Abstract
Follicular development, incidence of double ovulations, and steroid hormone levels were examined in Hokkaido Native Pony (HNP) and Thoroughbred (TB) mares during control and FSH-stimulated cycles. Daily FSH treatment (HNP 8 mg FSH/day and TB 16 mg FSH/day; i.m.) was initiated on Day 7 of the estrous cycle following PGF2α, administration on Day 6 (ovulation = Day 0). Follicular development and ovulation was monitored by transrectal ultrasonography. Peripheral blood concentrations of progesterone and estradiol-17β were evaluated by EIA. FSH treatment had no effect on the maximum follicular diameter attained during the estrous period and the diameter of the biggest follicle before ovulation. The incidence of a second follicle >10 mm before ovulation was 0/12 (HNP) and 7/9 (TB) during control cycles. During FSH-stimulated cycles, 2/7 (HNP) and 5/6 (TB)mares had a second follicle >10 mm before ovulation. No double ovulations could be observed in HNP mares during 16 control cycles and 12 FSH-stimulated cycles. In TB mares, double ovulations occurred in 2 out of 14 control cycles and 6 out of 11 FSH-stimulated cycles (P<0.05). Progesterone levels after ovulation were not influenced by FSH treat-ment. Mean concentrations of estradiol-17β during estrus were not different in control and FSH-stimulated cycles. However, maximum estradiol-17β levels were higher in control cycles (6.4 pg/ ml vs. 4.1 pg/ml; P<0.05). It is concluded that treatment with daily injections of FSH is effective in inducing double ovulations in Thoroughbred mares.
