CH4 emissions from ruminant livestock are currently estimated to be ~100 Tg/year and represent the biggest anthropogenic source. The loss of CH4 from ruminant livestock is a problem not only with respect to GHG emissions but also to farmers, in that feed converted into, and released as, CH4 is feed not being converted into meat and/or milk.
CH4 is produced in the guts of ruminant livestock as a result of methanogenic bacteria. The composition of the animal feed is a crucial factor in controlling the amounts of CH4 produced. A single sheep can produce ~30 l CH4/day while a dairy cow can produce up to 200 l/day.
As with rice agriculture, CH4 emissions arising from ruminant livestock are dependent on human demand. With a continuing expansion of meat and dairy product consumption around the world, the demand for ruminant livestock, and thus the size of this CH4 source, has grown rapidly. Intensive rearing methods, developed to provide large amounts of meat and dairy products at low prices and to a wide consumer base, have led to very high densities of ruminant livestock and strong local CH4 sources.
The best-studied and applied CH4 reduction strategy in ruminants has been that of altering the feed composition, either to reduce the percentage that is converted into CH4 or to improve the meat and milk yield. Improvements in the overall quality of animal feed may allow meat and dairy production to be maintained at the same level with fewer animals, and so result in less total CH4 emissions. Recent ruminant CH4 reduction strategies have included the introduction of methanogen inhibitors, both biological and chemical, with the animal feed, to kill off or at least reduce the activity of the methanogenic bacteria or the protozoa that they are associated within the gut.
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