Sustainability & efficiency

Genetic responses of heat tolerance based on milk β-hydroxybutyrate, milk yield and somatic cell score in Holsteins

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Abstract

Heat stress (HS) in dairy cows reduces milk yield, reproductive efficiency, and longevity. With climate change, improving heat tolerance (HT) has become an increasingly important breeding objective. In Japan, HT has been evaluated using HT MY,SCS, which reflects the decline in milk yield (MY) and the increase in somatic cell score (SCS) under HS. However, milk β-hydroxybutyrate (BHB) strongly reflects the metabolic balance associated with heat load and energy partitioning. Therefore, HT detected from changes in milk BHB (HT BHB) may serve as a more sensitive indicator of the genetic potential for adaptation to HS. The objective of this study was to clarify the genetic relationships between HT and economically important traits (productivity, fertility, longevity, and type traits) in the Japanese national breeding program and to compare the expected genetic response in HT when each indicator is emphasized in selection. This analysis used weather records and test-day production records collected from first-parity Holstein cows in Hokkaido between 2017 and 2022 through the dairy herd improvement program. Pedigree information was obtained from the Holstein Cattle Association of Japan. The temperature-humidity index (THI) was calculated using the daily mean temperature from the nearest weather station and the relative humidity from representative observation stations in each of the 14 regions of Hokkaido. A random regression test-day model was applied. The model included fixed herd-test-day effects, a lactation curve based on age at calving × month of calving, and additive genetic and permanent environmental effects modeled as random regressions on days in milk using second-order Legendre polynomials. Linear regression coefficients for THI were fitted for both additive genetic and permanent environmental effects. Estimated breeding values (EBVs) for HT BHB were calculated using the variance components from this model, while published EBVs for HT MY,SCS were obtained from the National Livestock Breeding Center. EBVs from 443 Holstein bulls were analyzed to estimate Pearson correlations between HT indicators and production, fertility, longevity, and type traits. Genetic responses per generation were predicted for the Nippon Total Profit Index and HT under several assumed weighting scenarios. Both HT indicators showed weak but unfavorable correlations with milk yield and milk components, suggesting that higher HT may be associated with lower production potential. In contrast, both HT indicators showed favorable correlations with fertility and herd life, suggesting that selection for HT may improve functional longevity. The unfavorable correlations between HT BHB and milk production traits were weaker than those observed for HT MY,SCS, indicating smaller indirect effects. Predicted responses showed that selection emphasizing HT BHB could improve HT more efficiently. In conclusion, HT BHB, derived from metabolic indicators in milk, is a useful and efficient indicator for improving heat tolerance in Holstein cattle.

Keywords: 2026

How to Cite:

Abe, H., Hagiya, K., Ishida, S., Kawakami, J., Nakahori, Y., Osawa, T., Yamaguchi, S., Yamaguchi, S. & Yamazaki, T., (2026) “Genetic responses of heat tolerance based on milk β-hydroxybutyrate, milk yield and somatic cell score in Holsteins”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2285378. doi: https://doi.org/10.31274/wcgalp.23666

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