Inheritance of the SLICK1 allele of the prolactin receptor gene improves thermotolerance of lactating Holstein cows under humid heat stress conditions. The aim of this study was to investigate whether pre-and postweaning Holstein heifers carrying the SLICK1 allele would show physiological responses indicative of higher tolerance to heat stress in high-and low-humidity climates. A total of 101 heifer calves of two age groups heterozygous for the SLICK1 allele and 103 wild-type half-siblings were evaluated during July 2020 in 3 dairy farms in central California and 2 in south Florida. Dry bulb temperature and relative humidity data were recorded during evaluation and used to calculate the temperature-humidity index (THI). Physiological measurements were obtained between 1600 and 1900 h in California, and 1200 and 1400 h in Florida and included rectal temperature, respiration rate, skin temperature, and sweating rate. Data were analyzed via Generalized Linear Mixed Models including the main effects of genotype, state, group, sire, farm within state, and interactions, with THI included as a covariate. The correlations between THI and dependent variables were analyzed via linear regression. The average 24-h THI was higher in Florida compared with California (90 vs. 72, respectively); the main driver of the higher THI in Florida was the high relative humidity (average 85.6% in Florida vs. 36.7% in California). In Florida, the rectal temperature of slick calves was 0.4°C lower than non-slick calves (39.5 ± 0.1 vs 39.9 ± 0.1°C); no differences were detected between slick and non-slick calves in California. Regardless of genotype, heifer calves in Florida had higher respiration rate, higher rectal and skin temperatures, and lower sweating rate than in California. This study is the first to evaluate physiological responses of calves carrying the SLICK1 allele under heat stress conditions in different climates. Our findings demonstrate that the presence of this allele is associated with lower rectal temperatures in pre-and post-weaning Holstein females. According to the physiological parameters evaluated, calves raised in Florida appeared to be under more severe heat stress; in those conditions, the SLICK1 allele was advantageous to confer thermotolerance as evidenced by lower rectal temperature in slick animals.

The SLICK1 mutation in the prolactin receptor (PRLR) results in a short-hair coat and increased ability to regulate body temperature during heat stress. It is unclear whether the mutation affects capacity for sweating. The objective of this observational study was to evaluate whether the SLICK1 mutation in PRLR alters characteristics of skin related to sweat gland abundance or function. Skin biopsies from 31 Holstein heifers, including 14 wild-type (SL −/− ) and 17 heterozygous slick (SL +/− ), were subjected to histological analysis to determine the percent of the surface area of skin sections that are occupied by sweat glands. We detected no effect of genotype on this variable. Immunohistochemical analysis of the forkhead transcription factor A1 (FOXA1), a protein essential for sweating in mice, from 6 SL −/− and 6 SL +/− heifers indicated twice as much FOXA1 in sweat glandular epithelia of SL +/− heifers as in SL −/− heifers. Results from RNA sequencing of skin biopsies from 5 SL −/− and 7 SL +/− heifers revealed few genes that were differentially expressed and none that have been associated with sweat gland development or function. In conclusion, results do not support the idea that the SLICK1 mutation changes the abundance of sweat glands in skin, but do show that functional properties of sweat glands, as indicated by increased abundance of immunoreactive FOXA1, are modified by inheritance of the mutation in PRLR.

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The objectives of this study were to evaluate physiological responses to heat stress of Holstein heifers carrying the SLICK1 allele of the PRLR, i.e., the slick mutation. Slick and non-slick females were produced in central CA (n = 3 farms) and south FL (n = 3 farms) by inseminating cows with semen from two heterozygous slick Holstein sires. In July 2020, a subgroup of heifers of both genotypes (slick=93; non-slick=110) and two age groups (Group 1: 6–8 months old; n = 131; Group 2: 9–51 days old; n = 72) in both states were evaluated for rectal temperature (RT), surface temperature (ST) and sweating rate (SR). Dry bulb temperature, relative humidity and dew point were recorded every 15 min during testing for temperature humidity index (THI) calculation. Statistical models included the effects of genotype, state, farm within state, group, sire, THI, and interactions. Significant effects were considered as those with P < 0.05. Although the THI in CA was lower than FL (80 ± 0.1 vs 84 ± 0.2; P < 0.01), both were above 77 (estimated heat stress threshold for young animals). Slick heifers raised in FL had lower RT compared to non-slick siblings (39.6 ± 0.1 vs 40.1 ±0.1; genotype P < 0.01); this difference was not observed in CA (39.1 ± 0.04 vs 39.1 ± 0.05; genotype P = 0.5; state x genotype P < 0.01). In both states, slick heifers tended to have lower ST (34.7 ± 0.2 vs 35.1 ± 0.2; P = 0.07); CA heifers had lower RT and ST and higher SR (P < 0.01) regardless of genotype. Current results show that presence of the SLICK1 allele can improve the ability of Holstein heifers to regulate body temperature, particularly in high humidity conditions. Next steps will include analysis of performance as study animals enter the reproductive phase. Funding sources: Holstein Association USA and L.E. “Red” Larson Endowment.

Our objective was to evaluate the reproductive performance of Holstein nulliparous heifers carrying the SLICK1 mutation and half-sisters not carrying the mutation. Heifers were born between November and March of 2020 to two Holstein sires heterozygous for the dominant SLICK1 allele and raised in four dairy farms in the San Joaquin and the Tulare basins of the California Central Valley. Heifers were bred from January to September of 2021 according to farm protocols. Data were obtained via farm records and analyzed with the MIXED and LOGISTIC procedures of SAS; proportions were obtained via the FREQ procedure. Models included the fixed effects of genotype, sire, and farm; animal was considered a random effect. Response variables included overall pregnancy rate, age at first breeding, services per conception, pregnancy rate to first insemination, calving rate and pregnancy length. As shown in Table 1, pregnancy and calving rates were similar between slick and non-slick heifers. Overall, services per conception were similar between slick and non-slick heifers (1.8±0.2 and 2±0.2 services for slick and non-slick heifers, respectively; P = 0.3); however, pregnancy to first service tended to be greater in slick heifers (P = 0.07). Age at first breeding was not affected by genotype or sire, but there was an effect of farm due to one dairy starting the breeding program at an older heifer age. Pregnancy length tended to be longer for slick heifers (P = 0.08). This is the first study to evaluate the reproductive performance of slick Holstein females, although it is limited in sample size. Results indicate that the presence of the SLICK1 allele could provide an advantage to reproductive efficiency of Holstein heifers by improving the rate of pregnancy to first service. The authors thank the participating dairy farms. Support: Holstein Association USA Research Grant Program.