Can live weight be used as a proxy for enteric methane emissions from pasture-fed sheep?

To test the hypothesis that sheep live weight (LW) could be used to improve enteric methane (CH(4)) emission calculations, mature ewes of 4 different breeds representative of the UK sheep industry were studied: Welsh Mountain, Scottish Blackface, Welsh Mule and Texel (n = 8 per breed). The ewes were housed and offered ad libitum access to fresh cut pasture of three different types, varying in digestibility: (a) a relatively high digestibility monoculture of perennial ryegrass (Lolium perenne), (b) a medium digestibility permanent pasture comprising a range of grass species, and (c) a relatively low digestibility native grassland pasture comprising mainly Molinia caerulea. Individual LW, feed dry matter intake (DMI), and CH(4) emissions in chambers were measured. The linear functional relationship between DMI and CH(4) emissions was positive (r = 0.77) with little breed effect. The relationships between LW and DMI, and LW and CH(4) emissions were also positive but weaker, regardless of pasture type. It is concluded that change to LW was a poor indicator of DMI and has limited value in the prediction of enteric CH(4) emissions from mature ewes.