Factors Associated with Pregnancy Success and Calf Growth via Multiple Ovulation and Embryo Transfer in...

This study evaluated factors affecting pregnancy outcomes, full-term calving success, and pre-weaning performance following embryo transfer in Holstein-Friesian cattle under commercial farm conditions. A total of 604 in vivo-derived embryos (fresh and frozen-thawed) were transferred into 384 heifers and 220 cows, and pregnancies were diagnosed by ultrasonography. Calves were monitored from birth to weaning. Pregnancy rate per embryo transfer (PR/ET) was higher in heifers than in cows, indicating superior reproductive performance of heifer recipients under field conditions. Recipient corpus luteum (CL) stage II yielded favorable pregnancy outcomes compared to CL stage I, whereas CL stage III comprised a small subgroup (n = 8) exhibiting a descriptive PR/ET of 87.5% that required cautious interpretation due to the limited sample size. Fresh embryos were associated with higher PR/ET than frozen-thawed embryos. Embryo developmental stage did not significantly affect initial pregnancy establishment or full-term calving rates, although stage 6 embryos (blastocyst) showed a non-significant numerical trend toward higher calving rates. Post-diagnosis gestation loss was a primary limiting factor, particularly in multiparous cows. Regarding pre-weaning performance, calves derived from frozen-thawed embryos exhibited a slightly higher birth weights than those from fresh embryos, whereas weaning weight and pre-weaning average daily gain did not differ between embryo types. Overall, recipient category and embryo type are key factors influencing pregnancy success, whereas postnatal performance is less affected. These findings highlight the importance of recipient selection and embryo management strategies for improving reproductive efficiency under commercial dairy farm conditions.