Abstract
Resistance of cows to heat stress is under genetic control. Recently, two novel breeding values for heat tolerance in lactating Holsteins have been developed based upon the change in conception at first service (CFS_THI) and milk yield (Milk_THI) with temperature-humidity index. The goal here was to test whether variation in standardized transmitting abilities (STA) for heat tolerance was associated with phenotypic variation in cow regulation of body temperature during heat stress. In this observational study, data on afternoon rectal temperatures (1400-1700 H) during the hot months of the year (May-September) were collected from lactating cows on farms in Florida (n = 3), California (n = 1), and Türkiye (n = 1) over a period spanning 2007 to 2023. Analyses were performed using records where the dry bulb temperature at cow side was ≥ 25 °C and STA reliability > 0.05. Data on 4,674 and 4,678 values of rectal temperature from 3,617 and 3,620 cows were analyzed for CFS_THI and Milk_THI, respectively. Standardized transmitting abilities were analyzed two ways-by comparing rectal temperatures of cows above and below a STA of 100 and by comparing cows in each quartile of the dataset. Rectal temperature was lower for cows with higher STA for CFS_THI but not for cows with higher STA for Milk_THI. Another analysis considered the percent of rectal temperatures > 39 °C (i.e., a body temperature more likely to result in compromised physiology and health). Higher STA for both CFS_THI and Milk_THI were associated with reduced percent rectal temperatures > 39 °C. In conclusion, results of this study demonstrate the link between genetic resistance to heat stress with respect to maintenance of fertility and milk yield with ability to regulate body temperature during heat stress. There is some evidence that this link is greater for effects of heat stress on fertility than on milk yield.