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

Cyclic Spanish Merino ewes were treated on Day 13 of the estrous cycle with 12 mg, im, FSH-P in saline (n = 9) or propylene glycol (n = 24), currently with 100 microgram, im, Cloprostenol (Day 0). From Day -6 to Day 0, the ewes were observed daily by transrectal ultrasonography; after Day 0, ultrasonography was performed every 12 h for 72 h. Sizes and locations of greater than equal to 2 mm follicles were recorded at each observation. The ovulation rate was determined by laparoscopy on Day 7 after estrus. The number of ovulations ranged from 0 to 6 in ewes treated with FSH-P in saline and from 0 to 16 in ewes receiving FSH-P in propylene glycol (P < 0.05). In the latter group, the response was bimodally distributed; about half of the females had 1 ovulation, whereas the remainder had >4 with a mean of 7 ovulations. The ovulation rate was associated with 2 characteristics of the largest follicle present at treatment (Day 0). First, if the largest follicle on Day 0 had not changed in diameter from Day -1 to Day 0, then 7 of 9 ewes had >3 ovulations; if the largest follicle had either increased or decreased, only 8 of 24 ewes had >3 ovulations (P < 0.05). Second, there was a linear trend (P < 0.07) for ovulation rate to decrease as the persistence of the largest follicle at treatment increased; no ewe in which the largest follicle on Day 0 remained present for more than 36 h ovulated more than 6 follicles. As with the ovulation rate, the numbers of large follicles on Days 1.5, 2 and 2.5 varied with the interaction of change in diameter of the largest follicle on Day 0 from Day -1 to Day 0 and with vehicle. In summary, the superovulatory response was affected by the change in diameter from Day -1 to Day 0 of the largest follicle on Day 0 and the period required for that follicle to regress after treatment with FSH-P and cloprostenol.