Physiological Effect of Heat Stress on Canadian Holstein Dairy Cows Classified based on Immune Response Capacity

There is a pressing need to identify high producing dairy cows resilient to physiological challenges from climate change. Cows that have high estimated breeding values for Cell-Mediated Immune Response (CMIR) and high Antibody-Mediated Immune Response (AMIR) are at lower risk of developing disease (e.g. mastitis, metritis, hoof lesions) and are less likely to require antibiotics, compared to herd mates. However, no published studies have yet examined the physiological impact of climate change on immuno-phenotyped dairy cows. We hypothesize that high immune response (IR) cows are more resilient to climate change compared to average or low IR. To date, we have investigated the thermoregulation abilities of 10 average IR, lactating cows (8 multiparous, 2 first lactation) randomly selected from two commercial herds in southwestern Ontario. Rectal tempeartures (RT) from these cows were recorded during morning and evening milking over three days at the end of a heatwave in September 2017. The first two days were 10-18°C above seasonal average (20°C) while the third day was 5-10°C below average. A simple ANOVA was performed using evening RT, which indicated a significant difference (p=1.41 e-7) between cows for mean evening RT, with the highest mean RT and highest ambient temperature occuring on the same day. There is significant positive correlation (r=0.68, p=1.38 e-6) between RT and ambient temperature during the study. The low p-value (p=0.07) in an asymptotic test (t= 15.89) suggests that the between-cow variation in RT is not due to chance. This variation in thermoregulation between cows during heat stress is under further investigation comparing average (control group) IR to high and low IR cows across multiple herds. Genetic selection for increased climate resilience may be warranted as more information is acquired on the heritability and benefits of this trait in Canadian Holstein, particularly in the context of immunity.