INCREASING EXOGENOUS PROGESTERONE DURING SYNCHRONIZATION OF ESTRUS DECREASES ENDOGENOUS 17-BETA-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 concen trations of 17beta-estradiol (E2) and on conception rates in bovine fe males. Heifers (n = 100) and cows (n = 100) received P4-releasing intr avaginal devices (PRIDs) to produce two different circulating concentr ations Of P4. All animals received a single PRID 10 days before (Day-1 0) 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 plasm a), the original PRID remained in place for 10 days. In animals that r eceived the larger dose of P4, (2 PRIDs, target concentration of 5-8 n g/ml of plasma), an additional PRID was inserted on Day -9. To maintai n 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, respective ly. Prostaglandin F2alpha (25 mg) was administered to all animals on D ays -9 and -3 to remove the endogenous source of P4. Following PRID re moval, animals were artificially inseminated 12 h after signs of behav ioral estrus were observed. A treatment-by-day interaction (p < 0.0001 ) was observed for concentrations Of P4 in circulation of both heifer s and cows. Animals that received 2 PRIDs had greater (p < 0.001) conc entrations of P4 by Day -8 of treatment than animals that received 1 P RID. 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). C ows treated with 2 PRIDs maintained concentrations of E2 similar to th ose at the initiation of treatment (treatment-by-day, p < 0.0001). In contrast to cows, concentrations of E2 did not increase in heifers tre ated with 1 PRID. After PRID withdrawal, both heifers and cows receivi ng 1 PRID exhibited behavioral estrus earlier (p < 0.006) than animals receiving 2 PRIDs. Cows receiving 1 PRID had a lower (p < 0.02) conce ption rate than did cows receiving 2 PRIDs (53.3% and 76.6%, respectiv ely). Conception rate in cows was negatively correlated (r = -0.39;P < 0.0001) to mean concentration of E2 prior to P4 withdrawal. There wer e rio 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 after the cascade of events require d to establish pregnancy and thereby reduce conception rates in cattle .