Advancing the time of ovulation in the mare with a short-term implant releasing the GnRH analogue deslorelin.

A small, biocompatible and short-term implant releasing 1.5 mg or 2.25 mg of the GnRH analogue deslorelin was evaluated in 140 Hanoverian (warm blooded) mares during the 1990 breeding season (Study I). Mares in oestrus and with a follicle 40 +/- 2 mm in diameter were assigned alternately to treatment (70) or remained as untreated controls. Implants were administered subcutaneously, and intervals to ovulation determined by rectal examination and ultrasound at 12-h intervals. Since results with both doses of deslorelin were similar, data were pooled. Deslorelin implantation resulted in ovulations in 65 of 70 mares within 48 h (93%), while only 5 of 70 control mares ovulated within the same time period (7%) (P < 0.01). Most induced ovulations (63%) occurred 36-48 h after implantation. In Study II, 4 groups of 12 Hanoverian mares each were treated with 3,000 or 5,000 iu hCG, or a 2.25 mg deslorelin implant, or received placebo. All treatments resulted in 100% ovulations within 48 h, versus 25% ovulations in controls (P < 0.01), and 63%, 75% and 86% of these ovulations occurred 36-48 h after treatment with 3,000 and 5,000 iu hCG and deslorelin respectively. Hormonal response to deslorelin in treated mares, sampled every 12 h, consisted of elevations of FSH and LH concentrations of > 200% and > 300% baseline values at 12 h (P < 0.001), of 67% and 79% at 24 h (P < 0.01) and of 35% and 49% at 48 h (P < 0.05), respectively. No local reactions at the implantation site were observed.