Herd-Scale Enteric Methane Emission from Cattle Measured Using CH4:CO2 Ratio Method.

Methane is a greenhouse gas that is naturally produced during the digestive process of ruminants. Most of this enteric methane is emitted by cattle grazing on pasture, but measurement is challenging. We measured enteric methane emitted from a small herd of cows grazing on dormant tallgrass prairie using the CH₄:CO₂ ratio method. The method is based on the idea that the ratio of measured eructated CH₄ to measured respired CO₂ equals the ratio of the actual enteric emission to the actual respired CO₂. Twelve head of 575-kg cows grazed two 40-m by 40-m paddocks during the 4-day study. The mass fluxes of both gases were measured using two eddy covariance systems, located immediately to the north or south of a grazed paddock. Winds blew predominantly from northerly or southerly directions. Eddy fluxes were calculated on 15-min time steps with measurements from a closed path infrared CO₂ sensor, an open path laser CH₄ sensor, and a 3-axis sonic anemometer. Upwind fluxes were subtracted from downwind fluxes to remove the pasture soil as a source or sink of CH₄ or CO₂. The mean CH₄:CO₂ ratio was 0.053, but there was considerable variability, with a SD of 0.075. Per capita methane emission over the four days of the study ranged from 0.14 to 0.44 kg d^-1 cow^-1 and averaged 0.31 kg d^-1 cow^-1. The method holds promise to measure herd-scale enteric CH₄. Variability can be reduced with better estimates of actual respired CO₂.