Evaluation of methods for suppressing estrus and ovulation in mares: sustained release injections of altrenogest versus deslorelin acetate

Abstract

Two experiments assessed the potential of altrenogest, an artificial progestogen, and deslorelin acetate, a gonadotropin-releasing hormone (GnRH) agonist, for suppressing estrus and ovulation in cyclic mares. In the first experiment, mares were administered a luteolytic dose of prostaglandin-F2á on d 6 of diestrus, and were then treated with 1 of 3 formulations of altrenogest in slow-release vehicle (6 mares/group): 1) Biorelease altrenogest LA150 (225 mg total as a 1.5 mL injection); 2) Biorelease altrenogest LA225 (225 mg total as a 1.0 mL injection); or 3) Biorelease altrenogest LA225 (450 mg total as a 2.0 mL injection). Six control mares received vehicle. Compared to control mares (10.0 days), time to ovulation was greater (P < 0.01) for all mares receiving altrenogest (17.5 days overall); there was no difference (P > 0.5) among groups receiving altrenogest. Control mares (6 out of 6) first displayed estrus an average of 4.5 days after prostaglandin-F2á injection; of the 18 mares receiving altrenogest, 11 displayed first estrus at an average of 13.5 days (P < 0.02 relative to controls) and there was no difference (P > 0.19) among groups. It was concluded that the 3 formulations of altrenogest were equally effective in delaying estrus and ovulation in cyclic mares. It is suggested that a 10-day injection interval could be used to keep mares out of heat for extended periods of time. The second experiment was similar to the first, except that there were 2 groups: 1) control mares that received a vehicle injection and 2) mares that received an i.m. injection of 1.5 g of deslorelin acetate in biodegradable microparticles. Administration of deslorelin did not affect (P > 0.1) the day of first onset of estrus or the day of ovulation. There was a tendency (P = 0.08) for deslorelin treated mares (3 out of 7 vs. 0 out of 6 controls) to not show estrus at the expected time, even though they experienced luteolysis and ovulation. It was concluded that injection of 1.5 g of deslorelin acetate was not an effective method for suppressing estrus and ovulation in cyclic mares.