Increasing exogenous progesterone during synchronization of estrus decreases endogenous 17 beta-estradiol and...

The objectives of this study were to determine the effects of dose of exogenous progesterone (P4) prior to artificial insemination on concentrations of 17 beta-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 F2 alpha (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).(ABSTRACT TRUNCATED AT 250 WORDS)