A comparison of systems for measuring methane emissions from sheep

Two experiments were conducted, at ADAS Drayton in the autumn and winter 1996/1997, to compare methane (CH 4 ) emissions from sheep housed either in a polytunnel system or in open-circuit respiration chambers. In each system, the sheep received maintenance levels of either cut grass or high temperature dried grass pellets (HTDG). All experiments in the tunnel were conducted on concrete to avoid any interactions of the CH 4 with the soil/plant environment. The results suggested that CH 4 production from the open-circuit chambers was greater than from the tunnel system (26.9 ±0.46 v. 31.7±0.35 1./kg dry matter intake (±S.E.) for open circuit respiration chambers and tunnel, respectively). Recovery tests gave similar results for both systems (95.5-97.9% for tunnels and 89.2-96.7% for chambers), and confirmed that both methods give good quantitative recovery of added CH 4 , and can therefore be assumed to provide reliable estimates of emissions from animals. There is no technical explanation, therefore, for the different estimates of emissions provided by the two systems. Further studies are required to understand the reasons for the differences and in particular, the possible links between animal behaviour induced by the two systems and CH 4 emission rates.