ENTERIC METHANE EMISSIONS OF BEEF COWS GRAZING TALLGRASS PRAIRIE PASTURE ON THE SOUTHERN GREAT PLAINS

Methane (CH4) is an important greenhouse gas, and about 20% of the carbon dioxide equivalent (CO2e) greenhouse gases emitted by U.S. agriculture are attributed to enteric CH4 produced by grazing beef cattle. Grazing cattle are mobile point sources of methane and present challenges to quantifying the enteric methane emission rate (MER). In this study, we applied three methods to measure herd-scale and individual-animal MER for a herd of beef cows grazing a native tallgrass prairie: a point source method that used forward-mode dispersion analysis and open-path lasers and cow locations, an open chamber breath analysis system (GreenFeed), and an eddy covariance ratio method that used the ratio of CH4 and CO2 mass fluxes. Three campaigns were conducted during the early season (July), late season (October), and dormant season (February). The point source and GreenFeed methods yielded similar MER (. SD) values during the early season campaign: 0.38 +/- 0.04 and 0.34 +/- 0.05 kg d(-1) cow(-1), respectively. However, the MER values from the two methods diverged in subsequent seasons. The GreenFeed MER decreased through the late and dormant seasons to 0.23 +/- 0.03 and 0.19 +/- 0.03 kg d(-1) cow(-1), respectively. In contrast, the point source MER stayed the same during the late season and increased during the dormant season to 0.41 +/- 0.07 kg d(-1) cow(-1). The CH4:CO2 ratio method, which was used only during the dormant season, yielded a MER of 0.29 +/- 0.05 kg d(-1) cow(-1). The point source and GreenFeed methods measured different MER (integrated herd-scale versus a subset of individual animals) and likely sampled methane emissions at different times during the day. We conclude that the point source method tended to overestimate emissions, and the GreenFeed method tended to underestimate emissions. Enteric methane emissions from beef cows over the three grazing seasons averaged 0.39 and 0.25 kg d(-1) cow(-1) as measured by the point source and GreenFeed methods, respectively. An annualized enteric MER for a beef cow herd grazing tallgrass prairie was 0.32 kg d(-1) cow(-1). Quantifying enteric methane emissions from grazing beef cows remains a challenge because of the mobile, often dispersed behavior of grazing cattle and the dynamic interactions of forage quality, dry matter intake, and changing physiological state of cows during the year.