Use of a GnRH antagonist, antarelix, associated or not with hCG, to control ovulation in cyclic pony mares.

The GnRH antagonist antarelix (Teverelix) was administered to mares (0.01 mg/kg, i.v., twice a day) during the periovulatory period. In Experiment 1, 20 mares were divided into a treated (A3d-) and a control (Control-) group. A3d- mares received antarelix for 3 days from the day when the dominant follicle (F1) reached 32 mm (D0). In Experiment 2, 10 mares were divided into a treated (A6d+) and a control (Control+) group. A6d+ mares received antarelix for 6 days from D0 and hCG was injected in all animals (1600 IU, i.v.) on D1. Pregnancies were determined 13 days after ovulation. In both experiments, antarelix interrupted or totally abolished the LH surge. In Experiment 1, 5/10 of the A3d- mares (with maximum LH concentrations of 11.6 ng/ml at the beginning of treatment) ovulated at the same time as the Control- mares; the other five mares (with LH concentrations under 5.4 ng/ml) ovulated 13.4+/-0.6 days later. In Experiment 2, all the A6d+ mares ovulated at the same time as the Control+ mares. In treated mares which ovulated during the treatment, progesterone concentrations and fertility did not differ from control mares. These results demonstrate that in mares: (1) a small elevation of endogenous LH can induce ovulation, (2) ovulation can be postponed approximately 13 days after a 3-day antarelix treatment if initiated just before the preovulatory LH surge, (3) ovulation can be induced by hCG on depressed levels of endogenous LH, (4) the inhibition of the post ovulatory LH surge has no effect either on the corpus luteum or on fertility.