Inventory of methane emissions from US cattle - Wiley Online Library

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Jun 27, 2001 - the West. The beef cow group released the most methane (-2.5 Tg yr -•) followed by. -1 ß. -1 ... production, but the number of cows needed to supply the U.S. consumer .... Four animals for each animal classification were randomly selected from .... Table 1 lists the emission rates measured for four beef cows.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106,NO. D12, PAGES 12,633-12,642,JUNE 27, 2001

Inventory of methane emissionsfrom U.S. cattle H. Westberg and B. Lamb Laboratory for Atmospheric Research, Washington StateUniversity,Pullman,Washington

K. A. Johnsonand M. Huyler Department of AnimalSciences, Washington StateUniversity,Pullman,Washington

Abstract.Manycountries, including theUnitedStates, arein theprocess of inventorying greenhouse gasemissions asa prerequisite for designing controlstrategies. We have developeda measurement-based inventoryof methaneemissionsfrom cattlein the United

States.Methaneemission factorswereestablished forthemajorlivestock groups usingan internaltracermethod.Thegroups studied included cows,replacement heifers,slaughter cattle,calves,andbullsin thebeefsector andcowsplusreplacement heifersin thedairy industry.Sincemethane emission is dependent onthequalityandquantityof feed,diets werechosenthatarerepresentative of thefeedregimesutilizedby producers in theUnited States.Regionalcattlepopulations, obtained fromU.S. Department of Agriculturestatistics, werecombinedwith themethaneemissionfactorsto yieldregionalemissionestimates.The

methane totalsfromthefiveregions werethensummed to giveanU.S. inventory of cattle emissions for 1990,1992,1994,1996,and1998. Annualreleases rangedfrom6.50Tg in 1990to a highof 6.98 Tg in 1996. On a regionalscaletheNorthCentralregionof theUnited Stateshadthelargestmethaneemissions fromlivestockfollowedby theSouthCentraland

theWest.Thebeefcowgroup released themost methane (-2.5-1Tgyr-•)followed by -1 ß

slaughter cattle(-1.7 Tg yr ) anddairycowsatabout1.5Tg yr . Methane released bycattle

in the United Statescontributesabout1163of the globalcattlesource. 1. Introduction

emissions (The Climate Change Action Plan, October 1993),

The concentration of methane in the atmosphere has increased dramatically over the last century [IntergovernmentalPanel on Climate Change (IPCC), 1990]. Methane is a greenhousegas (GHG) with global warming potentialsrelative to CO2 of 62 (20 years) and 24.5 (100 years) [IPCC, 1995] and is somewhatunique among the GHGs because it reacts with hydroxyl radical in the troposphere.This reactionimpactsozone formationand the overall oxidizing capacity of the lower atmosphere. The increase in troposphericmethane levels correlatesclosely with the global expansion in the human population. Consequently,it is believed that approximately 70% of methaneemissionsare anthropogenic.Landfills, fossil fuel usage,and agriculturalpracticesgenerateand releasemethane to the atmospherein amountsthat will increaseas the number of peoplein the world grows. Methane releasedto the atmosphereby domesticruminant livestock is consideredto be one of the three largest sources on a global scale [Cicerone and Oreroland, 1988]. In the

the majormethaneemissionsourcesarebeingexaminedwith respectto reductionstrategies.Furthermore,the United States has been a party to the international effort to control GHG releasesto the atmosphere. This has resulted in the Kyoto Protocol, which dictates a set of targets and timetables for reducing GHG emissions. The suggestedcommitmentfor the United

States is a 7% reduction

based on a 1990 "baseline"

level. If the United Statesratifies the agreement,it will have until the year 2010 to comply. In the meantime, it is importantto establish1990 "baselineemissionlevels" and to look at changesthat have occurredsincethat time. The emissionof methaneby cattlerepresentsa carbonloss pathwaythat resultsin reducedefficiency of feed energyuse. If the energy lost in generatingmethane could be used for weight gain or milk production,it would be beneficial to the cattle producer and provide a means of reducing methane emissionsto the atmosphere. For example, the U.S. dairy industry has increased per cow milk production very dramatically over the past 25 years. An increasein per cow methane emissions has accompanied the enhanced milk United States, emissions from ruminants accounted for 21% production, but the numberof cowsneededto supplythe U.S. of the total methanereleasedto the atmosphereduring 1990 consumer market has decreased substantially. Thus the [EnvironmentalProtection Agency (EPA), 1993]. This was higher methane emission rates of individual cows has been second only to landfill emissions which were estimated to more than offset by the decreasein dairy cow numbers. This contribute 36% of the 1990 methane emissions. Because of the United States commitment to control greenhouse hasresultedin estimatedmethanereductionof 0.2 Tg over the 30-year period between 1960 and 1990 (EPA unpublished data, 1993, available at www.epa.gov/outreach/rlep/ Copyright 2001 by theAmericanGeophysican Union. sustain.htm). Paper number 2000JD900808. 0148-0227/01/2000JD900808 $09.00

Past studies with

ruminant

animals

illustrate

that methane

production is dependenton the quantity and quality of the 12,633

12,634

WESTBERG

ET AL.: INVENTORY

OF METHANE

diet. Generally, highly digestiblefeedsyield lower methane emissionswhen comparedto poor qualitydiets. Thusfeedlot cattle fed high-grain diets convert about 2% of their energy intake to methane, while as high as 12% of gross energy intake (GEI) can be lost as methanewhen cattle are fed other types of diets. Dietary manipulationmay then provide a mechanism

for reduced

methane

emissions

from

EMISSIONS

FROM U.S. CATI'LE

production systems.A tracermethoddeveloped in our laboratory [Johnson et al., 1994a]wasusedto quantify methane emissions. The method allowsemissions testing fromcattlethatarein anambient environment typicalof those employed in U.S. production asopposed to confinement in a respirationcalorimetrychamber.Sufficientdataare available

domestic

to derivealgorithms thataccurately predictmethane emission ratesfromthedominantcattletypesin theUnitedStates.We Various methodshave been employedto inventoryglobal have combinedthe emissionrate datawith cattlecensusand

livestock [Johnson and Johnson, 1995].

methane emissions from ruminant animals. The early dietaryinformation to assemble an inventory of methane estimate of 100Tgyr-•byEhhalt[1974]tookintoaccount the emissions fromthe U.S. cattleindustry.Thisinventory amount of biomass consumed by ruminants and converted providesa baselineagainstwhichmethaneemission reduction that to an annual releaseto the atmosphere. Crutzenet al., strategies for livestockcanbejudged. [1986] based their estimate of ruminant methane emissions

(74 Tg) on a relationship published by Blaxter and Clapperton [1965]. The statisticalrelationshipemployedin this approach was developed from calorimetry chamber experiments using sheep. The Blaxter and Clapperton equationis best used to estimatemethaneemissionswhen feed types and feedinglevels are the sameas thoseusedto develop the statisticalmodel. It generallypredictsmethane emissionlossesin the rangeof 5 to 8% of grossenergyintake. As mentioned above with cattle, methanereleasescan vary between2 and 12% of GEl dependingon diet. Johnsonet al. [1993], using an extensive database,reviewed the ability of the Blaxter and Clapperton equation to predict methane emissions. The relationshipbetweenpredictedand observed

2. Experimental Method 2.1.

Animals

Four animalsfor eachanimalclassification wererandomly selected froma populationof cattlehousedat theWashington State University (WSU) Department of Animal Sciences animal facilities.

The classifications

measured were beef

cows,beef/dairyreplacement heifers,dairycows,beef stocker calves,feedlotsteers,growingbulls,andsucklingcalves.The

beefcowsaveraged 4 yearsof ageand585kgin bodyweight. Dairycowswere673 kg in weightandwereproducing 22.0 kg of milkperday. Stockercalvesandfeedlotsteers (544kg) were 12 and 17 monthsof age and were gaining0.5 and0.9

methane emissions wasverypoor(r2- 0.23). Theequationkg bodyweightper day,respectively.Growingheiferswere

was especiallypoor at predictingthe extremesof emissions, approximately 18 monthsof ageandgaining0.7 kg of weight

whichis not surprising sinceit wasdeveloped froma small perday. Growingbull calveswere498 kg in bodyweightand subsetof datathat did not encompass the extremeendsof 9 monthsof age. We did not measuremethaneproduction emissions. from mature bulls. Instead, we used the emissionsestimates

The current,best estimateof annual global methane from maturecowsin the inventoryfor bull emissions.The emissions dueto entericfermentation in ruminants is 85 Tg suckling calveswere4 months of ageandweighed 206 kg. with a rangeof uncertainty of 65 - 100 Tg [IPCC, 1995]. All cattleweregentledandaccustomed to thesampling halter Recently, IPCChasdeveloped guidelines fortheprediction of andcanister.Animalswererestrained aslittleaspossible.In methaneproductionthat attemptto be simplerand more fact, most animals would stand still while canisters were accuratethan the Blaxter and Clappertonequation. The changedin pens or on pasturewith no restraintneeded.If

equation usedis CH4emission (kgyr-•)= [Intake (MJd'•) x restraintoccurred,it waswith a ropehalterlooselytiedto a

Ymx 365dyr-•} / [55.65 MJkg'• CH4] , where Intake isgrossfence. The bullsandcalvesweremeasuredusingthe room

energyintake and Ym is a diet and production specific tracerapproach andwerehousedin a penasa groupandnot percentage of grossenergylost. TheYm factorfor developed restrained. The WSU Institutional Animal Care Committee countries is 6.0 + 0.5% for animalsconsuming forage-basedapprovedall experimentalprotocols. dietsand4.0 + 0.5% for animalsconsuming grain-based diets (