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
Résumé
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 (<0.7) was obtained for combined traits, namely, CH4/CO2, CH4/DMI and CO2/DMI. Higher precision would have been obtained if the first 2 weeks were not used but considered as further adaptation. In that case, about 50 spot-measures recorded during a 2-week test would be sufficient for a precise individual measure of CH4 emissions. For genetic evaluation, test duration of 5 weeks may be recommended for the simultaneous recording of CH4 emission and feed intake.