Enteric methane emissions of beef cows grazing tallgrass prairie pasture on the southern Great Plains

Abstract. Methane (CH4) is a greenhouse gas that is naturally produced during the digestive process of ruminants. About 20% of the U.S.inventory of CH4 is emitted by grazing cattle. Our objective was to quantify herd-scale and individual enteric CH4 emissions of beef cows during two grazing seasons with different levels of forage quality. We measured enteric CH4 emitted from a herd of cows grazing on tallgrass prairie using herd-scale micrometeorological methods and static voluntary single animal head chambers. Methods included multiple open path lasers coupled with forward and backward dispersion analysis, eddy covariance ratio method using carbon dioxide (CO2) as a tracer gas, and an open head chamber breath analysis system. We also evaluated two empirical models of methane emission. Research was conducted on a tallgrass prairie in central Oklahoma during early season (summer) and dormant season (winter) grazing periods, with forage quality highest in summer and lowest in winter. Enteric methane emissions from lactating cows grazing early season forage converged on values from 320 to 350 g d-1 cow-1 and from 0.55 to 0.64 g CH4 kg-1 BW. Methane emissions from late gestation cows grazing dormant winter forage were 25 to 40% greater, and converged on values from 400 to 500 g d-1 cow-1 and from 0.58 to 0.72 g CH4 kg-1 BW. Forage quality was a major determinant of enteric methane emission, with high quality early season forage yielding lower methane emissions. The empirical models showed promise, but whenever possible, measured, rather than calculated, inputs are needed to provide best model estimates.