This chapter introduces enteric CH4 emissions from animals. Some animals are herbivores or plant eaters. A subgroup assists their digestive process by regurgitating their food and chewing it a second time. This is called chewing their 'cud' or ruminating, so these animals have been dubbed ruminants. Domesticated ruminant livestock include sheep, cattle, goats and deer. Ruminants have a four-part stomach with two anterior chambers, including the rumen, forming a relatively large fermentation vat. Ruminants cannot digest the cellulose in plants, but these chambers contain an abundant, diverse microbial community 'subcontracted' for the job. Up to 75 per cent of the ruminant's energy supply comes from the products of microbial metabolism of dietary carbohydrate (Johnson and Ward, 1996). This symbiosis includes the microbial community gaining some of the feed's energy and the ruminant must gather the food and provide the community a stable environment (warm, wet and free of oxygen). The community completes their job by passing the fermentation products into two posterior chambers of the 'true' stomach for digestion, the prime energy source being volatile fatty acids. In principle, the bigger the ruminant, the bigger its energy requirement and fermenting vat, the longer its food can be 'cooked', the lower the feed quality that can be 'tolerated'.

Enteric CH4 is a by-product of feed fermentation in the rumen and, to a lesser extent, in the large intestine. Typically >80 per cent of the CH4 is produced in the rumen and the rest in the lower digestive tract (Immig, 1996). In sheep and cattle subjected to measurement, 92-98 per cent of the CH4 gas was emitted from the mouth, the rest via flatus (Murray et al, 1976; Grainger et al, 2007). For enteric CH4, gas emission has also been called gas eruction but not in this chapter. The rumen microbial community and 'ecosystem' is complex and its long evolution has led to a disposal mechanism for hydrogen through the reduction of carbon dioxide to CH4 by methanogens (McAllister and Newbold, 2008). A high partial pressure of hydrogen inhibits microbial growth and digestion in the rumen (Wolin et al, 1997). In summary, with its microbial community, the ruminant consumes carbohydrates to meet its energy requirement and CH4 emissions 'naturally' rid it of some of the hydrogen, potentially harmful in excess.

Recently, enteric CH4 emissions have gained unprecedented interest with respect to international public policy related to greenhouse gases. Our involvement has focused on the development of enteric CH4 emissions measurement methods, inventory calculations and the verification of technologies that have been proposed to mitigate or reduce the emissions. We thus reveal our disposition to a numerate approach, spanning significant temporal and spatial scales. Scaling raises the spectre of uncertainty in determining the strength of this source of CH4 emissions to the atmosphere. After the reader has been introduced to the determination of the emissions, we present a synopsis of the challenges facing ruminant scientists, policy analysts and farmers to develop effective ways to mitigate enteric CH4 emissions.

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