Control of ovulation in mares in the early breeding season with ovarian steroids and prostaglandin.

Two trials were conducted to (1) determine the degree of control of ovulation achieved by treating mares in late winter with progesterone and oestradiol-17 beta combined after prior exposure to an artificially increased photoperiod, and (2) to examine the effectiveness of such a procedure incorporated into equine breeding farm management systems. Following a 15-day treatment of 150 mg progesterone and 10 mg oestradiol-17 beta daily with 10 mg PGF-2 alpha on the last day of steroid treatment, 27 of 31 mares ovulated on Days 8-14 after the last injection in one trial. Conception rate for mares mated during that period was 77%, compared with 50% in untreated mares mated at first oestrus following the treatment period. In a second, larger field trial involving the same treatment for a 10-day period, 103 of 128 mares ovulated on Days 9-16 after the last injection and 62% of the mares mated on those days conceived. These results indicate that the combined steroid treatment provided reasonably precise and practical control of ovulation time in mares adequately stimulated by increased photoperiod without detrimental effect on fertility.