandunfairoutcomesbenefittinghistoricallyhigh-emittingcountries,sectors,evendowntotheindividualcompanyorfarmlevel.This“grand-fathering”canbeavoidedbytakingapreindustrialbaseline,asnotedabove,butthisraiseschallengesfortheequitableallocationofresponsibilityforthewarmingwithincountries.Thisunderscoresthatreflectionsonequityandfairnessarecen-traltoanyapplicationofstep-pulsemetricsonthenationalorcorporatelevelsincethechoiceofthebaseline(whichhasasignificantimpactonthemetric-reported“equivalent-emissions”)isanormativedecisionandnotaphysicalone.Thesespe-cificequityconsiderationsdonotapplytotheuseofpulsemetricssuchasGWP100,astheytreateveryunitofagivenGHGequallyandindependentlyfromtheemitterandthepointintimeatwhichtheemissionoccurred.Ontheotherhand,Lynchetal.(2020)alsoarguethatGWP100net-zerotargetsimplicitlysetabaselinetargetforCO2-inducedwarmingatwhateverlevelwasreachedpriortoarrivingatnet-zero,irrespectiveofhowmuchwarminganemit-termaycontinuetocausethroughtheirpastemissions,andhenceofongoingresponsibilityforclimatedamages.SimilarconcernsaboutcontinuedwarmingfromhistoricalCO2emissionswerethebasisofthe“Brazilianproposal”fortheKyotoProtocoltosetemissionreductiontargetsbasedonhistoricalcontribu-tionstoglobalwarming.However,settinganet-zeroemissionstargetbasedonGWPbaselinedonpresent-dayemissionswouldcompoundratherthanresolvesuchinequities,sinceitwouldretaintheimbalancecausedbyhistoricalCO2emissions,andaddadditionalbutseparateinequitiesbyallowingemitterswithcurrentlyhighCH4tocontinueemittingCH4atahighrateintotheindefinitefuture,whereasemitterswithcurrentlylowCH4emissionswouldbeforcedtoremainatthoselowlevels.Metricselectionandappropriatedeploymentmaybechosentoreflectcer-tainequityconsiderations,butrequiresausertorecognizeandchooseacertainperspectiveonthefundamentalconcernsraised,suchaswhetherandhowtoset189Methaneemissionsinlivestockandricesystemseffort-sharingexpectationsbasedonthemitigationpotentialofcontemporaryemissionreductionsoronanactor’scontributiontooverallglobalwarming,orhowtoallocateresponsibilityforwarmingfromhistoricalemissionsfromdiffer-entgasesacrosstoday’semittersthatmaynotsharethesameemissionsprofile.Theassumptionsbehindtheselectionofmetricsandtheinterpretationsregardingmat-tersofequityshouldbereportedtransparently.9.4METRICSELECTIONGUIDEThissectionaimstoprovideanexampleofhowapractitionercouldapproachthedecision-makingprocesstoidentifyasuitablemetricforanyparticularquestionorusage,basedontheinformationandlearningcontainedelsewherewithinthisreport.Differentmetricsincorporatedifferenteffectsonclimatethatresultfromemissions,andmayreporttheseeffectscoveringvarioustimeframesorwithrespecttovariousreferenceconditions.Differentmetricsmaythereforebeusefulfordif-ferentpurposes.Thisreportdoesnotrecommendthesoleuseofoneparticularmetricforallpurposes,asthechoiceofmetricwilldependonthespecificquestionbeingasked,andmayalsorequirevaluejudgementsbasedontheprioritiesofthepractitionerororganization.Werecommendthatpractitionersfollowtheguidancesetoutbelowandconsiderhoweachpointisrelevanttotheirparticularneeds.Twoexamplesareprovidedtoshowthatthewayaquestionisframedinfluencestheselectionofappropriatemetric.9.4.1PointstoconsiderExampleboxes:Section9.4.1.2describesacasestudy(Example1)assessingtheimpactofadairyfarmwhichcouldstartusingafeedadditivetoreduceCH4emissions.ThroughoutSection9.4.1,inboxessuchasthisone,wehaveincludedthestepsrelevantforeach“pointtoconsider”,whichweretakeninconsideringthefirstexample.AfullaccountofthiscasestudycanbefoundinSection9.4.2andintheAppendix,whichalsofeaturesdetailedmodellingwork.9.4.1.1DefineyourquestionThisisthefirstandmostimportantstep.Ifaparticularmetricistobeusedasthetoolforanevaluation,thentheobjectiveoftheevaluationmustbeclearlydefined.Iftheendgoalisunclear,thenanappropriate(orinappropriate)metriccannotbeidenti-fied.Sometimestheultimategoalmaynotbeimmediatelyapparent.Forexample,practitionersmaybeaskedtodefineanemissionsreductiontarget.Butwhatistheiroverarchinggoal?Thesegoalsmightbe:•tominimizeemissionsofspecifiedGHGs;•toachievesomeexternally-determinedtargetforaggregatedGHGemissionsbasedonapredefinedmetric;•tolimit(atachosenlevel)orundotheorganization’soverallcontributiontoglobalwarming;•toidentifyatargetthatincludesbudgetaryconsiderations;190Metricsforquantifyingtheimpactofmethaneemissions•alloftheabove;and•other.Therecouldbeahierarchyofgoals,suchasidentifyingstrategiestoreachaclimatemitigationtargetfirst,andthenrankingthebeststrategiesbasedonfairness,equityoreffectivenesscriteria.Ifthesemotivationsaremadeapparent,itcanbecomeclearerwhichapproachisthemostappropriateonetoaddressaparticularquestion.Whenthereisamultitudeofconcurrentgoals,articulatingthemcanhelpidentifymetricssuitableorunsuitableforeachgoal.Thiswillrevealwhetherthereisonesuitablemet-ricorwhetherdifferentmetricsareneededtoaddressthedifferentgoals.SeeSection9.1.1.Example1:Adairyfarmerwantstoassessthebenefitsofusingaparticularfeedadditiveontheirherd.Thequestionisdefinedasfollows:IfIstartusingthefeedadditive,whatwillbetheimpactonclimatechangecomparedtonotusingtheadditive?Thisactionismotivatedbythedesiretoreducethefarm’senviron-mentalimpactinthecomingdecades.Thecurrentemissionsareknown,andthefarmercanassumethattheseemissionswillremainstableandcomparethistotheemissionsthatwouldbegeneratedifthefeedadditiveweretobeintroduced.9.4.1.2ExistingrequirementsformetricsThismayalreadybeincludedintheanswertothefirstquestion.However,ifthatisnotthecase,arethereanyregulationswhichrequiretheuseofaparticularmetric?Althoughaparticularmetricmaybemandatoryinsomecases,itisworthconsider-ingwhetheritfullymeetsyourneeds.Ifitdoesnot,anothermetricormodellingexercisemaybeneededtoinformyourplansorpolicies.Forexample,asensitivityanalysisusingseveralmetricscanguideyouinsettingyourtargetstoensurethattheimpactsatdifferenttimescalesandontemperatureoutcomesaredulyconsidered.Thissensitivityanalysiscanbeparticularlyusefulifyouroverallaimistogainabetterunderstandingofthewiderenvironmentalimpactofdifferentstrategies.SeeSection9.1.2andSection9.1.3.Example1:ThefarmeralreadyusesGWP100inanexistingGHGfootprintcalculator,butturnstoothergreenhousegasmetricstoinformaninternalstrategicanalysis.9.4.1.3TimeframeAlternativeemissionmetricscandiffergreatlyinhowtheyreportgreenhousegasesas“equivalent”tooneanother(seeSection9.1forfurtherdetails).Thesedifferencesariseprimarilybecausedifferentgreenhousegasesshowadistincttime-dependenceintheirimpacts.Emissionmetricstypicallysetapredefinedtimehorizontocon-straincomparisonsandprovideasinglemeasureofequivalence,wheredifferent191Methaneemissionsinlivestockandricesystemstimehorizonswillresultindifferentvaluations.Inordertomakeajudgementonasuitablemetric,thetimeframeunderconsiderationtoansweryourquestionormeetyourgoalmustthereforebeexplicitlyconsidered.Isyourpriorityminimizingyouroperation’scontributiontoglobalwarmingin2050,2100oratanotherspecifictime,atallofthesetimesandinanyoftheinterveningyears,oroveranindefiniteperiodsoastocoverthefullimpactsanticipatedfromanyemissions?Whenthereareshort-livedclimatepollutantsbeingassessed,theuseofapairoftimehorizonswithoneforashort(e.g.20years)timehorizonandanotheroneforalong(e.g.100years)timehorizonwillshowthedifferenceinthetemporalimpactsofclimatepollutants.Thisimprovestransparencyasnosingle-termmetriccaneffec-tivelycapturethetime-dependencyoftheimpactsofshort-livedclimatepollutantsandlong-livedclimatepollutants(SLCPsandLLCPs).Ockoetal.(2017)comparesusingbothashort-andlong-timehorizonmetrictotheconventionalreportingofsystolic-diastolicbloodpressure–eachvalueismeaningfulontheirownbuttheyaremorevaluablewhenreportedtogether.TheLifeCycleInitiative,jointlyhostedbytheUnitedNationsEnvironmentProgramme(UNEP)andtheSocietyofEnvironmentalToxicologyandChemistry(SETAC),recommendsreportingboththeGTP100,toindicatelonger-termclimateimpacts,andtheGWP100toindicateshorter-termclimateimpacts,andoptionallytheGWP20forverynear-termclimateimpacts(Jollietetal.,2018).Proposedlong-termmetricsforthemetricpairingincludeGTP100(CherubiniandTanaka,2016;Cherubinietal.,2016;Levasseuretal.,2016;Jollietetal.,2018)andGWP100(Ockoetal.,2017).Proposedshort-termmetricsforthepairingincludeGWP100,GWP20andGTP20(Cherubinietal.,2016;CherubiniandTanaka,2016;Levasseuretal.,2016;Ockoetal.,2017;Jollietetal.2018).Anotheroptionwouldbetousethelong-termimpactmetricsCGTPorGWPtoevaluateendpointtempera-tures100yearsfromnow.Ascommonlyused,theywouldreportadditionalwarmingrelativetoachosenyear.Thisinformationcouldstillbecomparedwithashort-termmetricifappliedto20yearsfromnow.Theuseoftwoormoremetricswithdifferenttimehorizonsorformulationscanhelpunderstandhowrobustagivenmitigationstrategyisacrossarangeoftimehori-zonsandgivenvariousunderlyingmotivations.Forexample,ifagivenmitigationstrat-egyresultsinclimatebenefitswhenbothGWP20andGTP100areusedasalternativemetrics,thenthiswouldberegardedasahighlyrobuststrategy;whereasifagivenmiti-gationstrategywoulddeliverclimatebenefitsforonemetricbutwouldincreaseclimatechangeunderanothermetric,thenadditionalthoughtmaybewarrantedtodeterminewhetherthestrategyshouldbeadopted.Notethat,evenifitdoesnotyieldbenefitsaccordingtoallmetrics,itmaystillmakesensetoadoptastrategyusingwhichevermetricismostalignedwithanorganization’sobjectivesandtimehorizonsforanaction.Arelatedconcepttoconsideristhatof“discounting”,wherebyfuturebenefitsorimpactsarevaluedatadecliningratecomparedtothepresent(seeSection9.1.6).Thetimehorizoncanalsobechoseninlinewiththediscountratesusedforotherstrategicdecisions.Differentmetricsandtimehorizonseffectivelycorrespondtodifferentdiscountrates(SarofimandGiordano,2018;MallapragadaandMignone,2020).Highdiscountratesplacelessvalueonimpactsfurtherintothefuture,emphasizinginsteadtheimpactofshorter-livedpollutants.Economicconsiderationscanprovidefurtherinsightintothechoiceofmet-ric.AsindicatedinChapter2andAnnexIIofWorkingGroupIIIoftheIPCC’sSixthAssessmentReport(IPCC,2022b;Dhakal,MinxandToth,2022),thereis192MetricsforquantifyingtheimpactofmethaneemissionsincreasingevidencesupportingtheuseofGWP100underpathwaystowardtheParisAgreementgoalsasanapproximationofeconomicallyoptimalmetricsatleastuntilthemid-century(Tanakaetal.,2021).MetricsforCH4derivedfromcost-benefitandcost-effectivenessframeworkshavevaluesthatlieroughlybetween20and40,whichismoreconsistentwithGWP100thanwithGTP100orGWP20.Whilethissup-portstheadoptionofGWP100intheParisRulebookandtheuseofGWP100inthiscontext,itshouldbenotedthatthiswasaninadvertentoutcomebecauseGWP100isbydefinitionnotintendedtocaptureeconomicoptimality.Ausermaynotwishtodefineanytimehorizonordiscountrate,butinsteadtrytodirectlydemonstratehowglobalwarmingimpactsfromemissionswillvaryovertimeunderarangeofmitigationstrategies.Inthiscase,approachessuchasCGTPorGWPmaybeapplied,toreportrelativeimpactsnotjustatapredefinedtime,butspanninganynumberofyearsofinterest.Thisissimilartoprovidingmultiplemetricsand/oralternativetimehorizonstogiveinsightintothetemporalevolutionofdifferentclimatepollutants,butwithoutreportingafulltemporalevolution,asdescribedabove.Whenthesemetricsareused,thestartingpointforthetimeseriesiscriticalasitprovidesthebaselinelevelofwarmingagainstwhichanyfuturechangeintemperatureisexpressed.Inotherwords,usingtheterminologyintroducedinSection9.1.4,thesemetricswouldyieldinformationabouttheadditionalimpactofongoingemissionsrelativetothebase-lineyear,butnotastothemarginalimpactofongoingemissions.Whenappropriateandpractical,theuseofclimatemodelstoestimatetheclimateimpactisasuitablealternativetometrics(Farquharsonetal.,2017),asExample1illustrates.Thisisbecauseitoffersamorecomprehensiveandtransparentwayofdescribingcomplexclimateimpactthanasimplemetric.Itcouldbeusedeitheronitsownorasajustificationforselectingtheassessmentusingasinglemetricthatismostconsistentwiththesemoredetailedanalyses.SeeSection9.1.5andSection9.1.6,andSection9.2.1.toSection9.2.3.Example1:Thefarmerprimarilywantstoknowtheclimatebenefits(i.e.lowertemperatures)ofagiveninterventioninthespanofadecade.Theywouldalsowanttoknowwhatimplicationsitmighthave(i.e.highertemperature)atanypointbeforeorafter.9.4.1.4ContextandcounterfactualbaselineThecontextinwhichtheimpactsofanyemissionsareassessedmustbeconsideredbytheuser.Areyouinterestedinthetotalimpactsofanemissionscenarioyouareconsidering,potentiallycombiningtheimpactsofpastemissionswiththoseofcurrentemissions,andinhowthesecombinedimpactsmightrelatetoanoverallclimateobjective?Ordoyouonlywishtoassessthepotentiallyavoidablefutureimpactsthatwilloccurduetopresent-dayandimmediate-futureemissions?Pulsemetrics(e.g.GWPorGTPonanytimehorizon)capturetheimpactofanemissionrelativetonoemission.Inotherwords,thesemetricstellustheextenttowhichagivenemissioncontributestoglobalclimatechange(howmuchwarmertheclimateisbecauseofthissource),andconversely,howtheirspecifiedclimateimpactscouldbeavoidedifwedidn’treleaseanygivenemission(marginalwarming,193MethaneemissionsinlivestockandricesystemsseeSection9.1.4).Theycanalsobeusedtocomparetheclimateimpactsofalter-nativemitigationstrategiesbyevaluatingtheCO2-equivalentemissionsbasedontwoscenarios(e.g.withandwithoutaparticularmitigationstrategybeingimple-mented)anddeterminingthedifferencebetweenthoseemissionscenarios.ThisisdemonstratedthroughExample1(Section9.4.2.1).Step-pulsemetricsliketheGWPcapturethetemperatureimpactofanemissionrelativetothetemperatureimpactatabaselineyear(additionalwarming;seeSection9.1.4).However,ifyouarecalculatingequivalentemissionsfromabaselineyear,youshouldalsogiveproperconsiderationtowhatthatbaselineis(seeSection9.4.1.5).AsillustratedinSection9.1.4,thisconsiderationresultsinquitedifferentperspectivesforshort-andlong-livedGHGs,butalsoanswersafundamentallydifferentquestionsinceitpresentsthewarmingimpactonlyrelativetowarminginahistoricalreferenceyear.Forlong-livedGHGs,eachindividualemissionhasabroadlyadditiveimpact,andsotheoccurrenceofanyemissioncausesfurthertemperatureincreasesbeyondtheconditionsofthebaselineyear;theonlywaytonotablyreducetemperaturesbelowthebaselinewouldbethroughactiveGHGremoval.Forshort-livedGHGs,temperaturesdropbelowthoseofthebaselineyearsimplyasaresultofdecliningwarmingfrompriorshort-livedgasemissionsastheyareremovedfromtheatmosphere;thebaselinetemperaturewouldbemaintainedbyshort-livedgasemissionscontinuingatvirtuallythesamelevelfromthebaseyearonwards.Itisimportanttoclarifythatthisdoesnotinanysenseimplythatemissionsofashort-livedgreenhousegas(i.e.CH4)everresultinanactivecoolingoftheclimate.Areductioninemissionsofshort-livedGHGscanreducethetemperatureincreasesthattheyhadpre-viouslycaused,uptothepointofcompletelyphasingoutemissionsofthisshort-livedGHGandtherebyreversingmostofthetemperaturecontributionthattheyhadmade.Bothpulseandstep-pulsemetricscanusea“noemissions”ora“nofurtherpoli-cies”counterfactualbycalculatingtheCO2-equivalentemissionsofagivenmitigationscenario,andconsideringthedifferencebetweenthatandthecounterfactualscenario(e.g.“noemissions”or“nofurtherpolicies”).Whendifferentmetricsleadtothesamedecision,thecaseformakingthatdecisionismorecompelling.Shouldtheuseofdif-ferentmetricsleadtoadifferentoutcome,itwouldbeworthconsideringagainthecontext,thecounterfactualscenarioandthecriteriausedtomakethedecision.SeeSection9.1.4.Example1:Wewishtocomparea“businessasusual”scenariowitha“feedadditive”scenario.Wewouldalsoliketoknowtheclimateimpactofthesetwoscenarios,relativetoa“nofarm”scenario.9.4.1.5ComparabilityandtransparencyThecomparabilityofmetricsandwhethertheassessmentboundariesaffectingthemetricsaretransparentisimportantfortheselectionprocess.SinceGWP100isthemostcommonlyusedmetric,includingforreportingundertheParisAgreement,doingtheimpactassessmentreportwithGWP100isoftenperceivedasameansofensuringthatitcaneasilybecomparedwithvariousotherassessments(Levasseuretal.,2016).Shouldothermetricsbeselected,thenalsousingGWP100for194Metricsforquantifyingtheimpactofmethaneemissionstheassessmentcanimproveitscomparabilityandtransparency.Iftheassessmentisgreatlyaffectedbythechoiceofmetric,thenexplainingwhytheassessmentresultsaredifferenttothoseusingGWP100canimproveusers’understanding.TheboundariesorcounterfactualsoftheassessmentdonotchangetheamountofCO2-equivalentassignedtoatonneofemittedCH4whenusingGWPandGTP.Inthesepulsemetrics,anyunitofemissionisaccountedforthesameway,irrespec-tiveofthesourceorthepointintimeofemissions.Incontrast,theamountofCO2-warmingequivalentassignedtoatonneofemittedCH4usingGWPasdefinedbyForsteretal.(2021)dependsontheemissionsinthepresentand20yearsago.Thismeansthatitisdependentontheemissionhistoryofanindividualemitter(seeExample2)and,ifappliedonlyfromthepresentdayrelativeto20yearsprior,willonlyindicatetheadditionaleffectoftheemissionsonthetemperaturetrendatpresent.Inotherwords,usingGWPtocalculateCO2-warmingequivalentemissionswillindicatewhetherpresent-dayCH4emissionsarecausingthetemperaturetoriseorfall,butitwillnottellyouwhattheabsolutelevelofwarmingcausedbytheCH4emissionsis.Itmaythereforebeincompleteormisleadingtonoteonlythedirectionoftravel,andnottheabsolutelevelofwarming.Forexample,ifCH4emissionsdeclinedbyabout0.3percentperyear,basedonGWP,theCO2-warmingequivalentemissionswouldbezero,nomatterwhetherthatyear’semissionwas10tonnesor1milliontonnes.However,theabsolutelevelofCH4emissions(the10or1milliontonnes)determineshowmuchagivensourcecontributestoglobalwarming,andisrelevantforassessingwhetherthatlevelofemissionmightbedeemedacceptable.OnestraightforwardoptionwouldbetouseGWPinconjunctionwithanabsoluteannualCH4emissionexpressedintermsofametricreflectingmarginalimpacts,suchasGWP100.Step-pulsemetricslikeGWPdependnotonlyonchangesinemissionstoday,butalsoonthelevelofemissions20yearsago.Thiscausesnoproblemifthehistoricemissiontimeseriesisreasonablysmooth.However,inreal-worldapplicationstheremaybeconsiderableyear-to-yearvariabilityinCH4emissions,whichcancauseannualCO2-warmingequivalentemissionsusingGWPtobemorevariablethanthosecalculatedwithapulsemetric(MeinshausenandNicholls,2022).Thecumulativeimpactcalculatedovertimewouldremainaccuratedespitethisyear-to-yearvariability.Whilethisvariabilitywouldaccuratelyreflecttheconsequencesofavariabletimeseriesofemissions,itmayhaveimplicationsforthefeasibilityofapolicybasedonsuchemissions,whichmayneedtobeconsidered.9.4.1.6OtherconsiderationsTheremaybeotherrelevantconsiderationswhenchoosingmetrics,whichareunrelatedtotheunderlyingclimatescienceofmetricsandclimatepolicyobjec-tives(whichisourfocushere).Forexample,non-climateimpactslikeairqualityanditseffectsonhumanhealthandfoodproduction(UNEPandCCAC,2021),thestageofdevelopmentofacountryorregion,theimportanceofasectortoaregionrelativetootheropportunitiesandthecomparative/competitiveadvantagefromanemissionsperspectiveoneregionhasoveranother.Thismustbefactoredinbasedonthejudgementofpractitionersandwhetherthereisawiderscopecoveringmorethanjusttheclimateimpactsofemissions.SeeSection9.1.7.195Methaneemissionsinlivestockandricesystems9.4.2ExamplesThissectioncontainstwoexamplesdesignedtoillustratesomeoftheconceptsaroundmetricsdiscussedabove.Example1andExample2areexploredquantita-tivelytogivethereaderinsightsintotheimplicationsofusingthedifferentmetricsforanalysingthesecasestudies.Example1showswithinhowlongatimescaleeachmetriccanrepresentthetemperatureoutcomebyusingemissionsfromfarms.Theanswerisnotveryobviousfromthedefinitionofametricalonebecauseanactualapplicationmaydealwithsustainedemissionsoveracertainperiod(likeinthefirstexample),whicharedifferentfrompulseemissionsusedtodefinemetricssuchasGWP100.Example2illustratestheimportanceofselectinganappropriatebaseline,especiallywhenstep-pulsemetricssuchasGWPareused.9.4.2.1Example1:EvaluationofemissionmetricsinrepresentingthebenefitsofusingafeedadditiveAdairyfarmerwishestouseemissionmetricstoquantifytheclimatebenefitsthatwillresultfromusingacertainfeedadditiveontheirherd.Theiraimistoimprovetheirenvironmentalfootprintoverthenextdecade.Thisindicatesthattheywanttocomparetheemissionswhenusingthefeedadditiverelativetowhatemissionswouldbewithoutthefeedadditive(seeTable9).ThefarmeralreadyusesGWP100inaGHGcalculator,sothereisaprecedentthere.Table9showstheemissionsassociatedwiththefarmtoday(“controlfarm”)andwhenthefeedadditivehasbeenused.Methaneemissionsdecreasewiththeintro-ductionofthefeedadditive,buttheCO2emissionsincrease(duetotheproduction/distributionofthefeedadditive,basedoncurrentfossilfueluseintheenergysup-ply).Whatistheclimateimpactofswitchingfromthecontrolfarmtousingthefeedadditive?DoestheeffectoftheincreaseonCO2emissionsoutweightheeffectofthereducedCH4emissions?Wewillexplorethesequestionsnext.Thefarm’saggregatedannualGHGemissionsusingGWP100beforedeployingthefeedadditiveis2179tCO2eqperyear(usingAR6metricvalues:6027+1.68273+100=2179tothenearestroundnumber).ThefeedadditivelowersCH4emissionsbutraisesCO2,withacombinedeffectofreducingthefarm’sannualemissionsto1644tCO2eq(4027+1.68273+105=1644).Thesetotalsmayalsobedividedbytheoutputleavingthefarm(e.g.litresofmilk)toexpresstheemis-sionsasper-productratherthanper-farmfootprint(subjecttoanyallocationsthatmayberequiredaspartofthelifecycleassessment,suchasallocatingashareoftheemissionstootherco-productssuchasbeeforleather).Toreiteratethecontextout-linedabove,theseGHGfootprintstellustheclimateimpactsofthefarm’sannualemissions,relativetoascenarioinwhichthoseemissionswerenotmade(the“mar-ginal”impactoftheseemissions,asdiscussedinSection9.1.4).Implementingthefeedadditivethereforereducesthemarginalclimateimpactsofthefarm,asassessedusingtheGWP100(i.e.specifically,thetotalradiativeforcingforonehundredyearsTable9.AnnualemissionsassociatedwiththefarminExample1Controlfarm’sannualemissionsCH4tN2OtCO2tFeedadditivefarm’sannualemissions601.68100Source:Authors’ownelaboration.401.68105196MetricsforquantifyingtheimpactofmethaneemissionsTable10.Changeinannualemissionsfromusingthefeedadditivecomparedtothecontrolfarm,aggregatedusingGWP,GTPandGWPUnitCH4N2OCO2AggregatedTonnesofeachgassavedperyear-2005N/AGWP100CO2eqtonnessavedperyear-54005-535GWP20CO2eqtonnessavedperyear-159405-1589GTP100CO2eqtonnessavedperyear-9405-89GTP20CO2eqtonnessavedperyear-104005-1035GWPCO2eqtonnessavedperyear-253705-2532(forthefirst20years;2020-2039)GWPCO2eqtonnessavedperyear-15705-152(after20yearsofstabilizednewemissions;2040onwards)Source:Authors’ownelaboration.followingeachyear’semissionsisreducedbyusingthefeedadditive).Below,weconsiderwhyandhowdifferentmetricapproachesmayprovidedifferentquantifi-cationsofthesebenefits.Table10showsthechangeinCO2eqemissionsthatoccurswhenimplementingthefeedadditiveacrosstheherd(i.e.thedifferencebetweenannualemissionsunderbusinessasusualandimplementingthemeasure),ascalculatedusingdifferentmet-rics.ThereisarangeofvaluesofCO2eqemissionsfromthedifferentmetrics(alsoshowninFigure13),aseachmetriccapturesadifferentaspectoftheimpactofthoseemissionsontheclimatesystem.Forthepulsemetrics,thedifferenceinannualequivalentemissionsbetweenthetwoscenariosisthesameeveryyear.NotethatCO2eqemissionsarecalculatedusingtheIPCC’sAR6valuesforGWPandGTP(forexample,27forGWP100CH4)–withtheexceptionofCO2eqemissions,basedonGWP–whichusetheIPCC’sAR5valueofGWP100(thatis,28forGWP100CH4)asthisisconsistentwiththeGWPformulausedinAR6(Smith,CainandAllen,2021;footnoteinSection7.6.1.4ofIPCC[2021]).ForGWP,thereisagreatervalueplacedonthedifferenceinthefirst20yearsafterthefeedadditiveisintroduced(greaterthanthevalueplacedbyGWP20),4andthenasmallervalueplacedonthedifferencebeyondthattime(moresimilartothevalueplacedbyGTP100).5Howcanthesamedifferenceof20tonnesofCH4peryearvaryovertime?IfweweretotageachmoleculeofCH4fromthisfarmintheatmosphere,whenwereducetheCH4emissionby20tonnes,theamountoftaggedCH4leftintheatmospherewoulddeclineoveraperiodofapproximately20to40years.Itwouldthenstabilizeatanewequilibrium.Inotherwords,theimpactofreducingCH4emissionsontheatmosphereoccursinthefewdecadesthatimmedi-atelyfollowthechangeinemissions.Laterthanthat,theannualchangesinatmo-sphericmethanelevelsaremuchsmaller.GWPreflectsthiswithitstwoterms.Pulsemetricseitheraveragethesetime-varyingeffectsoveraspecifiedtimeperiod(e.g.GWP100),oronlyassessthemataparticulartimeperiod(e.g.GTP100).4TheCO2-equivalentemissionsavoidedannuallyinthefirst20yearsaftertheswitcharecalculatedasfollows,usingGWPbasedontheequationinSection9.1.3:28×(4.53×40-4.25×60)-28×(4.53×60-4.25×60)=-2537tCO2eq/year.5TheCO2-equivalentemissionsavoidedannuallymorethan20yearsaftertheswitcharecalculatedusingGWPbasedonthesameequation,butwith40tonnesofmethaneeveryyear:28×(4.53×40-4.25×40)-28×(4.53×60-4.25×60)=-157tCO2eq/year.197MethaneemissionsinlivestockandricesystemsToexplorewhatthatmeansinpracticeandtodemonstratehowthesemetricsrepresentthedifferentemissions,wehaveusedasimpleclimatemodel,theaggre-gatedcarboncycle,atmosphericchemistryandclimate(ACC2)model(TanakaandO’Neill,2018andTanakaetal.,2021;seetheAppendixfordetails).First,wehavemodelledascenariowherethecontrolfarmemissionsoccurbetween2000and2100,andshowntheimpactofthoseemissionsonglobalmeansurfacetem-perature(blacklineinFigure13I,b).Wehavethenmodelledtheemissionsforthescenarioinwhichthefarmstartsusingthefeedadditivein2020(blacklinesinFigure13II,d).Inbothcases,therearenoemissionsbefore2000.Thedifferenceintempera-turebetweenthesetwoscenarios(controlemissionsandfeedadditiveemissions)isalsoshownbytheblacklineinFigure13III,f.Thefeedadditivelowersbyaboutaquarterthelevelofglobalwarmingthatthisfarmwouldcauseby2100.ThisclearlydemonstratesthatthelevelofglobalwarmingduetotheincreasedCO2emissions(fromproducingthefeedadditive)issmallerthantheamountbywhichtheCH4reductionslowerthetemperature.Withmodelledtemperatureasthemetric,thereareclearbenefitstousingthefeedadditivecomparedtonotusingit.Theblacklinesshowthetemperaturechangefrommodellingtheseemissionscenariosrelativetothetemperaturein2000,whichwillallowustoillustratehowdifferentemissionmetricsrepresentthetemperaturechange,usingtheblacklinesasabenchmark.ThedifferencesinCO2eqemissionsbetweenthetwoscenariosareshowninTable10andhavebeenconvertedtoCO2eq.Totesthowwelltheyapproximatethetemperatureoutcomes,wehaveputtheCO2eqemissions(calcu-latedusingeachmetric)intothesimpleclimatemodelasCO2emissions.Themodelledwarmingarisingfromtheseemissionsisshownbythecolouredlinesinpanelsb,dandfofFigure13.Notethatthesearenotthetemperatureoutcomesofthefeedadditivescenario,butwhatwouldhappenifyouemittedthesameamountofCO2astheCO2eqemissionsdefinedbyeachmetric.Thisallowsustoillustratevariationsbetweenthedifferentmetrics,andcouldhelptoinformanevaluationofwhichmetricis“best”touseforaquestionorobjectiverelatedtoglobalwarming.PanelsbandddemonstratethelogicbehindwhyGWPplacesalargeCO2eqvalueonchangestoCH4emissionsfor20years,andasmallvaluethereafter.WhenyoumodeltheGWPemissions(yellow),thetemperaturecurveapproximatesthemodelledtemperaturefromtheoriginalemissions(black).Comparedwiththeotheremissionmetrics,GWPisclosesttoashort-termmetriclikeGWP20forthefirst20yearsandGTP100beyondthat(Figure13,panelsaandc).Itcanbeinter-pretedasGWPapproximatingthemodelledtemperatureoutcomesbyusingthesetwovaluesinonemetric.JustasCO2emissionsactcumulatively,GWPattemptstoreportCH4emissionsinsuchawaythatcumulativeemissionsalsolinkdirectlytotemperatureimpacts.Therefore,eventhoughthebenefitperyeardeclinesafter20years,thesignificantreductioninclimateimpactsachievedoverthefirst20yearspersists.Inthisexam-ple,thetotal(cumulative)avoidedGWP-calculatedCO2eqemissionsoveranyperiod(FigureA2cintheAppendix)couldalsobemultipliedbyaquantityknownasthetransientclimateresponsetocumulativeemissions(TCRE)–afactorthatscalescumulativeCO2emissionstotheresultingtemperaturechange(MacDougall,2016)–toestimatetheamountofavoidedwarmingfromthisinterventionattheendofthattimeperiod.Thisapproachcannotbeappliedtocumulativeemissionsusingthepulse-emissionmetricssuchasGWP100.198MetricsforquantifyingtheimpactofmethaneemissionsFigure13Scenarioscomputedusingtheaggregatedcarboncycle,atmosphericchemistryandclimate(ACC2)model.I.Controlfarmscenariocomparedtonofarmscenario(a)ChangeinCO2-equivalentemissions(tCO2e)(b)(×10-6)Changeinglobalwarning(°C)100000.2080000.15600040000.1020000.050-20000.0019802000202020402060208021001980200020202040206020802100II.Feedadditivefarmscenariocomparedtonofarmscenario(c)ChangeinCO2-equivalentemissions(tCO2e)(d)(×10-6)Changeinglobalwarning(°C)100000.2080000.15600040000.1020000.050-20000.0019802000202020402060208021001980200020202040206020802100III.Feedadditivefarmscenariocomparedtocontrolfarmscenario(e)ChangeinCO2-equivalentemissions(tCO2e)(f)(×10-6)Changeinglobalwarning(°C)2000202020402060208000.00-500-0.01-1000-0.02-1500-0.03-2000-0.04-2500-0.05-3000-0.06210019802000202020402060208021001980NometricuseGWP100GWP20GTP100GTP20GWPExample1:Evaluationofmetricsbasedonimpliedtemperaturesfromeachmetric-aggregatedCO2-equivalentemission.CO2-equivalentemissionsfromthecontrolfarmandfeedadditivefarmscenariosaggregatedusingdifferentemissionsmetrics(a,c)andresultingchangesinglobalwarmingcalculatedthroughthesimpleclimatemodelACC2usingtheselevelsofCO2emissions,comparedtothecaseinwhichnosuchfarmsexist(b,d).BlacklinesshowtheresultsbycalculatingtheavoidedwarmingseparatelyfromCO2,CH4andnitrousoxideemissions(thatis,emissionsarenotaggregatedintoCO2-equivalentemissionsforthe“nometricuse”warmingcalculations).Thecorrespondingresultsforthedifferencebetweenthecontrolfarmandthefeedadditivefarmareshowninthelasttwopanels:(e)CO2-equivalentemissionsavoidedeachyear(basedoneachmetric)byusingthefeedadditivebeginningin2020comparedtothosefromthecontrolfarm,and(f)avoidedwarmingcalculatedfromACC2usingCO2-equivalentemissionsbasedoneachmetric.Notabene,emissionsassociatedwiththelandinthecaseofhavingnofarmarenotconsideredhere(Manzano,P.&White,S.2019.Intensifyingpastoralismmaynotreducegreenhousegasemissions:Wildlife-dominatedlandscapescenariosasabaselineinlife-cycleanalysis.ClimateResearch,77:91–97.https://doi.org/10.3354/cr01555andFløjgaard,C.,Pedersen,P.B.M.,Sandom,C.J.,Svenning,J.&Ejrnæs,R.2022.Exploringanaturalbaselineforlarge-herbivorebiomassinecologicalrestoration.JournalofAppliedEcology,59(1):18–24.https://doi.org/10.1111/1365-2664.14047).Source:AdaptedfromTanaka,K.&O’Neill,B.C.2018.TheParisAgreementzero-emissionsgoalisnotalwaysconsistentwiththe1.5°Cand2°Ctemperaturetargets.NatureClimateChange,8(4):319–324.https://doi.org/10.1038/s41558-018-0097-xandTanaka,K.,Boucher,O.,Ciais,P.,Johansson,D.J.A.&Morfeldt,J.2021.Cost-effectiveimplementationoftheParisAgreementusingflexiblegreenhousegasmetrics.ScienceAdvances,7(22):eabf9020.https://doi.org/10.1126/sciadv.abf9020199MethaneemissionsinlivestockandricesystemsWhenusingGWP,thenetavoidedemissioneachyeartrapsthesameamountofadditionalenergyintheclimatesystemaswouldtheequivalentamountofCO2,integratedoutto100or20years,forGWP100orGWP20,respectively.ThedifferenceinmagnitudebetweenGWP100andGWP20valueshastodowiththefactthattheyareaveragedoverthe100-or20-yearperiods,andCH4hasmoreradiativeforcingimpactinthefirst20yearsfollowingtheemission.Importantly,GWP100andGWP20donotreflectthevaryingwarmingeffectswithinthesetimeperiods.Moreover,thisexampledealswithcontinuousemissions,whileGWPisdefinedusingpulseemissions.Thus,thetimehorizonofGWPisnotdirectlyrelatedtothetimescaleoftheemissionsinquestion.PanelsbanddshowthatGWP20(green),whichisdesignedtofocusonthewarmingpotentialovera20-yearperiod,approximatestheadditionalwarmingoverthefirst20yearswell,butafterthatoverestimatestheadditionalwarmingaswellasthetemperaturechangeduetothefeedadditive(panelf).GWP100(red),whichisappliedovera100-yeartimehorizon,underestimatesthetemperaturereductionforthefirsthundredyears.ForGWP100,thecumulativerelativewarmingbetweenthetwoscenarios(FigureA2intheAppendix)issomewhatunderestimated(redcomparedtoblack).IfusingGTP100orGTP20,thenetavoidedemissionseachyearwouldyieldthesamechangeintemperatureasfromtheequivalentamountofCO2,atatimepoint100or20yearsfollowingthatemission.However,asthisexampleshowsacontinu-ousemissionandnotapulseemission,themodelledtemperaturefortheoriginalemissionsandGTPemissions(blueandblack,panelsbandd)donotagree.Thereisa“sustainedGTP”metric,whichisbasedonthetemperaturechangeataspe-cifictimehorizonduetoaconstant1kgperyearincreaseinCH4emissions(seeSection9.1.2.2).ValuesofGTPsaresimilartothoseofGWP(Shineetal.,2005).Inthisexample,thisisborneoutbytheGWP100(red)temperatureintersectingwiththeactualtemperature(black)around100yearsaftertheemissionchangeoccurs.SimilarlytoGWP,thereisalargevariationbetweenGTPvaluesacrossdifferenttimehorizons,becausetheclimateimpactsfromCH4emissionsdeclinerapidlyafter20yearsfollowingtheemission.Thecumulativerelativewarming(FigureA2,panelscandfintheAppendix)fromGTP20showsgoodagreement(purplecom-paredtoblack)forthefirstcenturyapproximately.InExample1,allofthemetricsshowabenefittointroducingthefeedaddi-tive,whichreducesCH4emissionsbutatthesametimeincreasesCO2emissions.Consideringtherelativetemperaturechangewhenusingthefeedadditiveornot(Figure13f),GTP100indicatesaverymuchunderestimatedbenefit,whereasGWP20indicatesaverymuchoverestimatedbenefitbeyondabout40years.Meanwhile,GWPoverestimatesthetemperaturebenefitforthefirst50years,butshowsanaccurateagreementthereafter.TheGWPrepresentscomplexnon-linearclimateresponseswithonlytwotimescales(Allenetal.,2021),withcoefficientsbasedontheAR5impulse-responsemodel.TheresultsinFigure13suggesttheACC2modelrespondsslightlydifferentlytothemodelthatGWPapproximates,whichwouldexplainsomeofthedifferencesbetweentheyellowandblacklinesinFigure13,panelsb,dandf.However,allmetricsshowthatinthiscasethereisaclearbenefittousingthefeedadditive,whichisborneoutthroughmodellingtheactualchangesinemissions(black).Thisconclusionwillnotnecessarilyholdforeveryscenario,forexampleincasethefeedadditivewereassociatedwithsignificantlyhigherCO2emissions.200MetricsforquantifyingtheimpactofmethaneemissionsInconclusion,eachmetricprovidesadifferentquantificationofimpactsfromtheemissions.Hencetheimportanceofclearlydefiningthequestionorgoal,sothatanappropriatemetricormetricsmaybechosen.Intheabsenceofaspecifictimehorizonofinterest,multipletimehorizonscouldbeconsideredusingGWP20(toapproximatetemperatureimpactsoverthefirst30years)andGWP100(toapproxi-matetemperatureimpactsovera100-yearhorizon),aswellasGTP100(toapproxi-matetemperatureimpactsafteronehundredyears,forwhichtheGTPsversionofthemetricwouldbemostsuitableasitisasustainedchangetoemissionrates)orevenGWPoverthewholetimeseries.Step-pulsemetricscapturethetimevaria-tionsofimpactsontemperature,butpulsemetricsmayyieldanacceptableapproxi-mationofbenefits(e.g.cumulativerelativewarming)overspecifictimescales.FurtherdetailsforthisexamplemaybefoundintheAppendix.9.4.2.2Example2:Illustratingthepathdependencyofstep-pulsemetricsConsiderthatwehavethreefarmswhich,atpresent,havethesamenumberofcattleandemissionsasthecontrolfarminExample1(60tCH4,1.68tN2O,100tCO2).Despitehavingthesameemissionsin2020,allfarmshaveadifferentemissionshis-tory.FarmA(Abraham)hadstableemissionsforthewholetimesincebeingcreatedin2000.FarmB(Bethany)wasonlyestablishedin2020,andthushadzeroemis-sionsbeforethatyear.FarmC(Chris)hadtwicethecattle/emissionswhenitstartedin2000,butin2020itabruptlycuttheherd/emissionsinhalf.InapulsemetriclikeGWP100,thedifferenthistoryforthethreefarmerswouldnotaffectthevaluationoftheircurrentemissions:the2020CO2eqemissionsofallthreefarmerswouldamountto2178tCO2eq,asshownintheredlinesfrom2020onwardsinFigure14.TheyellowlinesinFigure14showtheCO2-weemissionscalculatedusingGWPforthewholetimeseriesofemissions,wheretheamountofCO2-weemissionscalculatedin2020dependsontheemissionsin2000,whichwerethendif-ferentforeachfarm.Thisreflectsthe“additional”natureofapplyingGWPtoascenario,inthatitisshowingtheadditionalimpactofemissionsatthatpointintime.TheCO2-welevelsforfarmAandfarmBareessentiallythesamebutshiftedby20years,asfarmBwasestablished20yearsafterfarmA.Forthefirst20yearsafterestablishment,eachofthesefarmsareallocatedtheirhighestamountofCO2-weusingGWP(over8000tCO2-we),whichisthenreducedtoaround1000tonnesCO2-weinthefollowingyears.Asaresultofthefarms’emissionsbeingdominatedbyCH4theabruptreductioninCH4emissionsinCH4forfarmCleadstohighlynegativeCO2-wevaluesforthe20yearsaftertheemissionsarehalved.ThisdoesnotmeanthattheremainingemissionsarenolongerGHGs(thoseemissionsdocausemarginalwarming),butratherthatthetemperatureincreasecausedbyemis-sionsuptotheyear2020ispartlyreversed.Withoutconsideringpriorwarming,applyingGWPcouldproduceseeminglycontradictoryresultsforfarmswiththesameemissionsatpresentbutwithdifferenthistories.ThisisbecauseGWP,inthisexample,isshowingthe“additional”effectsofthefarmsandnotthe“marginal”effects(seeSection9.1.4wherethistermino-logyisexplained).Example2goestoshowthataccountingforthefulltemperatureincreasecausedbyemissionsshouldbeacknowledgedinordertoavoidpotentiallymisleadingorinequitableoutcomes.ThiscouldbedonebyapplyingGWPtothefullhistoricaltimeseriesofemissions,orbyapplyingitrelativetothecasewheretherewaspreviouslynofarm(i.e.farmB)toshowthemaximumimpactofthefarm.201MethaneemissionsinlivestockandricesystemsFigure14CO2eqandCO2-weemissionsfromthethreefarmscalculatedusingGWP100andGWP(a)ChangeinCO2-equivalentemissions(tCO2eq)200001600012000800040000-400020202040Abrahamfarm206020802100-80002000(b)ChangeinCO2-equivalentemissions(tCO2eq)200001600012000800040000-400020202040Bethanyfarm206020802100-80002000(c)ChangeinCO2-equivalentemissions(tCO2eq)200001600012000800040000-400020202040Chrisfarm210020602080-80002000GWP100GWPThefullresultsbasedonallthemetricsconsideredhere,includingtemperaturecalculations,canbefoundinFigureA3oftheAppendix.Source:Authors’ownelaboration.202MetricsforquantifyingtheimpactofmethaneemissionsUsingalongertimeseriesforstep-pulsemetrics,insteadofasingleyearvaluetakeninisolation,willresultinamorecompleteassessmentthatdoesnotlosesightofthewidercontext.Alternatively,CO2-weemissionscouldbeevaluatedalongsideabso-luteannualCH4emissionstoindicateboththeadditionaleffectsoftheseemissionsandthelevelfromwhichtheyareincreasingordecreasing.9.4.3SummaryofkeyfeaturesandlimitationsofGWP,GWPandGTPOneaimofthissectionistosummarizesomeofthekeyfeaturesandlimitationsofGWP100,GWPandGTP.Whilebynomeanscomprehensive,itprovidesanout-lineandareminderfortheagriculturalpractitioneroftheissuesexploredinmoredetailthroughoutthisreport.9.4.3.1GWP100GWP100iscommonlyused,includingasthespecifiedmetricforreportingemissionstotheUnitedNations.Itprovidesanestimateofhowmuchenergyisaccumu-latedoverthe100-yeartimeperiodrelativetoanabsenceofemissions.ThistypeofmetricisthereforeusefuliftheimpactstotheclimatesystemasawholeoverthecomingcenturyarecomparedtonotemittingthoseGHGs(themarginalimpactsintheabovediscussion).ThesamecouldbesaidofGWP20,onlyforthe20yearsfollowingtheemission.GWP100andGWP20arederivedfrompulseemissions,andwhentheyareappliedtosustainedchangesinthelevelofemissionsovermultipleyears,themetrictimehorizons(100and20years)arenotindicativeoftheimpact(e.g.Example1inSection9.4.2.1).ThedisadvantageofGWP100isthatitdoesnotrelatedirectlytohowmuchCH4emissionschangethesurfacetemperatureovertime(theadditionalimpactsintheabovediscussion).Inparticular,itdoesnotreflectthefactthatreducingCH4emissionsdoesnotresultinadditionalwarming.Italsounder-representsthestrongwarmingeffectcausedbyintroducingnewCH4emissions.Consequently,thetemperatureoutcomesovertimefromanytrade-offsbeingconsideredwouldnotbeclear,because1tCO2eqofCH4doesnotcausethesameamountofwarmingatalltimesas1tCO2eqofN2OorCO2.SeealsoSection9.1.2.1.9.4.3.2GWPGWPisnotasingle-numbermetric,likeGWPandGTP.GWPapproximatesthewarmingthatarisesfromatimeseriesofshort-livedemissionslikeCH4,relativetothewarmingatthestartingpointofthattimeseries(termedadditionalwarming).Thisisarobustconceptatthegloballevel.However,anyevaluationatthenationalorcorporatelevelshouldalsopayheedtoeconomic,social,equityandpoliticalconsiderations(Allenetal.,2022a).TheminimumtimeseriesrequiredinthecaseofGWPistwodatapointsseparatedby20years,whereGWPcanbeusedtoevaluatetheeffectofthoseCH4emissionsrelativetotheemissions20yearsprior.Thismaybeadisad-vantageforsomeapplicationswhenthedataregardingemissionsfrom20yearspriorisnotavailable,orwhenassumptionsastowhattheymighthavebeencan-notbemade;italsomeansthatthevaluationofpresent-dayemissionsdependsstronglyonthelevelofemissionsfromthesamesourceproduced20yearsear-lier.Thisconditionforcestheusertospecifythequestionbeingasked(i.e.dowe203Methaneemissionsinlivestockandricesystemscareaboutthemarginalcontributionofanactivitytoglobalwarming,oronlyabouttheadditionalcontributionrelativetoagivenreferencedateand,ifso,why?),inordertoensurethatitiscorrectlyreflectedintheassumptionsmadeaboutpastemissions,whichrelatetoquestionsofresponsibility,equityandfair-ness.Ifthepresentdayisusedtoprovideabaseline,thenpathwaysareassessedrelativetothepresent-daylevelofwarmingfromCH4.Wewouldsuggestthatthislevelofwarmingbeexplicitlyevaluated,astheomissionofthisinformationcouldleadtowronglyassumingthatCH4emissionscouldcausecoolingcomparedtonoCH4emissionsoccurring.Instead,thebaselinelevelofwarming,whichisreflectedbyCO2-warmingequivalentemissionscalculatedusingGWP,canbereversedthroughCH4emissionreductions.Methaneemissionsreducingyearonyearwillcausetemperaturetodecreasecomparedtothebaselinelevelofwarming(additionalimpacts),butatthesametimecausehighertemperaturesthaniftheemissionsneveroccurred(marginalimpacts).Togiveamorecompleteanalysis,emissionscouldbeevaluatedstartingfromatimepriortothepresentday(e.g.atthetimewhentheorganizationorfarmwasestablished),orfromafuturepointintimethatmayberelevantforclimatepolicy(e.g.since1990oratanyfuturedatewhenaproposedclimatepolicyandassociatedaccountabilityforemissionswouldcomeintoeffect).GWPisausefulmetricifatimeseriesofemissionsisbeingevaluated,orcomparedtoanotheremissionscenario,basedontheirrespectiveimpactontem-perature(e.g.comparingthebenefitsfromseveralcompetingmitigationpathways).As1tCO2-wecalculatedwithGWPgeneratesapproximatelythesametempe-raturechangeovertime,nomatterwhichgasitrelatesto,trade-offscanbeassessedwithrespecttotheireffectonglobalwarmingusingGWP.SeealsoSection9.1.3.9.4.3.3GTPGTPcanbeusedtoestimatetheamountofwarmingthatwouldarisefromanemis-sionataspecifictimehorizon,comparedtoanabsenceofthatemission.Itisthere-foreusefultouseGTPifyouwanttocomparethetemperaturechangeataspeci-fiedtimewithandwithoutemittingtheGHGs.ThedisadvantageofGTPisthatthetimehorizonmustbespecified,andthereforemultiplecalculationswouldberequiredifmultipletimehorizonswereofinterest,orifmultipleyearsofemissionwererelatedtothesameend-pointyearthatwasofinterest.Forexample,ifoffset-tingaone-off1tCO2eqemissionofCH4withaCO2removalcalculatedusingGTP,thetemperatureimpactwouldbeequivalentatthatspecifictimehorizononly.WhenGTP100isappliedtoemissionsoccurringovermultipleyearsordecades,itdoesnotrepresentthetemperatureimpact100yearsfromthestartoftheexample.Forthis,the“sustainedGTP”metricwouldgiveabetterindicationoftemperatureoutcomes.SeealsoSection9.1.2.2.204ConclusionThereportcoversfourmaintopics:thesourcesandsinksofmethaneemissionsfromfoodandagriculture,thequantificationofmethaneemissions,themitigationofmethaneemissions,andthemetricsforquantifyingtheimpactofmethaneemis-sions.Thereporthighlightsthat:•Microbial-mediatedentericfermentativeprocessesinruminantlivestockcon-tributetoaround30percentofthetotalanthropogenicmethaneemissions,whiletheanaerobicdigestionofanimalmanureandotherorganicwastesandricepaddiescontributetoaround4.5percentand8percent,respectively.•Theatmosphericsinkthroughthechemicaldegradationofmethanebyhydroxylandchlorineradicalsinthetroposphereandstratosphereisrespon-siblefor90to96percentoftheglobalmethanesink,thesoilaccountsforabout4to10percentofthemethanedegraded,andtheoceanactsasasmallmethanesink.•Methanehasashorterlifetimethancarbondioxide,whichaffectsthequanti-ficationofgreenhousegasemissions,particularlyformethane.•Variousmethodsandmethodologiesareusedtomeasureandestimatemeth-aneemissionsfromruminantanimalsandthemanureproduced,includinggasexchangetechniques,head-stalls,thetracergastechnique,micrometeorologi-caltechniques,aircrafts,dronesandsatellites.Ineachofthesemethods,thereisatrade-offbetweeneaseofuse,repeatabilityandappropriatenessforhousedandgrazinganimals.•Thesuitabilityofagiventechniquefordeterminingmethaneemissionsfromricepaddiesalsodependsonmultiplefactors.•Severalmanagementpracticesthatinducetheincreasedredoxpotentialofsoilsuppressmethaneproductionandhencetheemissionsfromricefields.Thechoiceofoptionsdependsonthefeasibilityofthemanagementandpos-sibletrade-offs.•Decreasingentericmethaneemissionsfromruminantandricepaddiespro-ductioniscrucialtolimitingtheglobaltemperatureincreaseto1.5°Cby2050,andvariousstrategiesforentericmethaneabatementarebeinginvestigatedtothatend.•Thereportconsidersandanalysesthestateofplayoftheentericmethanemitigationstrategiesavailableatpresent(about30intotal),namelytheireffec-tiveness,safetyissues,theimpactonothergreenhousegasemissionsaswellaseconomic,regulatoryandsocietalaspects.•Mostresearchhasbeenconductedonconfinedanimals,andmoreresearchisneededtodevelopandevaluateanti-methanogenicstrategiesforgrazingsystems.•Continuousresearchanddevelopmentareneededtodeviseentericmethanemitigationstrategiesthatarelocallyapplicable,whilemoreinformationisrequiredtocalculatethecarbonfootprintofinterventionsonaregionalbasisandtoevaluatetheirimpactonnetgreenhousegasemissions.205Methaneemissionsinlivestockandricesystems•Greenhousegasemissionmetricsareusedtoquantifytheimpactofemis-sions(andthemitigationthereof)ontheclimatesystem.EachGHGemissionmetriccapturesaspecificclimateimpactoveraspecifictime;theequivalencebasedononemetricdoesnotimplyequivalencebasedonothermetrics.•Toprovidetherelevantinformation,themetricchoice–includingthetimehorizon–shouldtakeintoaccountthespecificmatterbeinginvestigatedandtherelevantpolicyobjectives.•Pulse-emissionmetrics(e.g.GWP100,GWP20,GTP100,GTP20)provideinfor-mationaboutfutureclimateimpactsofemissionunits,asopposedtotheabsenceofthoseemissions,whicharecalledthe“marginal”impacts.•Step-pulsemetrics(e.g.GWP,CGTP)provideinformationabout“addi-tional”impactsrelativetoaspecifieddate.•Dependingonthequestionposed,eitherpulse-emissionmetricsorstep-pulsemetricsmaybesuitable.Arangeofmetricscanbeusedtotestwhetherresultsareconsistentacrossdifferenttimescalesorwithrespecttodifferentimpacts.•Sincemetricsareusedastoolsbypolicymakers,itisimportanttoconsiderthemwithinthewidercontextoftheParisAgreement,definitionsofclimateneutrality,sustainableagricultureandequityconsiderations.206ReferencesAaheim,A.,Fuglestvedt,J.S.&Godal,O.2006.Costssavingsofaflexiblemulti-gasclimatepolicy.TheEnergyJournal,27:485–501.www.jstor.org/stable/23297097Abbott,D.W.,Aasen,I.M.,Beauchemin,K.A.,Grondahl,F.,Gruninger,R.,Hayes,M.,Huws,S.,Kenny,D.A.,Krizsan,S.J.,Kirwan,S.,Lind,V.,Meyer,U.,Ramin,M.,Theodoridou,K.,vonSoosten,D.,Walsh,P.,Waters,S.&Xing,X.2020.Seaweedandseaweedbioactivesformitigationofentericmethane:Challengesandopportunities.Animals,10(12):2432.https://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concentra-tions)onannual,decadalandcentennialtimescales.Suchmodelsdonotdealwithinterannualanddecadalvariabilityoftheearthsystem,norwiththeseasonalcyclewithinayear.Theydonotgenerallyprovideprojectionsataregionalscale.ACC2consistsofacarboncycle,atmosphericchemistry,physicalclimateandeconomymodules.Intheexamplesgivenhere,ACC2isusedasasimpleclimatemodel,withouttheeconomymodule,whichisrequiredwhenACC2isusedasanintegratedassessmentmodel.TheinputsintoACC2aretheemissionscenariosofgreenhousegasesandairpollutants.TheoutputsfromthemodelaretheprojectionsofatmosphericconcentrationsandradiativeforcingofCO2,CH4andN2O,amongother,andglobal-annual-meantemperaturechangesrelativetopreindustriallevels.ThephysicalclimatemoduleofACC2isanenergybalancemodelcoupledwiththeoceanheatdiffusionmodelDOECLIM(Kriegler,2005).Thecarboncyclemoduleisaboxmodelcomprisingthreeoceanboxes,fourlandboxesandacou-pledatmosphere-mixedlayerbox.Themodelcapturesthekeynon-linearitiesoftheglobalcarboncycle.TheoceanCO2uptakesaturateswithrisingatmosphericCO2concentrationduetothethermodynamicbalanceinvolvingcarbonatespecies(Hoossetal.,2001;Bruckneretal.,2003).ThelandCO2uptakefromthebiosphereincreaseswithrisingatmosphericCO2concentrationduetotheCO2fertilizationeffect.TheatmosphericchemistrymoduleaccountsforthetroposphericO3produc-tionfromCH4emissions.ThelifetimeofCH4isrelatedtotheOHconcentration,whichitselfdependsontheCH4concentrationandpollutantemissions,providingapositivefeedbacktothelifetimeofCH4.ThelifetimeofN2OisinverselyrelatedtotheN2Oconcentration,providinganegativefeedbacktotheN2Olifetime.Itisimportanttonotethateachforcingterm(orspecificallyatmosphericCO2,CH4andN2Oconcentrations)iscalculatedseparately,withoutanygasaggregation,usingemissionmetrics,unlessindicatedotherwise.Theequilibriumclimatesensitivityisassumedtobe3°C,thebestestimateofWorkingGroupIfortheIPCC’sSixth6NotethatthemetricsandformulaforGWPhavebeendevelopedusingdifferentsimpleclimatemodels,andnotACC2.Thediscrepancybetweenthemodels(i.e.IPCC’simpulse–responsefunctions)usedtoderivemetricvalues(includingtheGWPequation)andthemodel(i.e.ACC2)usedtoinvestigatethetemperatureimplicationsofmetricsmayexplainsomeofthedifferencesbetweenthetemperaturesrelyingonmetrics(colouredlines)andthetemperaturespurelyderivedfromthemodel(blacklines).309MethaneemissionsinlivestockandricesystemsAssessmentReport(IPCC,2021).OtheruncertainparametersareoptimizedbyusinghistoricaldataandobservationsbasedonaBayesianapproach(Tanakaetal.,2009b).Tocalculatethetemperatureeffectsofemissionsfromindividualsmallfarmsinourexamples,anassumptionisrequiredforthebackgroundemissions.Weadoptedtherepresentativeconcentrationpathway(RCP)4.5W/m2,anemissionscenarioinwhichtheradiativeforcingisstabilizedat4.5W/m2intheyear2100(Mossetal.,2010).Thus,inourexamples,emissionsfromindividualfarmsaremodelledontopoftheRCP4.5scenario.TheemissiondataforRCP4.5usedinouranal-ysisisconsistentwiththatusedintheintercomparisonprojectforsimpleclimatemodels(Nichollsetal.,2020).WhenweaddedfarmemissionstotheRCP4.5sce-nario,weassumed1000timeslargerfarmemissionsthantheoriginalmagnitudes.Thenthetemperaturedifferenceduetofarmemissionscalculatedfromthemodelwasdividedby1000.Table11showsthechangeinabsoluteCO2eqemissionswhenusingthefeedadditiveforindividualfarmsaggregatedusingGWP,GTPandGWP.Wecheckedthesensitivityoftheresultswithrespecttothescalingfactorandcon-firmedthattheresultsdonotdependonthescalingfactorwithinalargerangeinclud-ing1000.TableA1.Absoluteemissionswhenusingthefeedadditive,relativetonoemissions,aggregatedusingGWP,GTPandGWPUnitCH4N2OCO2AggregatedTonnesofeachgasperyear401.68105N/AGWP100CO2eqtonnesperyearGWP20CO2eqtonnesperyear10804581051644GTP100CO2eqtonnesperyearGTP20CO2eqtonnesperyear31884581053751GWPCO2eqtonnesperyear(forthefirst20years)188391105684GWPCO2eqtonnesperyear20804981052683(after20yearsofstabilizednewemissions)50744581055637314458105877NotethatCO2eqemissionsarecalculatedusingtheIPCC’sAR6valuesforGWPandGTP(forexample,27forGWP100CH4),withtheexceptionofCO2eqemissionsbasedonGWP,whichusetheIPCC’sAR5valueofGWP100(thatis,28forGWP100CH4)asdescribedintheGWPformula(Smith,CainandAllen,2021;footnoteofSection7.6.1.4inIPCC[2021]).NotethatachangetousingtheAR6valueofGWP100intheGWPformulawouldremainwellwithintheuncertaintiesandnotaffecttheresultsinanymeaningfulway.Source:Authors’ownelaboration.310AppendixFigureA1AdditionalresultsforExample1(a)(×10-6)Changeinglobalwarming(°C)0.200.15Nometricuse0.10GWP1000.05GWP200.001980200020202040206020802100(b)(×10-6)Changeinglobalwarming(°C)0.200.15Nometricuse0.10GTP1000.05GTP200.001980200020202040206020802100(c)(×10-6)Changeinglobalwarming(°C)0.200.150.10NGoWmPetricuse0.050.001980200020202040206020802100Modelledglobalwarmingfromthecontrolfarm(solid)andthefeedadditivefarm(dashed)scenarios(inblack).ColouredlinesshowmodelledglobalwarmingfromCO2emissionsderivedusingdifferentmetricsofequivalence.Panelsa,bandcshowGWP-basedequivalence,GTP-basedequivalenceandGWP-basedequivalence,respectively.Source:Authors’ownelaboration.311MethaneemissionsinlivestockandricesystemsFigureA2DetailedresultsforExample1(evaluationofemissionmetricsinrepresentingthebenefitsofusingafeedadditive)I.Controlfarmscenariocomparedtonofarmscenario(a)ChangeinCO2-equivalentemissions(b)(×10-6)Changeinglobalwarming(°C)(c)(×10-6)Changeincumulativewarming(°C/yr)10000(tCO2eq)0.45080000.340600040003020000.20200.110-200020502100215022000.0205021002150220002050210021502200200020002000II.Feedadditivefarmscenariocomparedtonofarmscenario(d)ChangeinCO2-equivalentemissions(e)(×10-6)Changeinglobalwarming(°C)(f)(×10-6)Changeincumulativewarming(°C/yr)10000(tCO2eq)0.45080000.340600040003020000.20200.110-200020502100215022000.0205021002150220002050210021502200200020002000III.Feedadditivefarmscenariocomparedtocontrolfarmscenario(g)ChangeinCO2-equivalentemissions(h)(×10-6)Changeinglobalwarming(°C)(i)(×10-6)Changeincumulativewarming(°C/yr)0(tCO2eq)0.000-500-0.04-2-1000-0.06-1500-0.08-4-2000-0.10-2500-6-8-10-30002050210021502200-0.122050210021502200-122050210021502200200020002000NometricuseGWP100GWP20GTP100GTP20GWPThisfigureshowstheresultsforalongertimescale(until2200),includingcumulativewarming,aproxyofclimatedamage.Seethemaintextforfurtherdetails.Source:Authors’ownelaboration.312AppendixFigureA3DetailedresultsforExample2(illustratingthepathdependencyofstep-pulsemetricsinrepresentingtheimpactofthreefarmerswithdifferenthistoricalemissions)I.Abrahamfarmcomparedtonofarm(a)ChangeinCO2-equivalentemissions(b)(×10-6)Changeinglobalwarming(°C)(c)(×10-6)Changeincumulativewarming(°C/yr)20000(tCO2eq)0.550160000.4401200080000.33040000.22000.110-4000-800020502100215022000.0205021002150220002050210021502200200020002000II.Bethanyfarmcomparedtonofarm(d)ChangeinCO2-equivalentemissions(e)(×10-6)Changeinglobalwarming(°C)(f)(×10-6)Changeincumulativewarming(°C/yr)20000(tCO2eq)0.550160000.4401200080000.33040000.22000.110-4000-800020502100215022000.0205021002150220002050210021502200200020002000III.Chrisfarmcomparedtonofarm(g)ChangeinCO2-equivalentemissions(g)(×10-6)Changeinglobalwarming(°C)(i)(×10-6)Changeincumulativewarming(°C/yr)20000(tCO2eq)0.550160000.4401200080000.33040000.22000.110-4000-800020502100215022000.0205021002150220002050210021502200200020002000IV.BethanyfarmcomparedtoAbrahamfarm(j)ChangeinCO2-equivalentemissions(k)(×10-6)Changeinglobalwarming(°C)(l)(×10-6)Changeincumulativewarming(°C/yr)9000(tCO2eq)0.0006000-0.02-230000-0.04-4-3000-0.06-6-6000-90002050210021502200-0.082050210021502200-82050210021502200200020002000V.ChrisfarmcomparedtoAbrahamfarm(j)ChangeinCO2-equivalentemissions(k)(×10-6)Changeinglobalwarming(°C)(l)(×10-6)Changeincumulativewarming(°C/yr)9000(tCO2eq)0.08860000.066300000.044-30000.022-6000-900020502100215022000.00205021002150220002050210021502200200020002000NometricuseGWP100GWP20GTP100GTP20GWPThisfigureshowstheresultsforalongertimescale(until2200),aswellasthetemperatureoutcomebasedonthesamemethodastheoneusedinExample1.Seethemaintextforfurtherdetails.Source:Authors’ownelaboration.313http://www.fao.org/partnerships/leap