PS11-14. Performance and Physiological Responses of Beef Calves Carrying the Slick PRLR Allele During the Post-weaning Period.

Abstract Natural mutations in the prolactin receptor gene (PRLR) result in slick hair phenotype and improved thermoregulation. Gene-editing technologies allow the introduction of novel variants into PRLR, providing a strategy to rapidly incorporate slick phenotype into cattle populations. The objective of this study was to evaluate performance and physiological responses of calves differing in PRLR genotype during the post-weaning period in South Texas. Bos taurus crossbred beef cows were artificially inseminated (AI) with semen from a single Angus sire who is mosaic for slick mutations in PRLR. Ten days after AI, Brangus bulls were introduced for the remainder of the breeding season. A total of 70 calves (33 females and 37 males) were weaned and vaccinated at 229 – 266 days of age (Day 0; October 2025). Genotyping identified 47 calves heterozygous for one of the five mutations producing the slick phenotype (SLICK), 15 wild-type calves without PRLR mutation (WT) and 8 calves with a large deletion in the PRLR resulting in a functional knockout (NULL). Rectal temperature (RT), BW, and blood samples were collected on Days 0, 3, 7, 14, 28 and 45. Chute and hair shed scores were collected on Day 3. Daily weather data indicated temperatures ranging from 8.0 to 33.2 °C and THI from 43.7 to 75.4. Adjusted 205-day WW was greater (P < 0.05) for SLICK (241.1 ± 3.9 kg) and WT (238.0 ± 7.1 kg) compared to NULL calves (202.9 ± 9.8 kg; P < 0.01); however, SLICK and WT did not differ (P = 0.70). Similarly, WW between male and female calves did not differ (P = 0.17). No treatment effects were detected (P = 0.24) for growth rate between Day 0 and Day 45 (-0.26, -0.43, -0.33 kg/d for NULL, SLICK, WT, respectively; SEM = 0.076). A treatment x day interaction (P < 0.01) was observed for RT. SLICK calves consistently exhibited lower RT compared with WT and NULL calves at Day 0 (39.3 vs. 39.8 and 40.5 °C), Day 7 (38.1 vs. 38.7 and 38.6 °C) and Day 14 (38.3 vs. 39.2 and 39.4 °C). Concentrations of haptoglobin increased (P < 0.05) at Day 3, although, no treatment effect was detected (P = 0.85). A treatment x day interaction (P < 0.05) was determined for concentrations of serum cortisol. Hair shedding score differed among genotypes (P < 0.01), with SLICK calves exhibiting lower scores (1.32) than WT (2.39) and NULL calves (4.47). Chute score was also affected by treatment (P < 0.01), with SLICK calves (1.67) exhibiting more desirable scores compared with WT (2.19) and NULL calves (2.10). These results indicate that the slick PRLR genotype improves thermoregulation and hair shedding, supporting its potential to enhance resilience of beef cattle in heat-stress environments.