INFLUENCE OF PHOTOPERIOD AND OVARIES ON SEASONAL REPRODUCTIVE ACTIVITY IN MARES

A study was done of the influence of length of photoperiod (control vs 16 h fixed photoperiod) and season (month) on: 1) changes in concentrations of plasma gonadotropins in ovariectomized (OVX) mares and 2) changes in plasma concentrations of gonadotropins and diameters of ovarian follicles preceding the onset of the ovulatory season in ovarian intact mares. In ovarian intact groups, intervals from start of project (January 6) to the first day of the ovulatory estrus, first appearance of a 30 mm follicle and onset of the ovulatory season (first ovulation) were shorter for the light-treated than for the control group. The changes in gonadotropin concentrations and follicular development preceding the onset of the ovulatory season (Day 0 = day of first ovulation) were not significantly different between light-treated and control groups. Mean plasma FSH levels fluctuated between Day -60 and Day -20 and then declined reaching a significant drop by Day -8. Mean plasma LH levels remained low until Day -8 and then increased reaching a maximum on the last day examined. The mean number of follicles 15-25 mm appeared to increase gradually from Day -60 to Day -20, remained elevated between Day -20 and Day -8 and then declined rapidly to a minimum on Day -1. The mean diameter of the largest follicle appeared to increase gradually from Day -60 to Day -8 and then increased rapidly between Day -8 and Day -1. The decline in FSH appeared to precede the decline in number of follicles 15-25 mm and the rise in LH. The rise in LH was temporally associated with the final growth and maturation of the preovulatory follicle. The ovulatory LH profile at the onset of the ovulatory season was similar in shape but lower in magnitude than the ovulatory LH profile during the middle of the ovulatory season. Removal of the ovaries during the anovulatory season did not evoke a postovariectomy rise in gonadotropin levels. However, there was a seasonal pattern of LH and FSH in ovariectomized mares and the LH levels increased approximately 2 months earlier in the light-treated group than in the control group. The rise in LH in the OVX groups occurred at approximately the same time as the onset of the ovulatory season in the intact groups.