Using the daily variation in body temperature of dairy cows in thermoneutral conditions to predict their response to hot weather

Weather conditions of high air temperature with or without elevated humidity induce heat stress in dairy cows, resulting in elevated body temperature, reduced DMI, and subsequent loss of production. The temperature and humidity index (THI) combines temperature and humidity to provide a single measure of heat load. Farmers will face longer and more severe weather, resulting in more frequent and severe periods of high THI in the future. We present a method to identify cows with lower body temperature responses to elevated THI through an understanding of the variation in body temperature on cool days and the relationship with body temperature on days of heat stress conditions. There was a strong quadratic relationship between the amplitude of body temperature on days of THI ≤65 and the maximum body temperature (R² = 0.71) as well as the amplitude of body temperature (R² = 0.86) on days of THI >68. Cows with a better ability to maintain body temperature within a narrow range under thermoneutral conditions appear to be more tolerant of exposure to hot weather. The amplitude of body temperature, regardless of weather characteristics, could be used to predict heat tolerance in dairy cattle. Selection for heat tolerance is essential for climate resilience in dairy production systems, and the incorporation of precision phenotyping could improve long-term outcomes by balancing resilience with productivity.