Assessing the CO 2 -prediction equation used in association with sniffers to estimate daily methane emissions in dairy cows with different feed efficiencies

Absolute methane (CH 4 ) production (g/day) from sniffer systems can be estimated by multiplying the measured CH 4 to carbon dioxide (CO 2 ) ratio with predicted CO 2 output (g/day) based on body weight and energy-corrected milk yield. However, this method may introduce bias in CO 2 prediction due to differences in energy use efficiency, affecting CH 4 estimates. Using respiration chamber data from 1060 individual records across 37 studies, we simulated CH 4 predicted from sniffers to examine the relationship of CH 4 and CO 2 estimates with feed efficiency, categorized by residual feed intake (RFI). Carbon dioxide production was overestimated in the low-RFI (efficient) cows (12.2 vs. 11.6 kg/day) and underpredicted in the high-RFI (inefficient) cows (12.3 vs. 13.4 kg/day). Consequently, CH 4 emissions were overpredicted in efficient cows (380 vs. 362 g/day) and underpredicted in inefficient cows (384 vs. 419 g/day; P < 0.01). When cows were grouped by CH 4 prediction error, observed CH 4 emissions and yields increased across error groups, while predicted values showed the opposite trend ( P < 0.01). These results indicate that CH 4 estimates based on predicted CO 2 may be systematically biased, which could affect animal ranking and genetic progress if used in selection indices without adjusting for feed intake or efficiency.