EFFECT OF CONSTANT ADMINISTRATION OF A GONADOTROPIN-RELEASING-HORMONEAGONIST ON REPRODUCTIVE ACTIVITY IN MARES - INDUCTION OF OVULATION DURING SEASONAL ANESTRUS

The potential of a gonadotropin-releasing hormone (GnRH) agonist (gose relin acetate), delivered constantly for 28 days via a subcutaneous de pot, to induce ovulation in seasonally anestrous mares, was investigat ed. Two experiments were conducted, in which a range of doses (30 to 2 40 mug/mare/d) was examined. Mares were selected on the basis of lack of substantial follicular development (follicle diameter < 20 mm deter mined ultrasonically) and low serum concentrations of luteinizing horm one (LH) and progesterone. Constant administration of the GnRH agonist -induced ovulation in anestrous mares, but a dose-response relation wa s not observed. Furthermore, with identical doses tested in consecutiv e or alternate years, considerable variation was observed in the ovula tory response. In general, ovulation in all treated mares was accompan ied by increased circulating concentrations of LH and a decrease in fo llicle-stimulating hormone values. Ovulation was preceded by an increa se in estradiol and LH concentrations. in mares in which ovulation did not occur, concentration of LH increased during agonist treatment, wh ereas that of follicle-stimulating hormone either increased or did not change. It was concluded that constant administration of GnRH agonist s may induce ovulation in mares during seasonal anestrus; however, per centage of mares ovulating and the lack of reproducibility of effect i ndicate that this approach is inappropriate for use as a reliable meth od to manipulate breeding activity in commercial broodmares.