Persistence of differences between sheep in methane emission under generous grazing conditions

Abstract

Four low and four high methane (CH 4 ) emitters were selected from a flock of 20 Romney sheep on the basis of CH 4 production rates per unit of intake, measured at grazing using the sulphur hexafluoride (SF 6 ) tracer technique. Methane emissions from these sheep were monitored at grazing for four periods (P): October, November, January and February 1999/2000. All measurements were carried out on perennial ryegrass/white clover pasture under generous herbage allowance, and the sheep were maintained on similar pastures during non-measurement periods. The tracer technique was used for all the CH 4 measurements and feed DM intake was calculated from total faecal collection and estimated DM digestibility. Data for liveweight (LW), gross energy intake (GEI) and CH 4 emission were analysed using split-plot analysis of variance. In addition, a between-period rank order correlation analysis was carried out for CH 4 emission data. Low CH 4 emitters were heavier ( P <0·05) than the high emitters in all the periods, but they did not differ ( P <0·05) in their gross energy intakes (GEI: MJ/kg LW 0·75 ). Low and high CH 4 emitters consistently maintained their initial rankings in CH 4 yield (% GEI) throughout the subsequent periods and the correlation analysis of rank order for CH 4 yield showed strong between-period correlation coefficients, although this was weaker in the last period. It is suggested that feeding conditions that maximize feed intake (e.g. generous allowance of good quality pasture under grazing) favour the expression and persistence of between-sheep differences in CH 4 yield.