Assessing individual differences in enteric methane emission among beef heifers using the GreenFeed Emission Monitoring system: effect of the length of testing period on precision

The GreenFeed Emission Monitoring system was used to measure individual greenhouse gas (GHG) emissions while recording feed intake of beef heifers. That technique provides spot-measures of methane (CH4) and carbon dioxide (CO2) fluxes at each visit to the GreenFeed feeder. A sampling variance is attached at each spot-measure due to circadian variation in GHG emission. Averaging spot-measures is required for reducing that sampling error when evaluating GHG emissions of individual cattle. The objective of the present study was to evaluate the length of test period and number of spot-measures for precisely assessing differences among beef heifers. The within-individual (σ2r) and across-individual (σ2i) variances of GHG-flux measures were estimated for 124 Charolais beef heifers fed a roughage diet during an 8-week test period, following 3–4 weeks of adaptation. High repeatability coefficients (>0.77) were obtained with 4-week test averages and ~100 spot-measures for CH4 and CO2 fluxes. Equivalent repeatability was obtained for dry matter intake (DMI). Lower repeatability (