Net methane emissions from grazing sheep

Methane emissions from ruminant livestock are responsible for 45% of New Zealand's combined CO2-equivalent greenhouse gas inventory, and arise principally from sheep. Using a flock of 6-month old sheep (20 ha(-1)) grazing abundant pasture, we compare micrometeorological measurements of net methane emission rates with measurements from individual sheep based on a sulphur-hexafluoride tracer technique. Individual sheep emission rates were highly variable and averaged 19.5+/-4.8 (SD) g CH4 sheep(-1) d(-1), or 39 +/- 9.6 mg CH4 m(-2) d(-1) on an areal basis. Emission rates were poorly correlated with animal live weight or dry matter intake but represented an average dietary energy loss of 3.6%. Methane fluxes from the surface were determined as half hourly averages by a flux-gradient technique using temperature and methane gradients. Soil methane consumption was measured using chambers and found to be negligible (< 0.09 mg CH4 m(-2) d(-1)) in comparison with the animal contribution. Daily net emission rates averaged 46 mg m(-2) d(-1) and exhibited a broad peak in the early afternoon which corresponded with animal activity patterns. On average, net emisssion rates were 40% higher during the day than at night. Stable nocturnal conditions led to a separation of the micrometeorological measurements from the methane source and hence highly variable results. Based on two corroborating techniques, the average net methane emission rate was c. 43 mg CH4 m(-2) d(-1) or 155 kg CH4 ha(-1) y(-1).