Follicular waves and circulating concentrations of gonadotrophins, inhibin and oestradiol during the anovulatory season in mares

Follicular waves and associated circulating hormone concentrations were studied during the anovulatory season in pony mares (n = 8). Follicles were monitored by ultrasonography and a blood sample was taken daily from 29 January until ovulation (mean, 28 April). A mid-anovulatory period (largest follicle, 16.0 +/- 0.5 mm in diameter) and transitional period (largest follicle, 22.4 +/- 0.5 mm) were distinctive in each mare. The two periods were delineated by an increase in the diameter of the largest follicle to greater than or equal to 21.0 mm. Follicular waves, identified by significant increases in the mean diameter of the second to sixth largest follicles, were detected during both the mid-anovulatory and transitional periods. The mean number of follicles greater than or equal to 15.0 mm in diameter and the diameter of the second to sixth largest follicles increased in association with statistically identified FSH surges. The pattern of the FSH concentration changes during surges did not change during the mid-anovulatory and transitional periods. During the declining portion of the FSH surge, follicle growth continued and circulating total inhibin increased, indicating suppression of FSH by inhibin from the growing follicles. Circulating oestradiol or LH did not change relative to wave emergence. Results indicated that follicular waves occurred during the second-half of the anovulatory season, even during the period of lowest follicular activity. On a temporal basis, follicular wave emergence was stimulated by surges in circulating FSH. However, the increase in follicle growth to greater than or equal to 21.0 mm in diameter for the wave at the beginning of the transitional period and for the subsequent waves was not attributable to a change in the characteristics of the associated FSH surges.