Pasture-Scale Measurement of Methane Emissions of Grazing Cattle.

Quantifying methane emission of cattle grazing on southern Great Plains pastures using micrometeorology presents several challenges. Cattle are elevated, mobile point sources of methane, so that knowing their location in relation to atmospheric methane concentration measurements becomes critical. Stocking density is typically low and cattle can disperse over a wide area, but they can also bunch into small areas. The methane concentration downwind of a herd may only increase slightly above upwind concentration. We describe a method to quantify pasture-scale methane emissions of grazing cattle that incorporates GPS tracking of individual cattle, multiple scanning open path methane lasers that divide a large pasture into smaller virtual paddocks, and forward Lagrangian dispersion analysis. Study area was a 27-ha pasture in central Oklahoma that was grazed by 50 cow-calf pairs. Each cow was fitted with a GPS tracking unit. Three open path methane lasers were mounted on motorized positioners programmed to scan 16 paths (from 206 to 396 m long) that crisscrossed the pasture, creating 16 virtual paddocks with areas that ranged from 1.1 to 2.1 ha. Three-axis sonic anemometers were used to measure wind speed and direction and variances of wind components. The dispersion model Windtrax was used in forward mode to establish the ratio of methane flux to methane concentration above background along each laser path for a set of prescribed conditions. The location of cattle was used to identify the closest measured upwind and downwind laser paths. Quality control metrics are presented and performance standards of the method established. Though data intensive and challenging, the method was able to provide reasonable estimates of methane emitted from grazing cattle.