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Synchronization of Estrus and Ovulation in the Mare with Methallibure^{1, 2,}

University of Wisconsin, Madison 53706

Abstract

Four experiments were conducted, using a total of 92 mares, to determine the effect of methallibure and HCG on estrus, ovulation and fertility. Methallibure administered orally at doses of .5 to 2.5 g per day for 18 or 20 days caused inappetence and required treatment via a bolus. A high frequency of mares receiving 1.5 g or more became lethargic or staggered during treatment. Estrus during treatment was not prevented in all mares at doses of .5 to 2 g per day. Ovulation did not occur during treatment except for four mares who refused drugged feed. Forty-seven of 50 mares came into estrus and 43 ovulated after methallibure treatment. Estrus was reasonably synchronized by methallibure treatment for mares within a given experiment and at one specific dose. The time intervals from the end of treatment to estrus were: 8.5±3.1, 6±2.9, 10.8±3.6 and 10.5±2.5 days, respectively, for methallibure doses of .5, .6, 1.5 and 2.0 g per day. HCG treatment shortened the interval from the end of methallibure treatment to ovulation in one of two experiments and hastened the time of ovulation during estrus in the other. It also slightly increased ovulation rate in three of three experiments. The pregnancy rates for all three experiments where mares were bred averaged 40, 41 and 42%. Neither methallibure or HCG treatment significantly altered the pregnancy rate from that of control mares.

¹ Research supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison; Ayerst Laboratories, Rouses Point, New York; and by the American Breeders Service, DeForest, Wisconsin.

² Paper No. 619 from the Department of Meat and Animal Science.