In the present study, in nonprolific Western white-faced ewes, a 6-day treatment with MAP from midcycle in the absence of luteal progesterone resulted in a significant increase in ovulation rate by approximately 50% when compared with control ewes or the pretreatment cycle of treated ewes. This increase was mainly the result of the ovulation of follicles from the penultimate follicular wave before ovulation, and some follicles from earlier waves.
The high ovulation rate in prolific Finn sheep is achieved by ovulation of —50% of follicles attaining >5 mm in diameter in the penultimate follicular wave, which ovulate along with the follicles from the final wave of the estrous cycle. The occurrence of two ovulatory waves has also recently been shown in % Rambouillet X % Booroola ewes. This contrasts with nonprolific Western white-faced ewes, in which very few ovulatory-sized follicles ovulate from the penultimate wave of the cycle. In the present study (experiments 1 and 2), the percentage of follicles that ovulated from the penultimate wave before ovulation in MAP-treated ewes was 70% and 60%, respectively. There is, therefore, a striking similarity in the pattern of ovulatory follicle recruitment between the Western white-faced ewes that received PGF2a and MAP, and that of normally cycling Finn sheep.