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Greenhouse Gas Production from Dairying: Reducing Methane Production Frank O’Mara Department of Animal Science and Production, University College Dublin, Belfield, Dublin 4, Ireland E-mail: frank.omara@ucd.ie Take Home Messages 8 Methane is 21 times more potent than CO2 as a greenhouse gas (GHG), and dairy cows typically produce 118 kg methane/year, which is over twice that produced by other non-lactating cattle. 8 Evaluation of strategies to reduce methane production should consider the effect
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   Advances in Dairy Technology (2004) Volume 16, page 295 Greenhouse Gas Production from Dairying:Reducing Methane Production Frank O’Mara Department of Animal Science and Production, University College Dublin, Belfield, Dublin 4,IrelandE-mail: frank.omara@ucd.ie    Take Home Messages 8 Methane is 21 times more potent than CO 2 as a greenhouse gas (GHG),and dairy cows typically produce 118 kg methane/year, which is over twicethat produced by other non-lactating cattle. 8 Evaluation of strategies to reduce methane production should consider theeffects on total farm greenhouse gas emissions. 8 Increasing productivity per cow will reduce methane emissions per kg ofmilk and total farm GHG emissions where milk production is fixed, althoughthe effect on total farm emissions needs further clarification. 8 A lower culling rate will reduce farm methane and total GHG emissions. 8 Diets with a high proportion of concentrates that promote a high propionatetype of ruminal fermentation are conducive to reducing ruminal methaneproduction, but the effect on total farm GHG emissions may be less. 8 Selecting forages and concentrates high in non fiber carbohydrates couldreduce methane emissions. 8 Breeding forage crops with high concentration of propionate precursorssuch as malate may be a long-term solution to reducing methaneemissions.    Introduction Although methane production from enteric fermentation in ruminants has beenstudied for many years, it is only recently that research has focused onreducing it in order to combat global warming. Methane from entericfermentation is a large component of livestock related greenhouse gasemissions. This paper gives a short outline of how methane arises during the  296 O’Mara process of ruminal fermentation, and outlines the typical emissions of dairycattle. Then, possible mitigation strategies are examined. While this paperfocuses on methane emissions from enteric fermentation, overall farm systememissions of total greenhouse gases must be taken into account to get acomprehensive picture. These will include methane and nitrous oxideemissions from animal manures, nitrous oxide emissions from soils, and CO 2  emissions from energy consumption. The data of Johnson et al. (2002)showing the relative contribution of these sources from the contrastingsituations of Wisconsin and New Zealand dairy farms, indicate that the type ofproduction system can have a major impact on the relative importance of eachsource (Figure 1). CH4Enteric CH4   Figure 1. Total farm emissions of greenhouse gases per kg milk from aWisconsin or New Zealand dairy farm (Johnson et al., 2002)    Ruminal Fermentation and the Production of Methane Methane is produced as a result of anaerobic fermentation in the rumen andthe hind-gut. Microbial enzymatic activity in the rumen (and salivary enzymes),hydrolyses much of the dietary organic matter to amino acids and simplesugars. These products are then anaerobically fermented to volatile fatty acids(VFA), hydrogen and CO 2 . Some of the CO 2 is then reduced throughcombination with hydrogen to produce methane:CO 2 + 4 H 2    CH 4 + 2 H 2 O  Greenhouse Gas Production from Dairying 297 Alternatively, hydrogen can by used in the formation of some VFA orincorporated into microbial organic matter. The stoichometry of the formation ofthe main VFA is shown in the following equations:2H producing reactions:Pyruvate  acetate (C2) + CO 2 + 2H2H using reactions :Pyruvate + 4H  propionate (C3) + H 2 02 C 2 + 4H  butyrate (C4) + 2H 2 OFrom this it can be concluded that if ruminal fermentation patterns are shiftedfrom acetate to propionate, both hydrogen and methane production will bereduced. This relationship between methane emissions and the ratio of thevarious VFA has been well documented (Hungate, 1966), and it providesopportunites to reduce methane emissions. Herein also lies the explanation asto why fibrous diets produce more methane than non-structural carbohydratediets: the fibrous diets promote higher acetate, resulting in more hydrogen andthus more methane.The methane in the rumen is produced by methanogenic bacteria andprotozoa. The role of protozoa in methane formation is interesting. It has beenestablished that virtually all of the bacteria attached to protozoa aremethanogens (Vogels et al., 1980) and that these bacteria are responsible forbetween 0.25 and 0.37 of the total methane produced (Finlay et al., 1994;Newbold et al., 1995). By removing the protozoal population throughdefaunation, the ruminal bacterial population is modified, VFA production isshifted from acetate and butyrate towards propionate, and methane emissionsare decreased. There is also a negative impact on fiber digestion (Demeyer etal., 1982) so care must be taken not to unduly disrupt rumen metabolism by thisroute.The hind-gut has been reported to account for between 0.13 and 0.23 of thetotal emissions by sheep (Murray et al., 1976; Kennedy and Miligan, 1978).However, it appears that most (0.89) of the methane produced in the hind-gut isabsorbed through the gut wall and excreted via the lungs (Murray et al., 1976).In the hind gut, protozoa are absent, and methane is produced bymethanogenic bacteria. Methane emissions from the hindgut are lower thanfrom the rumen and it has been speculated that this could be due to hydrogenremoval by reductive acetogenesis rather than methanogenesis (De Grave andDemeyer, 1988).  298 O’Mara    Methane Production of Dairy Cows High yielding dairy cows generally produce over 100 kg of methane/year fromenteric fermentation. In the absence of country specific emission factors, theIPCC (1996) recommend that a default value of 118 kg/year be used for highlyproductive commercial North American dairy cows. As methane is consideredto have a global warming potential 21 times that of CO 2 (IPCC, 1996), 118 kg ofmethane is equivalent to 2.478 tonnes of CO 2 in inventories of greenhouse gasproduction. Figure 2 illustrates that the emissions from dairy cows are overtwice that from other cattle (beef cows, bulls, calves, growing steers/heifers,and feedlot cattle). Typically, methane emissions from enteric fermentationrepresent about 6% of dietary gross energy, but this varies with diet from about2% (cattle in feedlots) to 12% (animals eating very poor quality forage)according to Johnson and Johnson (1995). 020406080100120kgmethane/yearDairy cowsOther cattleSheep   Figure 2. Default annual methane emissions of North American dairycows, other cattle and sheep (IPCC, 1996)    Strategies to Reduce Methane Emissions There have been many strategies proposed that could reduce methaneemissions and these have been comprehensively reviewed by Moss (1994).This paper will discuss some of the most pertinent strategies to high producingdairy herds, and will review some of the more promising developing strategies.
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