Ovarian endocrine activity and development of ovarian follicles during the postpartum interval in beef cows

Abstract

To assess differences between multiparous and primiparous beef cows, largest follicle diameter, serum estradiol-17β (E 2) and progesterone (P 4) were monitored during the postpartum interval. Beginning 6–10 days postpartum, nine primiparous and 10 multiparous Angus cows underwent ovarian ultrasonography and jugular venipuncture every 48 h until cows either exhibited a normal estrous cycle or, owing to the ending of breeding season, first estrus. No difference was observed in average daily gain (±s.d.) for primiparous and multiparous cows (0.76±0.27 and 0.51±0.43 kg, respectively). Within 2 weeks postpartum, 5–10 mm follicles were found in all cows. The interval from appearance of the first follicle ≥ 14 mm to ovulation was longer ( P<0.01) in primiparous than in multiparous cows (42.7 and 13.5 days, respectively). No difference in mean follicle diameter was found during the postpartum interval prior to ovulation or during the first postpartum estrous cycle. Length of first postpartum estrous cycle was not different between primiparous (17.0 days) and multiparous (16.9 days) cows. Average days postpartum to first ovulation for primiparous and multiparous cows was 112 and 46.1 days, respectively ( P<0.001). Estradiol-17β remained low throughout the postpartum interval with occasional increases corresponding to increases in follicular size. Average baseline P 4 concentrations were 0.04 and 0.03 ng ml −1 for primiparous and multiparous cows, respectively. All cows had increased P 4 preceded by a drop in largest follicle diameter, prior to first postpartum estrous cycle. Only those primiparous cows with unusually long (> 115 day, n=5) postpartum intervals were likely to have an increase in largest follicle diameter after parturition. Achieving an ovulatory size follicle (≥ 14 mm) is no indication of impending estrus in primiparous cows as it may be in multiparous cows. In addition, we found no difference in mean largest follicle diameter between primiparous and multiparous cows during the postpartum interval or first postpartum estrous cycle.