USE OF DESLORELIN SHORT-TERM IMPLANTS TO INDUCE OVULATION IN CYCLING MARES DURING 3 CONSECUTIVE ESTROUS CYCLES

Alternatives to human chorionic gonadotropin (hCG) for inducing ovulat ion in cycling mares over several consecutive cycles were explored. Pl acebo, one, three or five short-term implants each containing 2.2 mg o f gonadotropin-releasing hormone (GnRH) analogue (deslorelin) were adm inistered to cycling mares after identification of a follicle over 30 mm. Mares were treated over three consecutive cycles, and artificially inseminated during the third cycle only, Serum was assayed for concen trations of luteinizing hormone (LH), follicle-stimulating hormone (FS H), and progesterone during each cycle. All deslorelin doses decreased the interval to ovulation (4.0 days, 2.6 days, 2.4 days and 2.0 days) , increased the proportion of mares ovulating within 48 h (20.0%, 83.3 %, 73.3% and 85.7%), and decreased the diameter of the largest follicl e at ovulation (45.8 mm, 38.0 mm, 41.0 mm and 37.2 mm) for the placebo , 2.2 mg, 6.6 mg and 11.0 mg groups, respectively (P < 0.05). The inte rovulatory interval was lengthened in the 11.0 mg group compared with all other groups (21.2 days, 21.8 days, 26.4 days and 32.7 days for th e placebo, 2.2 mg, 6.6 mg and 11.0 mg groups, respectively, P < 0.05). No differences (P > 0.05) were detected in serum concentrations of pr ogesterone or pregnancy rate among the groups. Cycle number had no eff ect (P > 0.05) on the reproductive parameters or serum concentrations of progesterone. Concentrations of LH were higher in Cycle 3 in the 6. 6 mg and 11.0 mg groups, and FSH concentrations were decreased in Cycl es 2 and 3 in treated mares. Deslorelin was efficacious for inducing o vulation in cycling mares with no diminished activity over three conse cutive cycles. However, ovarian suppression and changes in serum conce ntrations of gonadotropins were noted at higher doses.