Methane Emissions from Grazing Beef Cattle Quantified with Three Methods.

Grazing cattle are mobile point sources of methane (CH₄) and present methodological challenges to quantify emissions. Stocking density is low and cattle can bunch up or disperse over a wide area. The CH₄ concentration downwind of a herd may only increase slightly above upwind concentration. Our objective was to develop a micrometeorological method to quantify pasture-scale CH₄ emissions from grazing cattle and compare results with those from two other methods. The study pasture was a 26.4-ha tallgrass prairie at El Reno, Oklahoma grazed by 50 cow-calf pairs during early summer, 2014. Each beef cow was fitted with a satellite tracking unit. Three open path CH₄ lasers on motorized positioners scanned 16 paths that crisscrossed the pasture. Sonic anemometers measured wind speed and direction and variances of wind components. We used dispersion analysis to model CH₄ dispersion from the cattle. Most 15-min per capita emission rates (PCER) ranged from 150 to 600 g CH₄ d^-1 (cow-calf)^-1. Mean PCER over five days was 359±61 g CH₄ d^-1 (cow-calf)^-1, which included a negligible soil source/sink. Independent, concurrent measurements from two GreenFeed systems and an Intergovernmental Panel on Climate Change Tier 2 estimate of enteric CH₄ emission were 385±57 and 340±57 g CH₄ d^-1 (cow-calf)^-1, respectively. Methane emission of the cow-calf pair as a fraction of body weight (BW) for the three methods ranged from 0.49 to 0.56 g CH₄ d^-1 kg^-1 BW. We concluded that the enteric CH₄ emissions from beef cows with calves grazing early season tallgrass prairie was 0.52±0.11 g CH₄ d^-1 kg^-1 BW, which yielded an emission factor of 238 g d^-1 AU^-1 (animal unit, 454 kg cow with calf eating 0.026 kg dry matter kg^-1 BW). These results provide baseline data to validate modeling efforts, improve inventories and fine-tune life cycle analyses.