Net methane emissions from grazing sheep

Methane emissions from ruminant livestock are responsible for 45% of New Ze aland's combined CO2-equivalent greenhouse gas inventory, and arise princip ally from sheep. Using a flock of 6-month old sheep (20 ha(-1)) grazing abu ndant pasture, we compare micrometeorological measurements of net methane e mission rates with measurements from individual sheep based on a sulphur-he xafluoride tracer technique. Individual sheep emission rates were highly va riable 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 w ith animal live weight or dry matter intake but represented an average diet ary 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 met hane 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 a verage, net emisssion rates were 40% higher during the day than at night. S table nocturnal conditions led to a separation of the micrometeorological m easurements from the methane source and hence highly variable results. Base d on two corroborating techniques, the average net methane emission rate wa s c. 43 mg CH4 m(-2) d(-1) or 155 kg CH4 ha(-1) y(-1).