The relative milk production of dairy cattle in tropical Costa Rica that are heterozygous and homozygous for the SLICK1 allele.

SLICK1 is an allelic variant of the prolactin receptor () that is found in Senepol beef cattle. The presence of a single copy of this allele produces a short hair coat and confers heat tolerance. We aimed to determine the effect of 2 copies of this allele on milking performance of dairy cattle. Data were obtained from a commercial dairy farm in Costa Rica where crossbreeding of the dairy herd with Senepol had been undertaken for more than 15 yr. Random regression modeling was used to generate lactation curves for individual cattle. Homozygous SLICK1 cows produced 9% more milk (356 L), predominantly over the first 200 d of a 305-d lactation. Milk production of wild-type cattle in the same herd was much lower than SLICK1 carriers, but the number of cows in this group was too low (n = 13) to provide a conclusive assessment of milking performance.