Regulation of estrus and ovulation in mares with progesterone or progesterone and estradiol biodegradable microspheres with or without PGF2 alpha

A single injection of a microsphere preparation, designed to deliver 1.25 gm progesterone and 100 mg estradiol-17 beta at a controlled rate, for a duration of 12 to 14 days, produces accurate control of estrus and fertile ovulations in mares. Treatment is followed by PGF2 alpha injection 14 days after steroid injection. The objectives of the present study were to determine whether estradiol added to the progesterone treatment or PGF2 alpha administered at the end of the steroid treatment regimen, would improve synchronization of estrus and ovulation. A total of 45 cyclic horse mares was randomly assigned to 1 of 5 treatment groups as follows: Group 1 (control, n = 9) sterile microsphere vehicle + sterile PGF2 alpha vehicle 14 days after treatment with microsphere vehicle; Group 2 (n = 9) progesterone and estradiol microspheres + PGF2 alpha 14 days after treatment with microspheres; Group 3 (n = 9) progesterone and estradiol microspheres + PGF2 alpha vehicle 14 days after treatment with microspheres; Group 4 (n = 9) progesterone + PGF2 alpha 14 days after treatment with microspheres; and Group 5 (n = 9) progesterone + PGF2 alpha vehicle 14 days after treatment with microspheres. Addition of estradiol (P < 0.05) or PGF2 alpha (P < 0.05) to the treatment regimen increased synchronization efficacy by reducing variation in days to ovulation. All treatments significantly reduced variation in days to estrus compared with that of the controls; however, mares in the progesterone groups had an increased incidence of silent or shortened estrous behavior (less than or equal to 2 days) following treatment. Estradiol added to the treatment regimen increased (P < 0.05) the number of mares with post treatment estrus > 2 days in duration compared with mares treated with progesterone (78 vs 33%, respectively). Therefore, estradiol and PGF2 alpha each appear to reduce variation in days to ovulation while estradiol seems to promote better expression of post-treatment estrous behavior.