Effect of dose of GnRH analog on ovulation in mares

Proper timing of insemination for optimal conception is accomplished by frequent palpations per rectum, by ultrasonography of the preovulatory follicle and/or by treatment with hCG or GnRH. Sustained release of GnRH from implants has been shown to hasten ovulation. Therefore, 2 studies were conducted to evaluate the efficacy of a GnRH analog, deslorelin, for hastening ovulation in nonlactating cyclic mares. The GnRH implant was 2.3x3.7 mm and released deslorelin for 2 to 3 days. In Experiment 1, 60 nonlactating, cycling mares were assigned to 1 of 5 doses: 0, 1.2, 1.7, 2.2 and 2.7 mg per implant. Mares were assigned sequentially on the first day of estrus (Day 1). Ovaries were examined per rectum and with ultrasonography every 12 h until ovulation. Once the mares obtained a follicle>30 mm, they were injected subcutaneously with a GnRH implant. The mares were inseminated every other day during estrus with semen from 1 of 3 stallions. Pregnancy was determined with ultrasonography. Experiment 2, 40 nonlactating, cyclic mares were assigned to 1 of 5 treatments (same treatments as in Experiment 1). Data were obtained on interval to ovulation, duration of estrus and pregnancy rates at 12, 18 and 35 d after ovulation. Time to ovulation was shorter (P<0.05) in GnRH-treated mares than in control mares in the Experiment 1. Mean time to ovulation was 68, 49, 48, 47, 44 h in Experiment 1, and 91, 66, 58, 46, 58 h in Experiment 2 for mares given 0, 1.2, 1.7, 2.2 and 2.7 mg/mare in the 2 trials. Averaged for both experiments, the proportion of mares ovulating within 48 h of treatment was 40, 75, 85, 90 and 90% for 0, 1.2, 1.7, 2.2 and 2.7 mg/mare. For both experiments, there was no effect of GnRH on pregnancy rate. In summary, a subcutaneous implant containing GnRH analog induced ovulation in most mares by 48 h of injection, and there was no advantage of doses higher than 2.2 mg/mare.