Increasing Exogenous Progesterone during Synchronization of Estrus Decreases Endogenous 17Î2-Estradiol and Increases Conception in Cows

The objectives of this study were to determine the effects of dose of exogenous progesterone (P4) prior to artificial insemination on concentrations of 17p-estradiol (E,) and on conception rates in bovine females. Heifers (n = 100) and cows (n = 100) received P4-releasing intravaginal devices (PRIDs) to produce two different circulating concentrations of P4. All animals received a single PRID 10 days before (Day -10) the start of the breeding season (Day 0). In animals that received the low dose of P4 (1 PRID, target concentration of 2-3 ng/ml of plasma), the original PRID remained in place for 10 days. In animals that received the larger dose of P4 (2 PRIDs, target concentration of 5-8 ng/ml of plasma), an additional PRID was inserted on Day -9. To maintain concentrations of P4 in the 2-PRID group, the PRIDs inserted on Days -10 and -9 were replaced with new PRIDs on Days -5 and -4, respectively. Prostaglandin F2,, (25 mg) was administered to all animals on Days -9 and -3 to remove the endogenous source of P4. Following PRID removal, animals were artificially inseminated 12 h after signs of behavioral estrus were observed. A treatment-by-day interaction (p < 0.0001) was observed for concentrations of P4 in circulation of both heifers and cows. Animals that received 2 PRIDs had greater (p < 0.001) concentrations of P4 by Day -8 of treatment than animals that received 1 PRID. In cows that received 1 PRID, concentrations of E, increased 2.4-fold from Day -10 (6.8 ± 1.0 pg/ml) to Day -2 (16.7 ± 1.4 pg/ml). Cows treated with 2 PRIDs maintained concentrations of E2 similar to those at the initiation of treatment (treatment-by-day, p < 0.0001). In contrast to cows, concentrations of E, did not increase in heifers treated with 1 PRID. After PRID withdrawal, both heifers and cows receiving 1 PRID exhibited behavioral estrus earlier (p < 0.006) than animals receiving 2 PRIDs. Cows receiving 1 PRID had a lower (p < 0.02) conception rate than did cows receiving 2 PRIDs (53.3% and 76.6%, respectively). Conception rate in cows was negatively correlated (r = -0.39; p < 0.0001) to mean concentration of E, prior to P4 withdrawal. There were no differences in conception rates between heifers that received 1 or 2 PRIDs. In summary, cows with low circulating concentrations of P4 (2-3 ng/ml) have elevated concentrations of E,. Prolonged exposure to elevated concentrations of E2 may alter the cascade of events required to establish pregnancy and thereby reduce conception rates in cattle.