Does the methane-mitigating feed additive 3-nitrooxypropanol affect additive genetic variance and ranking of methane...

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.