Association of heat tolerance with conformation and fitness traits in dairy cattle

Dairy cows are highly susceptible to heat stress, which has a detrimental effect on milk production, fertility performance, and animal welfare. These adverse effects will increase as dairy-producing regions continue to be affected by climate change. To address this, Australia implemented a genetic evaluation for heat tolerance in 2017, which was updated in 2024. The objective of this study was to assess the relationship between heat tolerance solutions from the official genetic evaluation and other important traits in Australian Holsteins and Jerseys. In addition to heat tolerance solution, genetic correlations were estimated for mean yield (cow solution for intercept) of milk, fat, and protein yields with 11 conformation traits, 2 body size composites, and 2 fitness traits using a multivariate animal model. To compare the correlation of heat tolerance solution with the traits considered in this study, we also calculated the correlations of heat tolerance adjusted for mean milk, fat, or protein production. Of the conformation traits considered, angularity had the strongest genetic correlation with the heat tolerance solution for milk of -0.26 ± 0.03 and -0.38 ± 0.04 for Holstein and Jersey, respectively. Comparatively, when the genetic correlation between angularity and the adjusted heat tolerance solution was corrected for milk, the correlations reduced to -0.11 ± 0.06 for Holstein and -0.12 ± 0.14 for Jersey. Both calving interval and survival had strong genetic correlations with the heat tolerance solution compared with other traits studied. When comparing heat tolerance solutions with heat tolerance solutions adjusted for mean production, the effect of milk, fat, and protein yield on the relationship between heat tolerance and conformation traits is driven by milk, fat, and protein yield. Production strongly influences the relationship of the current Australian phenotype for heat tolerance due to the calculation of heat tolerance phenotype via yield deviation under heat stress. Given these results, conformation traits have minimal value as a predictor of heat tolerance in Australian Holstein and Jersey cattle.