Agricultural sources of atmospheric methane compromise a large portion of the annual methane budget. Within these sources, cattle serve as one of the major contributors (the numerics of which are highly disputable). Two distinct processes account for the release of methane from cattle: The first being as a byproduct of ruminant digestion and the second being methane emissions from excreta. Thorough understanding of these processes is necessary if a reduction in methane production rates is to be accomplished.
Looking at ruminant digestion first, the typical beef and dairy cow each produce about 60 to 71 kg and 109 to 126 kg of methane annually (Johnson, 1995). Equivalently, these figures translate into an average of approximately 6% of the animals gross energy intake (GEI) is used to produce methane (Johnson, 1995). The goal of much nutrition research is to reduce this number which subsequently decrease methane emissions while increasing nutrition efficiency. Multiple factors contribute to ruminant methane production but two principle mechanism are responsible for most methane production variance. First, the magnitude and type of carbohydrates fermented in the reticulorumen plays a vital role and second, the production ratio of propionic acid to acetic acid is a principal indicator of total methane production.
The carbohydrates ingested by an animal will affect methane production. As the carbohydrate fraction of any fiber or starch is increased, methane production will decrease. Thus, feedstuffs such as beetpulp tend to reduce methane emissions and subsequently the percent of GEI used to produce methane. Another method of reducing methane emissions is to increase the percentage of grain in the animal's diet. U.S. feedlots commonly feed a 90% grain ration, yet the percent of GEI consumed by methane production falls to 3 to 4%, which translates into methane emission rate decreases.
Additional methods which decrease methane emissions include pelleting and grinding forages. This process can lead up to a 40% reduction in methane production (Johnson, 1995). The addition of lipids to an animal's diet also can lead to a reduction in methane production, typically a 29% decrease in methane production results from lipid addition to feedstuffs. Similar results occur with ionophore addition to feedstuffs. Alteration of ruminal microbes can lead to a 50% reduction in methane emissions, however, obtaining results this encouraging requires a specialized situation, namely the animal must be on a barley diet with a defaunation of the rumen (Johnson, 1995).
The second bovine-based methane source is excreta, of which the magnitude released is considerable lower than aructated methane. In a U.K. study of a 165 head dairy operation, the average animal generated 105 kg annually through ruminant processes versus 0.22 kg annually from excreta (Jarvis, 1995).
Jarvis, Lovell, and Panayides calculated methane emission rates from dung collected from various animals including dairy and beef cattle. Dung pats representative in volume and area of soil coverage to actual dung piles were placed in air tight containers and 10 ml samples of the air were periodically removed from the containers through a syringe and Suba seal and then analyzed with a gas chromatograph.
Results indicate that methane emission is usually highest immediately after deposition and decreases to insignificant levels after ten days. Total emissions from a single pat average 1702 mg for a grazing dairy cow foraging upon grasses and clover. Pats originating from a housed dairy cow being fed a silage and concentrates ration averaged 716 mg . Thus suggesting the possibility of dietary implications upon methane emissions. A study by Lodman et. al. revealed that methane emissions per kg of dung were 7.5 times higher when animals were fed a high grain ration (Jarvis et.al., 1995). Note, the increase in methane emissions from the dung pat does not counter the decrease in aructated methane when grain intake is increased due to the relative magnitudes of the average emission rates.
Methane emission rate decreases from cattle are possible, and with dung methane emissions being approximately only 0.2% of all bovine generated methane, it becomes obvious that decreases in aructated methane are likely to yield greater decreases in total methane emissions from cattle.
Johnson, K. A. and D. E. Johnson, 1995: Methane emissions from cattle. J. Anim. Sci., 73, 2483-2492.