Comparison of genetic heat tolerance in Japanese Holstein cows estimated by using milk yield, somatic cell score, or milk β-hydroxybutyrate

The heat tolerance (HT) of dairy cows has often been evaluated in research settings according to a reduction in milk yield and an increase in somatic cell score (SCS). Changes in these measures are attributed to the effects of heat stress on metabolism. However, evaluating HT by assessing milk β-hydroxybutyrate (BHB), a metabolism-related trait in dairy cattle, may be more informative than following changes in milk yield and SCS. Here, we estimated the genetic parameters of heat tolerance evaluated by using milk yield, SCS, or milk BHB by analyzing 3.7 million test-day records collected from 469,504 primiparous Holstein cows in Hokkaido, Japan, between 2017 and 2022, together with pedigree and meteorological data. The random regression test-day animal model contained herd-year, lactation curves of age-month at calving, genetic and permanent environment effects using second-order Legendre polynomials with linear effect of heat stress, and residuals. The heritability of HT was higher when defined according to milk BHB than when milk yield or SCS was used (at temperature-humidity index THI 72: 0.12 for milk BHB vs. 0.02 for milk yield or SCS; at THI 78: 0.25 vs. 0.04, respectively). In addition, genetic correlations among estimated breeding values for the different HT indicators showed contrasting patterns, with a strong positive correlation between HT based on milk yield and SCS, and weak correlations of HT based on milk BHB with other HT indicators. The use of milk BHB to evaluate HT in dairy cows can provide additional information on specific aspects of HS responses.