PSIII-8 Evaluation of Reproductive Efficiency of Holstein Nulliparous Heifers Carrying the Slick1 Mutation of the Prolactin Receptor (Prlr) Gene

Abstract

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.