EVALUATION OF FACTORS AFFECTING THE PREGNANCY SUCCESS OF FRESH EMBRYO TRANSFERS IN COMMERCIAL BEEF CATTLE

This study aimed to evaluate the impacts of certain factors and their relationship with estrus expression, recipient suitability, and pregnancy success of fresh in vitro fertilized embryo transfers in commercial beef cows. Embryos were produced using IVF procedures by Trans Ova Genetics, Inc. (Sioux Center, IA). Sex-sorted heifer semen from a registered Charolais bull (WR Foreman D602; AICA 883344) was used in the IVF procedures to fertilize Angus-type oocytes collected in a packing plant. Prior to embryo transfer, 157 commercial beef recipients ranging from three to ten years of age were estrus synchronized using a progesterone-based protocol. Visual heat detection was performed three times daily, six to eight days prior to embryo transfer to determine whether or not heat expression had occurred. Embryos were transferred (n = 143) by a licensed veterinarian at the Texas A&M University McGregor Research Station in McGregor, TX, on June 22nd, 2023. Fourteen recipients were deemed unsuitable for an embryo because they did not have a viable corpus luteum (CL) on either their left or right ovary and, therefore, did not receive an embryo. Pregnancy success from embryo transfer was determined by both rectal ultrasonography and evaluation of calves born. Factors evaluated in this study included heat expression prior to ET (Heat or No Heat), recipient age [Group 1 (2 and 3 years old), Group 2 (4 and 5 years old), Group 3 (6 and 7 years iii old), and Group 4 (9 and 10 years old)], embryo quality grade (Grade 1 vs. Grade 2), and uterine horn transfer side (left uterine horn; LUH vs. right uterine horn; RUH). All statistical analysis was done using R-studio version 4.3.1. Statistical tests run included Pearson’s Chi-squared and Fisher’s exact test. Statistical significance was determined at a 95% confidence level. Overall: i) there was an estrus expression rate of 60.5% (95/157), ii) the rate of recipient suitability for embryo transfer was 91.1% (143/157), and iii) the pregnancy success rate to ET was 52.4% (75/143). There were no significant differences in pregnancy rates observed between Heat and No Heat cows (p = .14), uterine horn transfer sides (LUH vs. RUH; p = .54), or embryo quality grades (Grade 1 vs. Grade 2; p = .24). Similarly, there were no significant differences detected between recipient age and rates of recipient suitability (p = .14). However, a significant interaction was detected at the 95% confidence level between recipient age and ET pregnancy success. Group 4 exhibited significantly higher pregnancy rates than Group 1 (67.6% vs. 43.9%, respectively; p = .02). Additionally, Group 4 also exhibited higher rates of heat expression prior to ET when compared to Group 1, Group 2, and Group 3 (82.4% vs. 60.0%, 41.9%, and 44.4%, respectively; p < .05). When comparing the effects of heat expression on recipient suitability, Heat cows received an embryo at a significantly higher rate than No Heat cows (96.9% vs. 81.3%, respectively; p < .01). Overall, Group 4 cows exhibited the highest overall rates of heat expression, recipient suitability, and pregnancy success. Further research is encouraged to further evaluate the relationship between age and rates of external heat expression, pregnancy success, and embryo recipient suitability