Source: Susan Kelly, MEATINGPLACE, 12/5/17

Scientists at Pennsylvania State University have concluded that current estimates of total livestock methane emissions may rely on outdated factors and do not fully consider feed intake and differences in animal diets or the facilities used to store manure.
These data gaps lead to large uncertainties in methane emission figures, the researchers said, noting that just how much gas the animals release is a subject of debate.

In the Penn State study, funded in part by ExxonMobil and published in the American Chemical Society’s peer-reviewed journal Environmental Science & Technology, the team analyzed feed intake data for cattle and manure storage practices for cattle, pigs and poultry at the county and state levels in the contiguous United States. 

Their analysis yielded total livestock methane emissions for 2012 (last census of agriculture) that are comparable to current U.S. Environmental Protection Agency estimates and to estimates from the global gridded Emission Database for Global Atmospheric Research (EDGAR) inventory. But estimates by location varied widely from those reported by EDGAR.

For example, the researchers found that the combined enteric and manure methane emissions from livestock in Texas and California were 36 percent less and 100 percent greater, respectively, than estimates by EDGAR. Based on their data, the researchers say that results from studies that use inaccurate distribution inventories to determine emissions sources should be cautiously interpreted.

According to EPA, the top three sources of anthropogenic methane in the United States are the combined energy sector (natural gas, petroleum systems, and coal mining; 40 percent of the total), livestock (36 percent), and landfills (18 percent).
In another recent study, sponsored by the U.S. National Aeronautics and Space Administration's (NASA) Carbon Monitoring System research initiative, researchers from the Joint Global Change Research Institute (JGCRI) found that global livestock methane (CH4) emissions for 2011 were 11 percent higher than the estimates based on guidelines provided by the Intergovernmental Panel on Climate Change (IPCC) in 2006.

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