Effect of fenprostalene, a PGF(2)alpha analogue, on plasma levels of estradiol-17beta and progesterone in cyclic heifers

Following a 40-day acclimatization period, 12 cyclic beef heifers entered a 95- to 101-day test period. Prior to fenprostalene treatment, all animals were studied through two normal estrous cycles. Plasma samples were obtained daily from all animals during the course of the study and were assayed for estradiol-17beta and progesterone. Group 1 heifers (n=6) were then treated with fenprostalene at mid-cycle during two subsequent cycles. This treatment was accomplished by treating the animals 11 days after the first clinically observed signs of estrus following Study Day 21 and treating them again 11 days later. Each treatment consisted of a subcutaneous injection of 1.0 mg fenprostalene. The animals were studied through two or three estrous cycles following the second injection. The Group 2 animals (n=6) were maintained as untreated controls through a corresponding period. Fenprostalene induced estrus in five of six treated heifers within 5 d following the first injection and in five of six heifers within 3 d following the second injection. The mean time to estrus was 3.4 d (+/- 1.1 d SD) following the first injection and 2.2 d (+/-0.8 days SD) following the second injection. No significant differences were found in the plasma levels of estradiol-17beta and progesterone when comparing fenprostalene-induced cycles to those that occurred naturally. The fenprostalene injection reset the estrous cycle without changing the nature of the cycle. The time of clinically detected estrus usually coincided with a sharp peak in estradiol-17beta concentration.