Greenhouse Gas Emissions by Livestock

Livestock GHG emissions equate to 1,725 Mio t CO2-eq. annually on a global scale. This is dominated by methane from ruminants such as cattle and sheep, which accounts for nearly one-third of global anthropogenic emissions of this gas and is the largest methane source globally. In the case of the U.S. livestock total CH4 emissions in 2005 were about 112 Mio t CO2-eq. (EPA, 2007b), mostly from beef and dairy cattle (by 95 percent). This is more than the total of the emissions from iron and steel and cement industry. In Germany lifestock methane emissions are close to one (0.87) Mio t CH4 (UBA, 2007) which is more than 20 Mio t CO2-eq. and equates to two percent of the total national GHG emissions.

The background of methane production by ruminant livestock is as follows: Nutrients from the food consumed by the animals are decomposed in their digestion systems. During this complex process which partly takes place in an oxygen free atmosphere, anaerobic bacteria produce methane as one of the end-products. As methane producers ruminants like cattle, sheep, goats, and camels dominate. In their rumen which is a type of a fore-stomach, bacteria break down the feed so that it can be absorbed and afterwards metabolized by the following intestinal organs. Ruminants are thus able to digest coarse plant material, such as grass, and other green crops containing high cellulose. Methane produced is exhaled by the animals.

The amount of methane generated primarily depends on the type of the digestive system of the animal. Other factors are amount and composition of the feed consumed. Energy rich feed results in more methane.

Cattle naturally emit between 150 and 250 liters of methane per day (40 to 65 kg per head per year). Under an energy rich feeeding regime, higher values may occur. The CH4 emission factors of dairy cows in Germany are in the range of 95 kg per head and year with an increase between 1990 and 2005 from 77 to 117.5 (UBA, 2007).

Other kinds of livestock have considerably lower individual emission rates, as given in table 11.3.

Table 11.3 Emission rates of livestock other than cattle (EPA, 2006)

(kg methane per head per year)

Horses

18

Sheep

8 - 10.5

Goats

5 - 6.5

Pigs

1.3

Poultry

0.09

Mitigation proposals imply improved feeding practices and use of specific dietary additives. Better feeding practices include measures such as optimization of protein intake, adding oils or oilseeds, or improving pasture quality especially in low developed agricultures. The use of additives aims at suppressing or avoiding methane generation during the digestion process. For many of the proposed agents a clear benefit was not proven and side effects were observed. In some cases the agents were banned. Vaccination against certain methanogenic bacteria was studied. In the case of sheep it was estimated that a reduction of methane emissions by 20 percent could be achieved. In the case of Australia where the tests were made, livestock GHG emissions by sheep and cattle amount 14 percent of the country's total GHG emissions. A reduction by 0.3 Mio t CO2-eq. annually was predicted (CSIRO, 2001). However a commercial vaccine is not yet on the market (IPCC, 2007c).

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