Hormonal mechanism of follicle deviation as indicated by major versus minor follicular waves during the transition into the anovulatory season in mares

Follicle growth and circulating hormone concentrations were compared between an interovulatory interval and the first 60 days of the anovulatory season in pony mares. Daily observations were made from November of three groups: (i) ablation of follicles of >/=6 mm in diameter at day 10 after an ovulation that initiated an interovulatory interval, as determined retrospectively (ovulatory group, n=8), (ii) ablation at day 10 after the last ovulation of the year (anovulatory-10 group, n=6); and (iii) ablation at day 60 after the last ovulation of the year (anovulatory-60 group, n=6). Follicular waves were defined as major (dominant follicle) and minor (no dominant follicle). The percentage of mares with major waves after ablation for the ovulatory, anovulatory-10 and anovulatory-60 groups was 100, 33 and 0%, respectively, and the percentage with minor waves was 0, 67 and 100%, respectively. Minor waves were also detected in 83% of anovulatory mares between day 20 and day 60. Growth of the largest follicle was similar for major waves and minor waves but only until the beginning of deviation in the major waves. FSH surges after ablation were similar for all groups and for surges detected during days 20-60. Concentrations of LH were greater in association with major waves than with minor waves. Both diameter of the largest follicle and LH concentrations for minor waves were greater after ablation at day 10 after the last ovulation of the year than after ablation at day 60. The results of this study indicate that major follicular waves developed in some mares early in the anovulatory season and that minor waves developed throughout the first 2 months. Despite similarities in the wave-stimulating FSH surge, differences in follicle growth occurred and were attributable, on a temporal basis, to differences in LH concentrations. A minor wave developed into a major wave when the largest follicle reached a diameter characteristic of the beginning of deviation in the presence of an adequate LH stimulus for continued growth of a dominant follicle.