Effect of xylazine on interovulatory interval and serum progesterone concentrations in the horse mare

The objective of this study was to determine the effect of the alpha2-adrenergic agonist, xylazine, on interovulatory interval and progesterone concentrations in the horse mare. Mares were assigned to one of four treatments: Group 1 (controls) received an intramuscular injection (i.m.) of 5 cc saline (n=6), Group 2 received 10 mg prostaglandin F2alpha (PGF2alpha) i.m. (n=5), Group 3 received 500 mg xylazine i.m. (n=6) and group 4 received an intravenous injection (i.v) of 350 mg xylazine (n=6). Treatment was administered on Day 10 of the estrous cycle (Day 0=Day of detected ovulation). There was no difference in length of interovulatory interval between PGF2alpha-treated mares and control mares (mean+/-SEM; 18.8+/-1.0 versus 21.7+/-1.6 d). When compared with either xylazine-treated group, PGF2alpha-treated mares had a shorter interovulatory interval (18.3+/-1.0 d versus 22.2+/-0.6 and 22.8+/-1.3 d, respectively; P<0.05). There was no difference in the length of interovulatory interval between control mares and either xylazine-treated group. At the time of treatment all mares had progesterone concentrations>10 ng/ml, therefore the onset of luteolysis was defined as the day of the estrous cycle when progesterone concentrations decreased below 10 ng/ml. In PGF2alpha-treated mares, this event occurred earlier than in any other group (Day 11.2+/-0.2 of the estrous cycle versus 16.0+/-1.3 for control, Day 15.7+/-0.2 for Group 3 and Day 15.2+/-0.6 for Group 4; P<0.002). It was concluded that a single treatment with xylazine, either by an intramuscular or intravenous route, had no significant effect on interovulatory interval or progesterone concentrations in horse mares.