How much can performance measures explain of the between-cow variation in enteric methane?

Enteric CH₄ produced from dairy cows contributes to the emission of greenhouse gases from anthropogenic sources. Recent studies have shown that the selection of lower CH₄-emitting cows is possible, but doing so would be simpler if performance measures already recorded on farm could be used, instead of measuring gas emissions from individual cows. These performance measures could be used for selection of low emitting cows. The aim of this analysis was to quantify how much of the between-cow variation in CH₄ production can be explained by variation in performance measures. A dataset with 3 experiments and a total of 149 lactating dairy cows with repeated measures was used to estimate the between-cow variation (the variation between cow estimates) for performance and gas measures from GreenFeed (C-Lock, Rapid City, SD). The cow estimates were obtained with a linear mixed model with the diet within period effect as a fixed effect and the cow within experiment as a random effect. The cow estimates for CH₄ production were first regressed on the performance and gas measures individually, and then performance and CO₂ production measures were grouped in 3 subsets for principal component analysis and principal component regression. The variables that explained most of the between-cow variation in CH₄ production were DMI (R² = 0.44), among the performance measures, and CO₂ production (R² = 0.61), among gas measures. Grouping the measures increased the R² to 0.53 when only performance measures were used, and to 0.66 when CO₂ production was added to the significant performance measures. We found the marginal improvement to be insufficient to justify the use of grouped measures rather than an individual measure because the latter simplifies the model and avoids over-fitting. Investigation of other measures that can be explored to increase explanatory power of between-cow variation in CH₄ production is briefly discussed. Finally, the use of residual CH₄ as a measure for CH₄ efficiency could be considered by using either DMI or CO₂ production as the sole predicting variables.