Summary. Individual follicles were monitored by ultrasonography in 15 mares during the transitional period preceding the first ovulation of the year and in 9 mares during the first interovulatory interval. During the transitional period, 7 mares developed 1–3 anovulatory follicular waves characterized by a dominant follicle (maximum diameter ≥ 38 mm) that had growing, static, and regressing phases. The emergence of a subsequent wave (anovulatory or ovulatory) did not occur until the dominant follicle of the previous wave was in the static phase. After the emergence of the subsequent wave, the previous dominant follicle regressed. The mean (± s.d.) length of the interval between successive waves was 10·8 ± 2·2 days. Before the emergence of waves (identified by a dominant follicle), follicular activity seemed erratic and follicles did not reach > 35 mm. During the interovulatory interval, 6 mares developed 2 waves (an anovulatory wave and a subsequent ovulatory wave) and 3 mares developed only 1 detected wave (the ovulatory wave). The ovulatory follicle at the end of the transitional period reached 20 mm earlier (Day − 15), grew slower (2·6 ± 0·1 mm/day; mean ± s.e.m.) but reached a larger diameter on Day − 1 (50·5 ± 1·1 mm) than for the ovulatory follicle at the end of the interovulatory interval (Day − 10, 3·6 ± 0·2 mm/day, 44·4 ± 1·0 mm, respectively; P < 0·05 for each end point). The interval from cessation of growth of the largest subordinate follicle to the occurrence of ovulation was longer (P < 0·05) for end of the transitional period (9·5 ± 0·7 days) than for the end of the interovulatory interval (6·8 ± 0·6 days). Results demonstrated the occurrence of rhythmic follicular waves during some transitional periods and the occurrence of 2 waves during some of the first oestrous cycles of the year.
Keywords: follicles; follicular waves; mares; reproductive seasonality