Abstract
Heat stress (HS) reduces dry-matter intake and causes negative energy balance (EB) in Holstein cattle, with consequent deterioration in milk production and wellness. Therefore, the effects of HS can be detected more directly from imbalances in EB than from the consequent changes in production or health traits. EB can be monitored by metabolism-related traits such as predicted EB (PEB), the fat-to-protein ratio (FPR), or β-hydroxybutyrate (BHB) in milk. We examined the days on which HS effects on the test-day PEB, FPR, or milk BHB were the greatest in first lactation. We collected weather records and test-day records. We considered the fixed effects of herd-year, test month, calving age, days in milk, temperature-humidity index (THI) from any one of test day to 14 days prior (15 models per trait), and random effects of animal and residuals in the models and compared the deviance information criterion (DIC) between models for each trait. For PEB, FPR, and milk BHB, the model gave the lowest DIC when including the effect of THI 1, 1, and 0 day before the test day. We observed that HS caused a decrease in PEB and an increase in FPR and milk BHB.