Effects of bulls on fertilization of bovine oocytes and their subsequent development in vitro

Abstract

This study investigated the pregnancy rate and calf mortality after transfer of embryos produced using sex-sorted semen. Data for 12,438 embryo transfers performed on dairy farms were analyzed. Of these, 10,697 embryos were produced using conventional semen (CONV embryos) and 1741 using sex-sorted semen from 97 bulls (SEX embryos), predominantly of Ayrshire and Holstein breeds. Of the CONV embryos, 27.4% were transferred fresh, whereas of the SEX embryos, 55.7% were fresh. Recipient attributes (breed, parity, number of previous breeding attempts, and interval from calving to transfer) were comparable for both embryo types, heifers representing 57.8% of recipients in the CONV group and 54.8% in the SEX group. Recipients that were not artificially inseminated or did not undergo a new embryo transfer after the initial embryo transfer and had registered calving in fewer than 290 days after the transfer were considered pregnant. Pregnancy rate for recipients receiving CONV embryos was 44.1%, and for those receiving SEX embryos, it was 38.8%. The odds ratio for pregnancy in recipients receiving CONV embryos was 1.34 compared with SEX embryos (P < 0.001). The proportion of female calves was 49.6% and 92.3% in CONV and SEX groups, respectively. Overall, calf mortality was comparable in both groups. Mortality was similar in CONV and SEX groups (6.6% and 7.7%, respectively) for female calves. For male calves, mortality was 9.2% in the CONV group but significantly higher, 16.0% (P < 0.05), in the SEX group. This study showed that transfer of embryos produced with sex-sorted semen decreased the pregnancy rate by about 12% compared with embryos produced using conventional semen. Mortality of male calves born from SEX embryos was higher than for those born from CONV embryos.