Heat stress during maternal recognition of pregnancy alters cortisol concentrations and fertility in Holstein cows.

Heat stress has a strong negative impact on reproduction in dairy cattle during summer. However, the effects of exposure to heat stress and their relationship with cortisol during maternal recognition of pregnancy have not been fully characterized. To understand how heat stress negatively affects pregnancy outcomes in cattle, and whether cortisol is involved in the establishment of pregnancy, we evaluated the relationship between the daily number of hours with a temperature-humidity index (THI) ≥ 65 and plasma cortisol concentrations on Day 18 post-insemination. In this study, 50 artificial insemination (AI) events (32 cows and 18 heifers) performed between April and October 2024 were evaluated. Plasma cortisol concentrations were measured on Day 18, and pregnancy was diagnosed on Day 30 post-AI. Logistic regression analysis revealed that exposure to THI ≥ 65 on Day 18 was negatively associated with pregnancy probability (β = -0.089; OR = 0.92; 95% CI: 0.83-0.99; P < 0.05) in cows, but not in heifers. Linear regression analysis showed a significant positive association between THI and plasma cortisol concentrations in cows (β = 0.183; P < 0.001); however, this relationship was not observed in heifers. Additionally, plasma cortisol concentrations were higher in non-pregnant cows than in pregnant cows on Day 18 (P < 0.01). Together, these findings suggest that heat stress exposure during maternal recognition of pregnancy may impair the establishment of pregnancy partially by increasing cortisol concentration in dairy cows, even under temperate conditions such as those in Hokkaido.