Domestic animals kept in pens produce massive mounds of manure that become anaerobic and generate CH4 and N2O. This source amounts to 1.0% of the world’s greenhouse gas emissions from human activities (U.S. Environmental Protection Agency 2006a). Composting, which regularly turns over manure and other organic material to promote aeration and the faster decomposition of aerobic respiration, diminishes the emissions of CH4 and other putrid products, which are generated during anaerobic respiration.
Alternatively, farmers cover the manure and other organic material to induce anaerobic conditions and capture the CH4 thus generated for use as a fuel. In Europe during 2006, such operations produced “biogas” equivalent to 1.3 million metric tons of oil (EurObserv'ER 2007), or about 0.5% of oil production from Western Europe (BP 2007). Some domestic animals — specifically, cows, sheep, goats, buffalos, and camels — have a rumen, a compartment in their digestive system that harbors microorganisms that help break down organic carbon compounds such as cellulose, which are difficult to degrade. About 3% of these microorganisms conduct methanogenesis.
Ruminants exhale the CH4 generated through this enteric fermentation, and it accounts for 4.4% of the world’s greenhouse gas emissions (U.S. Environmental Protection Agency 2006a). Thus, four times more greenhouse gases emanate from the mouths of ruminants than from the other end. A variety of mitigation options for livestock management are under investigation.  The CH4 from enteric fermentation represents a loss of carbon from the feed and, therefore, a loss of productivity. Methane emissions per animal product are higher when the diet is poor. Less fiber and more oils or organic acids decrease CH4 emissions per milk produced or weight gain, but such diets may be prohibitively expensive. Hormone treatments such as bovine somatotropin (bST) and growth hormone do not directly influence enteric fermentation, but they boost animal performance and thus reduce CH4 emissions per milk produced or weight gain. Another possibility is to administer antibiotics or vaccines that repress methanogenic microorganisms. Yet, broader use of hormones or antibiotics in domestic animals invokes health and environmental concerns.
 Steinfeld, H., P. Gerber, T. Wassenaar, V. Castel, M. Rosales, and C. de Haan (2006) Livestock's Long Shadow: Environmental Issues and Options, Food and Agriculture Organization of the United Nations, Rome, ftp://ftp.fao.org/docrep/fao/010/a0701e/a0701e.pdf. Smith, P., D. Martino, Z. Cai, D. Gwary, H. Janzen, P. Kumar, B. McCarl, S. Ogle, F. O’Mara, C. Rice, B. Scholes, and O. Sirotenko (2007) Agriculture. In: Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Metz, B., O. R. Davidson, P. R. Bosch, R. Dave, and L. A. Meyer, eds. Cambridge University Press, New York. pp. 497-540.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
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