Chapter 2: Livestock and grazed land emissions. U.S. Agriculture and Forestry Greenhouse Gas Inventory: 1990-2005. Technical bulletin 1921

Authors: Del Grosso, Stephen - Steve; OGLE, S - Colorado State University; WIRTH, S - Environmental Protection Agency (EPA); SKILES, S - Colorado State University

Submitted to: United States Department of Agriculture Technical Bulletin
Publication Type: Research Technical Update
Publication Acceptance Date: 5/2/2008
Publication Date: 8/1/2008
Citation: Del Grosso, S.J., Ogle, S., Wirth, S., Skiles, S. 2008. Chapter 2: Livestock and grazed land emissions. U.S. Agriculture and Forestry Greenhouse Gas Inventory: 1990-2005. Technical bulletin 1921. United States Department of Agriculture Technical Bulletin 1921. 11-32.

Interpretive Summary: Beef cattle were responsible for the largest fraction (65%) of GHG emissions from livestock in 2005, with the majority of emissions in the form of CH4 from enteric fermentation and N2O from grazed land soils. Dairy cattle were the second largest livestock source of GHG emissions (20%), primarily CH4 from enteric fermentation and managed waste. The third largest GHG source from livestock was swine (8%), nearly all of which was CH4 from waste. Horses, goats, and sheep caused relatively small GHG emissions when compared to other animal groups, because populations of these types are relatively small. Livestock contribute GHGs to the atmosphere both directly and indirectly. Livestock emit CH4 directly as a byproduct of digestion through a process called enteric fermentation. In addition, livestock manure and urine (“waste”) cause CH4 and N2O emissions to the atmosphere through increased decomposition and nitrification/denitrification. Managed waste that is collected and stored emits CH4 and N2O. Grazing animals influence soil processes (nitrification/denitrification) that result in N2O emissions from the nitrogen (N) in their waste, which increases N2O emissions. Forage legumes on grazed lands also contribute to N2O emissions because legumes fix nitrogen from the atmosphere which can become mineralized in the soil and contribute to nitrification and denitrification.

Technical Abstract: : A total of 259 Tg CO2 eq. of greenhouse gasses (GHGs) were emitted from livestock, managed livestock waste, and grazed land in 2005. This represents about 49% of total emissions from the agricultural sector. Compared to the base line year (1990), emissions from this source were about 2% lower in 2005. The 95% confidence interval for 2005 was estimated to lie between 239 and 306 Tg CO2 eq. Enteric fermentation was responsible for almost half (112 Tg CO2 eq.) of all emissions associated with livestock production, while grazed lands (96 Tg CO2 eq.) and managed waste (50 Tg CO2 eq.) accounted for approximately 40% and 20% of the total emissions. All of the emissions from enteric fermentation and about 81% of emissions from managed livestock waste were in the form of methane (CH4). Of the emissions from grazed lands 97% were in the form of nitrous oxide (N2O). Grazed lands do not often experience the anaerobic conditions required for CH4 production to exceed CH4 uptake. However, a small portion of manure from grazing animals is converted to CH4. Grazed lands were roughly neutral for CO2 emissions in 2005. The largest total emissions associated with livestock production were from Texas and California. Emissions were high in Texas primarily because of the large numbers of beef cattle, while dairy cattle emissions are responsible for most emissions in California. Emissions were also high in Iowa, Nebraska, Kansas, Oklahoma, and Missouri. [GRACENet publication]