Methane emission from free-ranging sheep: a comparison of two measurement methods

Methane emissions from a flock of 14, 1-year old sheep grazing on a grass a nd legume pasture were measured using a micrometeorological mass-balance me thod and a sulphur hexaflouride (SF6) tracer technique. The former measured the mean emission, over 45 min intervals, from all the sheep within a fenc ed 24 m x 24 m enclosure, from the enrichment of methane (CH4) in air as it passed over the sheep. The tracer technique measured emissions from a subs et of 7 individual animals over 24 h periods from measurements of CH4 and S F6 concentrations in air exhaled by the sheep, and from the known rate of r elease of SF6 from small permeation tubes placed in the animals' rumens. Bo th methods gave highly similar results for 4 out of 5 days. When the specie s composition of dietary intake was steady during the last two days of meas urement, the mean emission rate from the mass-balance method was 11.9 +/- 1 .5 (SEM) g CH4 sheep(-1) d(-1), while the rate from the tracer technique wa s 11.7 +/- 0.4 (SEM) g CH4 sheep(-1) d(-1). These rates are for sheep with mean live mass of 27 kg, with a measured dry matter intake of 508 g sheep(- 1) d(-1) and pasture dry matter digestibility of 69.5%. There was close agr eement between these measurements and estimates from algorithms used to pre dict methane emissions from sheep for the Australian National Greenhouse Ga s Inventory. (C) 1999 Elsevier Science Ltd. All rights reserved.