Efficacy of Long-Acting Formulations of Estradiol or Progesterone Plus Estradiol on Estrous Synchronization in Broodmares

A preliminary trial was performed to evaluate the ability of sustained release preparations of estradiol-17β or progesterone plus estradiol-17β to synchronize estrus in cyclic mares. Group 1 mares were treated with a 50 mg intramuscular (IM) injection of sustained release estradiol-17β, while group 2 mares were treated with estradiol plus 1.5 g of sustained release progesterone. All mares received an IM injection of 10 mg of prostaglandin-F2α (PGF2α) 10 days after steroid treatment. Mares were examined by transrectal ultrasonography on Days 1 and 10 of treatment and then at ≤2 day intervals to monitor follicle size. Once a follicle ≥30 mm diameter and uterine edema were detected, 0.5 mg of the GnRH analog histrelin was administered IM. Mares were examined daily thereafter to detect ovulation. Group 1 mares did not exhibit ovulation synchrony (ovulations occurred 12-22 days after steroid treatment), whereas ovulation synchrony was satisfactory in group 2 mares (interval to ovulation being 20.4 ± 1.5 days, range 17-22 days). Using sustained release preparations of progesterone plus estradiol-17β, with PGF2α administered on Day 10, could eliminate the need for daily injections of steroid preparations in oil when synchronizing estrus and ovulation.