Estimating Cattle Methane Emissions Using Eddy Covariance in Grazing Rangelands.

Globally, enteric methane from ruminants is the largest contributor to agricultural greenhouse gas emissions. In countries with extensive grazing systems, measurement of cattle emissions is challenged by low stocking rates, expansive paddocks and remote locations. Our study explored the capability of an eddy covariance measurement system for long term monitoring of methane emissions from grazing cattle in Australia. Two measurement systems allowed for flux recording from a central location within the grazing paddock and near a watering point where animals tended to congregate during the day. Time-lapse photographs were used to map animal positions, and a Lagrangian stochastic model facilitated conversion of the eddy covariance fluxes to a per-animal methane emission rate. Paddock measurements captured cattle only briefly as animals crossed the flux footprint while grazing. Placement of a feed supplement upwind of the measurement tower helped to retain cattle in the footprint and increase the number of periods containing animal emissions. The water point location was more suitable to daytime measurements as over 50% of the flux measurement periods included cattle emissions. Overall, enteric emissions at the paddock site were greater (253 g CH4 m-2 AE-1 d-1, SD 75) and more variable than emissions at the water point (158 g CH4 AE-1 d-1, SD 34). Combining results from both sites and accounting for cattle weights gave a CH4 production of 0.43 g/kg body weight which is in range of other reported emissions from grazing animals. Our study suggests that with an understanding of animal positions on the landscape and a model to aid interpretation, the eddy covariance technique is a viable option for long-term monitoring of cattle emissions in grazing environments.