Much of the world's livestock are ruminants--such as sheep, goats, camel, cattle, and buffalo--who have a unique, four-chambered stomach. In the chamber called the rumen, bacteria break down food and generate methane as a by-product. The production rate is affected by factors such as quantity and quality of feed, body weight, age, and exercise, and varies among animal species as well as among individuals of the same species. Chapter 4 of the EPA report provides an overview of livestock methane emissions.
In "Methane Emissions from Animals," Lerner, Matthews, and Fung (1988) estimate that of the annual global production of 400 to 600 Tg (1Tg=1 million tons) of methane, enteric fermentation in domestic animals contributes approximately 65 to 85 Tg. The authors present a global high-resolution database of methane emissions from animals (including cattle, water buffalo, sheep, goats, camels, pigs, horses, and caribou). Plates 1a-1h illustrate global distribution of animal populations and methane emission. Table 1 shows population statistics and associated methane emissions for seven animal types for 1984; table 2 presents total annual methane emissions by domestic animals for several countries in 1984; and table 3 shows latitudinal distribution of 1984 methane emissions by animal type and of total annual emissions by domestic animals.
Leng (1993) provides data on world ruminant population densities, estimated methane production rates, and factors influencing methane production in "Quantitative Ruminant Nutrition - A Green Science." The author estimates that ruminants on low quality feeds produce more than 75 percent of the total livestock methane emissions.