Stimulation of Ovulation in Seasonally Anovulatory and Vernal Transitional Mares with Estradiol and Domperidone: Dose and Combination Studies

Three experiments were conducted to test the efficacy of different doses of estradiol cypionate (ECP) and domperidone for inducing ovulation in seasonally anovulatory (January; Experiments 1 and 2) and transitional period (March; Experiment 3) mares. In the first two experiments, mares in Kentucky and Louisiana were administered domperidone (3 g in biodegradable particles) alone or after pretreatment with 100 or 150 mg of ECP; another group received ECP, domperidone, and progesterone, and a fifth group received ECP and progesterone only (the latter two in Kentucky). Control mares in both states received no treatment. The proportion of mares ovulating within 35 days (for mares treated in January) was greater (P = .0002) for those receiving any combination of ECP plus domperidone relative to mares not receiving the combined treatment. Addition of progesterone to ECP plus domperidone did not enhance (P = .7) the response relative to the combination alone; domperidone by itself, or ECP plus progesterone, did not alter the response relative to controls (P > .24). Experiment 3 was conducted in Louisiana as a 2 × 3 factorial, with two doses of domperidone (1.5 or 3 g) and three doses of ECP (0, 75, or 150 mg). There was no main effect of domperidone or ECP dose; a greater proportion (P = .055) of mares receiving any combination of ECP plus domperidone ovulated in 21 days compared with those receiving no ECP. In conclusion, pretreatment with ECP before injection of domperidone 10 days later can be used to increase the proportion of mares ovulating early in the year; within the limits of the present experiments, there appears to be no difference in doses of ECP of 75, 100, or 150 mg, and no difference in doses of domperidone of 1.5 or 3 g.