Net methane emissions from grazing sheep

Abstract

Summary Methane emissions from ruminant livestock are responsible for 45 % of New Zealand’s combined CO 2 ‐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 CH 4 sheep –1 d –1 , or 39 ± 9.6 mg CH 4 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 ( 4 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 CH 4 m –2 d –1 or 155 kg CH 4 ha –1 y –1 .

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