Animals produce methane through "enteric fermentation". In this process plant matter is converted by bacteria and other microbes in the animal’s digestive tract into nutrients such as sugars and organic acids. These nutrients are used by the animal for energy and growth. A number of by-products, including methane, are also produced, but they are not used by the animal; some are released as gas into the atmosphere. (Although carbon dioxide (CO2) is produced in similar quantities as methane, it is derived from sustainably produced plant matter and thus makes no net addition to the atmosphere.) The carbon in the plant manner is converted into methane through this general, overall chemical reaction:
microbial
Organic Plant Matter + H2O -----------> CO2 + CH4 + (nutrients
and
metabolism other products)
The amount of methane that an individual animal produces depends on many factors. The key variables are the species, the animal’s age and weight, its health and living conditions, and the type of feed it eats. Ruminant animals - such as cows, sheep, buffalo, and goats - have the highest methane emissions per unit of energy in their feed, but emissions from some non-ruminant animals, such as horses and pigs, are also significant. National differences in animal-farming are particularly important. Dairy cows in developing nations, for example, produce about 35 kg of methane per head per year, while those in industrialized nations, where cows are typically fed a richer diet and are physically confined, produce about 2.5 times as much per head.
There is a strong link between human diet and methane emissions from livestock. Nations where beef forms a large part of the diet, for example, tend to have large herds of cattle. As beef consumption rises or falls, the number of livestock will, in general, also rise or fall, as will the related methane emissions. Similarly, the consumption of dairy goods, pork, mutton, and other meats, as well non-food items such as wool and draft labor (by oxen, camels, and horses), also influences the size of herds and methane emissions. The figures below present recent estimates of methane emissions by type of animal and by region. Due to their large numbers, cattle and dairy cows produce the bulk of total emissions. In addition, certain regions - both developing and industrialized - produce significant percentages of the global total. Emissions in South and East Asia are high principally because of large human populations; emissions per-capita are slightly lower than the world average. Latin America has the highest regional emissions per capita, due primarily to large cattle populations in the beef-exporting countries (notably Brazil and Argentina). Centrally-planned Asia (mainly China) has by far the lowest per-capita emissions due to a diet low in meat and dairy products.
See also Fact Sheet 271: "Reducing methane emissions from livestock farming".
For further reading:
Intergovernmental Panel on Climate Change (IPCC), 1990. Greenhouse Gas Emissions from Agricultural Systems. Proceedings of a workshop on greenhouse gas emissions from agricultural systems held in Washington D.C., December 12 - 14, 1989. Published as US/EPA report 20P-2005, September, 1990 (2 volumes).
United States Environmental Protection Agency (US/EPA), 1990. Policy Options For Stabilizing Global Climate, edited by D.A. Lashof and D. Tirpak. Report # 21P-2003.1, December, 1991. US/EPA Office of Planning and Evaluation, Washington D.C. Data from computer files used for this report were used to create the tables.