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Livestock Contribution to US Greenhouse Gas Emission

Posted: June 11, 2008

There are conflicting reports on the contribution of ruminants to greenhouse gas emissions (GGE). In these environmentally sensitive times it will help to put things in prospective.

There are conflicting reports on the contribution of ruminants to greenhouse gas emissions (GGE). In these environmentally sensitive times it will help to put things in prospective.

A Nov 2006 FAO (the Food and Agriculture Organization of the United Nations) report (www.fao.org/newsroom/en/news/2006/1000448) states that “…the livestock sector generates more GGM as measured in CO2 equivalent – 18 percent – than transport”. In other words, ruminants (domesticated) on Earth emit more greenhouse gasses than all possible means of human transportation (cars, trucks, locomotives, boats of all kind, airplanes, spacecrafts, and don’t forget those lawn mowers) that burn hydrocarbons.

At first, this is truly hard to believe! However, these effects may depend on societal factors. For example, ruminants in the U.S. (97 million cattle and another 9 million sheep and goats; NASS, 2008) represent a very small fraction (3%) of all ruminants, productive or idle, around the world; according to FAO’s 2005-6 Yearbook, 1.522 billion cattle and buffaloes and 1.821 billion sheep and goats. Thus, the ratio of ruminants to transportation vehicles is much larger in developing than in developed countries. In the U.S., for example, there are a total of 251 million highway vehicles (www.bts.gov). India, on the other hand, has more than 283 million cattle and buffaloes and 182 million sheep and goats (FAO’s 2005-6 Yearbook), but only 59 million motor vehicles (web.worldbank.org). Therefore, what might be true for the world may not be true for the United States.

According to an EPA report (Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2005; www.epa.gov), total GGE in the U.S. (2005 data) were 7,260 Tg (teragram, or 1012 g) CO2 equivalent (CO2 Eq ) with CO2 accounting for over 84% of the emissions, methane (CH4) for about 7%, and N2O for 6%. By far, the biggest source of GGE in the U.S. is the energy sector – 85% of the total GGE, or 6,201 Tg of CO2 Eq. As a whole, agriculture accounted for only 7.4% (or 536 Tg of CO2 Eq.) of all GGE in the U.S. Proportionally, GGE from agriculture decreased since 1990 (when it was 9% of all GGE) primarily due to increased non-agricultural emissions. Over 69% of the total agricultural GGE came from agricultural soil management (primarily N2O; only 1.7% from manure management) and 21% (or 112 Tg of CO2 Eq.) from enteric fermentation (i.e., rumen and gut fermentation) as CH4. By comparison, CH4 emissions from landfills (i.e., non-agricultural activities) were 132 Tg of CO2 Eq., or 18% more than emissions from enteric fermentation. Again by comparison, total GGE from the transportation sector exceeded 1,900 Tg of CO2 Eq. in 2005.

This is 3.5 times more than total emissions from agriculture. The main GGE from livestock production agriculture, CH4 and N2O (enteric fermentation and manure management), amounted to a total of 163 Tg of CO2 Eq., which is only a fraction (8.5%) of the GGE from transportation in the U.S. Overall, according to the EPA report, livestock GGE (CH4 and N2O) account for only 2.2% of the total GGE in the U.S. in 2005. Thus, the society has to look much harder into conserving energy through reducing driving (particularly low-efficiency SUVs and pick-up trucks), building excessively large homes, unnecessary illumination of office buildings, continuous heating and air conditioning of office and residential buildings, maintaining golf courses and lawns (which, according to some, is a huge source of GGE), etc. On a national scale, these measures will have a much larger impact on GGE than reducing livestock population, respectively meat and milk production (and then, we won’t have to reach for an extra dime for our morning latte).

Alexander N. Hristov, Associate Professor of Dairy Nutrition, Department of Dairy and Animal Science