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The methane produced in a cow's rumen - the first of a cow's four stomachs - gets into the bloodstream and exits through the lungs, said Ken Olson, a range livestock nutritionist at Utah State University.
Almost all of it comes from breathing, though a tiny bit does escape when a cow belches, Olson said.
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There are several major sources of methane: rice paddies (methane-producing bacteria thrive in the underwater environment), swamps and wetlands (ditto), mining and oil drilling, landfills, termites (although there's still some controversy on this one), "biomass burning" (notably in the Amazon rain forest), and animals. Ninety percent of animal methane is produced by ruminants (i.e., cud-chewers). These include sheep, goats, camels, water buffalo, and so on, but most of all cattle, of which the world has an estimated 1.2 billion.
