Methane emissions from cattle grazing under diverse conditions: An examination of field configurations appropriate for line-averaging sensors

Micrometeorological techniques offer the possibility of a non-interfering measurement of enteric emissions from cattle in their natural environment, where animals do not need to be encumbered or handled. However, the grazing environment is a difficult application for these techniques. This study reports on an experimental design using an inverse dispersion method (IDM) to measure enteric methane (CH4) emissions, and its application to 15 rather distinct cattle trials in three types of feeding situations: summer grazing, winter swath grazing, and winter feeding. The IDM design was based on long and narrow animal paddocks with line-averaging sensors measuring CH4 concentration alongside the long axes of the paddock. Emissions were calculated based on the difference in concentration between the two measurement paths. The narrow paddock has many advantages for an IDM calculation: it avoids the need to monitor animal positions; it helps ensure measurable downwind concentration; and it increases the range of useable wind directions. Four different sensor configurations were used in the trials, differing in the number of concentration sensors (one or two) and sensor paths (two or four). Some configurations used sensor aiming motors to give multiple measurement paths and others used mirrors to create segmented paths (i.e., to go around a paddock corner). Cattle emissions measured with the IDM design showed good agreement across the 15 trials, consistent with high forage diets. When expressed in terms of CH4 yield (g/kg dry matter intake), the three feeding situations averaged 21.3 (summer grazing), 23.4 (winter grazing), and 23.9 (winter feeding). Based on the trial-to-trial consistency of the results, the similarity with other literature studies, and the success of a previous tracer-release study, we conclude that the narrow paddock IDM design provides a flexible and accurate method for calculating CH4 emissions from grazing cattle.