Reproductive Performance of Yearling Beef Heifers After Estradiol Benzoate Injection and Estrous Synchronization

Abstract

AbstractInjectable estradiol benzoate (EB) was evaluated for its effect on the reproductive performance of yearling beef heifers whose estrous cycles were synchronized using the melengestrol acetate/prostaglandin F2α (MGA®/PG) system. A total of 101 fall-born Simmental heifers received MGA (.5mg x hd−1 x d−1) for 14 d, with PG injected intramuscularly 17 d after the last day of MGA feeding. On d −10 and d 0 (d 1 = 1st d of MGA feeding), blood was collected for serum progesterone analysis. At 40h after PG injection, heifers were allotted, based on age and weight, to receive one intramuscular injection of either 1) 2ml of corn oil (control) or 2) 2ml of corn oil containing 400μg of EB. Heifers were artificially inseminated (AI) 12h after the first detected estrus. A second PG injection was given 10 d later to heifers that had not yet shown estrus. Artificial insemination continued for the first 35 d of the breeding season and was followed by 28 d of exposure to a single sire (63 d breeding season). Pregnancy was determined by palpation of the uterus 50 to 100 d after AI. First-service conception rate was verified by actual calving dates. At the beginning of the breeding season, similar percentages of heifers had serum progesterone >1 ng/ml on either d -10 or d 0 (78 vs 80%) and average serum progesterone (4.82 vs 6.14 ng/ml) did not differ in control and EB, respectively. Estradiol benzoate had no effect on the percentage of heifers showing estrus, first-service AI conception rate, overall AI conception rate, cumulative pregnancy rate or average calving date. Injecting yearling beef heifers with 400 μg of EB 40h after PG injection in an MGA/PG estrous synchronization system failed to improve further reproductive performance.