EFFECT OF DOSE OF GNRH ANALOG ON OVULATION IN MARES

Proper timing of insemination for optimal conception is accomplished b y frequent palpations per rectum, by ultrasonography of the preovulato ry follicle and/or by treatment with hCG or GnRH.Sustained r elease of GnRH from implants has been shown to hasten ovulation. Therefore, 2 s tudies were conducted to evaluate the efficacy of a GnRH analog, deslo relin, for hastening ovulation in nonlactating cyclic mares. The GnRH implant was 2.3 x 3.7 mm and released deslorelin for 2 to 3 days. In E xperiment 1, 60 nonlactating, cycling mares were assigned to 1 of 5 do ses: 0, 1.2, 1.7, 2.2 and 2.7 mg per implant. Mares were assigned sequ entially on the first day of estrus (Day 1). Ovaries were exmined per rectum and with ultrasonography every 12 h until ovulation. Once the m ares obtained a follicle >30 mm, they were injected subcutaneously wit h a GnRH implant. The mares were inseminated every other day during es trus with semen form 1 of 3 stallions. Pregnancy was determined with u ltrasonography. Experiment 2, 40 nonlactating, cyclic mares were assig ned to 1 of 5 treatments (same treatments as in Experiment 1). Data we re obtained on interval to ovulation, duration of estrus and pregnancy rates at 12, 18 and 35 d after ovulation. Time to ovulation was short er (P<0.05) in GnRH-treated mares than in control mares in the Experim ent 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 experim ents, there was no effect of GnRH on pregnancy rate. In summary, a sub cutaneous implant containing GnRH analog induced ovulation in most mar es by 48 h of injection, and there was no advantage of doses higher th an 2.2 mg/mare.