UC Davis

Lactating Holstein cows carrying the SLICK1 allele of the prolactin receptor gene have demonstrated increased thermotolerance during periods of heat stress in tropical and subtropical climates. We aimed to evaluate Holstein heifers heterozygous for the SLICK1 allele for their (1) preweaning growth, (2) physiological responses to heat stress in low and high humidity climates at pre- and postweaned ages, and (3) reproductive performance as nulliparous heifers. Upon evaluation of 35 slick and 36 non-slick half-sister calves born in California during the cool season, birth weights of slick calves averaged 40.0 ± 0.9 kg while non-slick calves averaged 38.0 ± 1.0 kg (P = 0.08); however, weaning weights and average daily gain did not differ between slick and non-slick calves. In July 2020, 50 calves in Florida (21 slick and 29 non-slick half-sisters) and 222 calves in California (80 slick, 81 non-slick half-sisters, and 61 unrelated cohort control) were evaluated for physiological responses under environmental conditions where the average THI was 90 in Florida and 72 in California. Data were collected midday in Florida and at three times in California: morning and afternoon for all animals, and at an additional timepoint at midday for the pre-weaned calves. Two separate data analyses were performed. The first analysis combined the data from the measurements obtained in slick and non-slick heifer calves in Florida with the measurements obtained in the afternoon in the slick and non-slick heifer calves in California. Slick calves in Florida had rectal temperatures that were 0.4ºC lower than the non-slick calves in Florida while no difference between genotypes was detected in the calves in California. Regression analysis to test for a correlation between rectal temperature and THI revealed a stronger correlation between rectal temperature and THI of non-slick calves (Adj. R2 = 0.36) compared to slick calves (Adj. R2 = 0.06). Moreover, the calves in Florida had greater rectal temperature, respiration rate, and sweating rate than those in California regardless of genotypes. The calves in Florida experienced more severe heat stress conditions, under which the presence of the SLICK1 allele was advantageous to improve thermotolerance in the calves. The second analysis was performed on the data obtained from slick, non-slick, and unrelated control cohort heifer calves in California in the morning, midday, and afternoon. Here again, regression analyses to test for a correlation between rectal temperature and THI during the afternoon when THI is highest, revealed a weaker correlation in slick calves (Group 1 Adj. R2 = 0.01; Group 2 Adj. R2 = -0.04) compared to non-slick (Group 1 Adj. R2 = 0.12; Group 2 Adj. R2 = 0.19) and control calves (Group 1 Adj. R2 = 0.10; Group 2 Adj. R2 = 0.07). This indicates that the slick heifers maintained a more consistent body temperature as THI rose compared to the non-slick and control animals. However, we did not detect differences between genotypes in the morning, midday, or afternoon. The preweaned calves began the day with lower rectal temperature, respiration rate, skin temperature, and sweating rate than the postweaned heifers, yet by the afternoon, preweaned calves had comparable or slightly higher measurements on all physiological parameters evaluated. We also evaluated the age at first breeding, pregnancy success, pregnancy success to first service, services per conception, age at calving, calving success, and pregnancy length for a subgroup of heifers in California. There were no differences in these reproductive parameters between heifers carrying the SLICK1 allele and their non-slick half-sisters. This project is the first to evaluate the preweaning growth of slick Holstein calves in a climate with low humidity. Additionally, this is the first evaluation of physiological responses to heat stress in relation to presence of the SLICK1 allele in young cattle (< 1 year of age) and cattle raised under heat with low humidity. Finally, this is the first report on the reproductive performance of nulliparous Holstein heifers carrying the SLICK1 allele.