The relation between patterns of ovarian follicle growth and ovulation rate in sheep

The number and growth rate of follicles within classes based on granulosa volume were determined for ovaries taken from groups of 4-5-year-old, fine-wool Merino ewes drawn at different times of the year from a single strain flock maintained at Armidale, N.S.W. The breeding season of the flock normally extends from February to October and the mean ovulation rate rises from about 0.5 in February to about 1.8-1.9 during April-May. Ewes sampled when they were anoestrous or had one (single-ovulatory) or two (twin-ovulatory) recent corpora lutea did not differ in respect to the mean total number of ovarian follicles, the mean number of follicles in individual classes, the time for follicles to complete their rapid growth stage, or the incidence of follicle atresia. However, the ovaries of twin-ovulatory ewes contained significantly more follicles in the two terminal classes within the rapid growth stage than did the ovaries of single-ovulatory or anoestrous ewes (2.2 v. 0.9 and 1.0). This difference was attributed to the differing numbers of follicles per day entering into the rapid growth stage (5.2, 4.5 and 3.7 respectively in twin-ovulatory, single-ovulatory and anoestrous ewes).