Animal Science Department


Date of this Version



Published in BIOLOGY OF REPRODUCTION 49, 214-220 (1993).


Copyright © Society for Study of Reproduction. Used by permission.


The objectives of this study were to determine the effects of dose of exogenous progesterone (P4) prior to artificial insemination on concentrations of 17β-estradiol (E2) 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 F (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 E2 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 E2 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 E2 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 E2. Prolonged exposure to elevated concentrations of E2 may alter the cascade of events required to establish pregnancy and thereby reduce conception rates in cattle.