Genetic parameters for enteric methane traits and their genetic connection with milk production in Danish Holstein cattle.

Due to their large contribution to anthropogenic GHG emissions, mitigating methane (CH₄) emissions originating from enteric fermentation of fiber in ruminants such as cattle, has become a current focus in animal breeding. This is because exploiting genetics by integrating CH₄ emissions in the breeding goal is a promising tool as it enables continuous and long-term improvement. However, a sustainable design of the selection index requires a thorough understanding of the new traits' genetic architecture and relationship with already existing breeding goal traits. This study attempted a detailed analysis of CH₄ traits, such as CH₄ and CO₂ concentrations in the cows' breath (CH₄_C and CO₂_C) as well as the ratio of these traits (CH₄:CO₂ ratio), and their genetic connection with milk production traits, using a sample of 4,019 Danish Holstein cows with 587,640 daily observations. Records of CH₄_C and CO₂_C were measured using sniffers installed on commercial Danish dairy farms. To estimate heritabilities and genetic correlations, we applied pedigree-based uni- and bivariate repeatability linear animal models while differentiating between growing (i.e., primiparous, PP) and adult (i.e., multiparous, MLP) cows. For the CH₄ traits, heritabilities were between 0.08 (se = 0.01) for the CH₄:CO₂ ratio in MLP cows and 0.19 (se = 0.03) for CH₄_C in PP cows. Repeatabilities were between 0.13 (se = 0.01) for the CH₄:CO₂ ratio and 0.42 (se = 0.01) for CO₂_C, both in MLP cows. Genetic correlations with milk production traits were in a moderate to high unfavorable direction and stronger for MLP cows, e.g., 0.44 (se = 0.09) between CH₄_C and ECM, which indicates increasing emissions with higher milk production. Our results confirm the unfavorable relationship between milk production and CH₄ emissions found in previous studies. Moreover, they raise the need for follow-up studies (1) to develop a CH₄ trait that is genetically independent from milk production to enable breeding for lower emitting cows without compromising milk production and (2) that include information on possible mediating factors of the genetic correlations found in this study (e.g., feed intake data) to further decipher the relationship between enteric CH₄ emissions and milk production.