Monitoring methane emissions from grazing cattle

The ability to accurately measure greenhouse gas (GHG) emissions is essential to understand the magnitude of the anthropogenic GHG sources and to gage our ability to reduce these emissions. Measuring a typical enteric methane emission from cattle in grazing systems is difficult since it depends on the amount and type of feed intake. As well, many of the available measurement techniques for estimating enteric methane emission cause a change in feed intake which alters the emission. Our study evaluates a non-intrusive technique to measure enteric methane emission from grazing cattle, reported as the major source of GHG in many countries. This study was conducted in Queensland, Australia, using a digital stepping motor with a mounted open-path laser to measure methane concentration across five paths above a 1 ha paddock containing 18 cattle fitted with a global positioning system. These data were used along with wind statistics in a dispersion model (WindTrax) to estimate a herd methane emission rate every 10-min. Results indicate that over a few hours the technique accounted for 77% of the methane released in a release-recovery trial. Enteric methane emissions were variable over the short-term but over the entire study of 17 d the herd averaged was 141 g animal-1 d-1. Further work is needed to document the impact of non-steady state conditions on methane emissions, such as cattle movement during the averaging intervals.