FOLLOWING a report published in Proceedings of the National Academy of Science that used a "top-down" approach to calculating U.S. methane emissions (Feedstuffs, Dec. 2, 2013), animal science researchers have published a letter in the same journal questioning the study's use of the top-down methodology.
In the letter, Alex Hristov of The Pennsylvania State University, Kristen Johnson of Washington State University and Ermias Kebreab of the University of California-Davis noted that animal scientists have generated large data sets of enteric methane production estimates per unit of feed or energy intake.
Methanogenesis in the rumen is substrate dependent, and methane production data derived from studies using respiratory chambers (or other techniques) expressed on a feed intake basis are representative of field emissions, if feed intake is known.
Hristov et al. used the U.S. Department of Agriculture's National Agricultural Statistics Service (NASS) livestock inventory estimates for 2013 and methane emission rates per unit of feed dry matter intake from two large data sets to estimate total methane emissions from enteric fermentation for the U.S.
According to their letter, methane production rates were estimated at 8-13 g/kg for cattle on feed or 20 g/kg of feed dry matter intake for all other categories. The methane emissions contributed by other ruminants or non-ruminant herbivores (sheep, goats, wild ruminants, horses and so forth) are small in the U.S. and were not included in this analysis.
Therefore, Hristov et al. estimated total methane emissions from enteric fermentation to be 6.241 teragrams (6,240,000,000 kg) per year, which is comparable to the current 2011 Environmental Protection Agency estimate of 6.542 teragrams per year.
NASS inventories for cattle, swine and poultry and Intergovernmental Panel on Climate Change manure methane emission factors were used to estimate emissions from manure management, Hristov et al. said. Using this approach, manure emissions in the U.S. were estimated at 1.604 teragrams per year, which is lower than the 2011 EPA estimate of 2.478 teragrams per year (with the latter figure perhaps being more representative of manure systems in the U.S.), the researchers said.
Thus, the conclusions from the earlier top-down study that EPA figures for livestock methane emissions are grossly underestimated appear to be unsubstantiated by the "bottom-up" approach, Hristov et al. concluded.
They added that there is a need for a detailed inventory of manure systems for all farm animal species and categories, which will help more accurately estimate greenhouse gas (and ammonia) emissions from animal manure in the U.S.