Administration of oxytocin immediately after insemination does not improvepregnancy rates in mares bred by fertile or subfertile stallions

It is probable that reduced pregnancy rates in mares bred to subfertile sta llions is attributable, in part, to the reduced number of normal spermatozo a that colonize the oviduct. Administration of oxytocin stimulates both ute rine and oviductal contractility. The hypothesis that oxytocin may enhance sperm transport to/into the oviducts, and thereby increase pregnancy rates, was tested in 2 trials. For both trials, fertile estrous mares with follic les greater than or equal to 35 mm in diameter were inseminated once at 24 h after administration of 1500 to 2000 U hCG. The inseminate dose was limit ed to 100 million spermatozoa in order to lower pregnancy rates and thus in crease the chance of detecting a treatment effect. Pregnancy status was det ermined by transrectal ultrasound examination 14 to 16 d after insemination . In Trial 1, 49 mares were inseminated with 4 mL extended semen from 1 of 3 stallions (1 fertile and 2 subfertile males). Immediately after inseminatio n, the mares were administered either 20 U oxytocin or 1 mL saline intraven ously. In Trial 2, 51 mares were inseminated with 4 mL extended semen from 1 of 4 stallions (1 fertile and 1 subfertile male used in Trial 1, and 2 ad ditional fertile males). Immediately after insemination, and again 30 min l ater, mares were administered either 5 U oxytocin or 0.25 mL saline intramu scularly. To test for effects of treatment with oxytocin and for the interaction betw een semen quality and treatment, a generalized linear mixed regression mode l was used that accounted for the split-plot design (treatment within stall ions), the random effect of stallion, the fixed effect of semen quality, th e binary outcome of a single breeding trial, and the varying number of tria ls per stallion/treatment groups. Three treatment protocols or regimens wer e used: placebo, 5 U oxytocin injected twice intramuscularly, and 20 units oxytocin injected twice intravenously. Semen was classified as high (fertil e stallions) or low (subfertile stallions) quality. No interaction between semen quality and treatment was detected (P > 0.10). The pregnancy rate of mares treated with oxytocin immediately after insemination was 30% (15/50) compared with 50% (25/50) for mares treated with saline immediately after b reeding. Administration of oxytocin did not affect pregnancy rates (P > 0.1 0). (C) 1999 by Elsevier Science Inc.