Does the methane-mitigating feed additive 3-nitrooxypropanol affect additive genetic variance and ranking of methane and yield traits in Holstein dairy cows?

This study aimed to quantify the effect of the methane-mitigating feed additive 3-nitrooxypropanol (3-NOP) on genetic parameters and potential genotype by environment (G × E) interaction for gas emission traits, that is, methane (CH4) and carbon dioxide (CO2) breath concentrations, as well as milk production traits in Danish Holstein cows. Weekly gas emission phenotypes on individual cow level, recorded with sniffers, and monthly test-day records from 2,404 cows in 5 commercial herds were analyzed using a reaction norm model in which additive genetic and permanent environmental effects were modeled as linear functions of 3-NOP supplementation. Mean CH4 concentration decreased from 398 to 214 ppm when 3-NOP was supplied to the cows, accompanied by reductions in CO2 concentrations as well as the CH4:CO2 ratio, and small changes in milk production traits. 3-Nitrooxypropanol substantially reduced additive genetic, permanent environmental, and residual variances for the CH4 concentration, but did not significantly change the heritability and repeatability for CH4, CO2, and CH4:CO2 ratio. For milk production traits, no effect was observed on the estimated variance components. Genetic correlations between environments of 3-NOP were high and near unity most traits, indicating no GT × ENV reranking. Our findings suggest that CH4 do not require separate trait definition across 3-NOP environments; however, accounting for the reduced variance structure is recommended to ensure unbiased genetic evaluation of CH4 phenotypes.