Resumption of follicular activity and interval to postpartum ovulation after exogenous progestins

In Experiment 1, treatment of dairy cows with progesterone using an intravaginal device from d 5 to 15 postpartum failed to alter onset or duration of the first estrous cycle. Compared with controls in Experiment 2, concentrations of FSH increased, and those of LH decreased, in cows treated with either norgestomet or progesterone for 6 d. Relationships of various measures of LH, but not FSH, were all positive with increasing days postpartum, and those of FSH produced positive polynomial regressions as days to first ovulation decreased. No significant changes in follicular status occurred during or after progestin treatments; however, a consistent relationship was observed between increasing diameter of dominant follicles and serum concentrations of estradiol-17 beta in most cows. Appearance of the ovulatory dominant follicle generally was detected 4 to 5 d before ovulation regardless of treatment and was first detected more frequently during treatment with progestins when concentrations of FSH were increased compared with controls. Concentrations of progesterone were higher on d 7 to 10 following removal of progestin treatments. Appearance of the first dominant follicle was detected during periods of increased average minimum concentrations of FSH, increased variation in LH secretion, and was related to subsequent duration of the first estrous cycle.