Control of ovulation in cycling mares with ovarian steroids and prostaglandin

A combined progesterone-estradiol-17beta treatment was given in two experiments conducted to examine its effectiveness in controlling ovulation time in cycling mares. In the first experiment, the combined steroid (150 mg progesterone, 10 mg estradiol-17beta daily for 10 days) alone or combined with prostaglandin on the first and last days of steroid treatment resulted in ovulation in 15 of 16 mares 9-13 days after last injection, 13 of them on days 10-12. A CL present prior to treatment in one mare that received no prostaglandin persisted through and for 14 days after treatment. In the second experiment the combined steroid treatment started on the first or second day of estrus blocked ovulation in only 5 of 13 mares. Thus prostaglandin is necessary at least at the end of treatment. In both experiments a total of 20 mares with no luteal function at the end of steroid treatment ovulated on days 9-13 after last injection, 18 of these on days 10-12. These results indicate that the combined steroid-prostaglandin treatment can result in ovulations in a very restricted interval with apparently a normal distribution.