Effects of follicular status at treatment on follicular development and ovulation in response to FSH in Spanish Merino ewes

摘要

Cyclic Spanish Merino ewes were treated 13 days after an induced oestrus with 12 mg FSH-P in saline (n = 9) or FSH-P in propylene glycol (PG) (n = 24), and were injected on the same day (day 0) with 100 g cloprostenol. Transrectal ultrasonography was done daily from day 6 to day 0 and then at 12-h intervals for 72 h. The size and location of follicles 2 mm were recorded at each examination. Ovulation rate, determined by laparoscopy 7 days after oestrus, was higher (P<0.05)in ewes that received FSH-P in PG (0-16) than in saline (0-6). In the PG group, ovulation rate was bimodally distributed; about half the ewes had 1 ovulation while the remainder had >4 ovulations (mean = 7). Ovulation rate was associated with the characteristics of the largest follicle present on day 0: of the 9 ewes in which the largest follicle on day 0 had not changed in diameter from day 1 to day 0, 7 had >3 ovulations; and of the 24 ewes that showed an increase or decrease in diameter of thelargest follicle, only 8 had >3 ovulations (P<0.05). There was a linear trend for ovulation rate to decrease as the persistence of the largest follicle at treatment increased; none of the ewes in which the largest follicle at day 0 persisted for >36 h ovulated >6 follicles. As in the case of ovulation rate, the number of large follicles on days 1.5, 2 and 2.5 showed an interaction of vehicle with change in diameter of the largest follicle from day 1 to day 0. It is concluded that superovulationin response to a single intramuscular injection of FSH-P in PG is greatest when the largest follicle present at treatment does not change in diameter from that of the previous day but regresses rapidly after treatment.