Follicular and gonadotrophic changes during transition from ovulatory to anovulatory seasons.

Observations of follicular and gonadotrophic changes were conducted on 14 Pony mares for 31-day periods beginning at the penultimate (second last) ovulation and at the last ovulation of the season. There were no main effects or interaction of day or period on the numbers of small follicles (less than 20 mm). Concentrations of FSH, numbers of large follicles (greater than 20 mm), and oestrous behaviour each showed an effect of day (P less than 0.01), but no effect of period. Concentrations of LH and diameter of the largest follicle each showed an interaction (P less than 0.01) between day and period attributable primarily to smaller values on Days 18--24 after the last ovulation than after the penultimate ovulation. It was concluded that, of the factors considered, failure of ovulation at the onset of the anovulatory season was due to lack of an adequate ovulatory LH surge and final growth of a preovulatory follicle.