EFFECT OF CONSTANT ADMINISTRATION OF A GONADOTROPIN-RELEASING-HORMONEAGONIST ON REPRODUCTIVE ACTIVITY IN MARES - PRELIMINARY EVIDENCE ON SUPPRESSION OF OVULATION DURING THE BREEDING-SEASON

During the breeding season, the effect of constant administration of a n agonist analog of gonadotropin-releasing hormone (GnRH; goserelin ac etate) on reproductive activity of mares was determined. Twenty-four m ares undergoing estrous cycles were allocated at random to 6 groups (n = 4/group) and, on May 29 (day 0), received no treatment (group 1, co ntrols), 120 mug (group 2), 360 mug (group 3), 600 mug (group 4), or 1 ,200 mug (group 5) of GnRH agonist/d for 28 days via a depot implanted subcutaneously. The final group of mares (group 6) was treated with 1 20 mug of GnRH agonist/d for 84 days (3 occasions at 28-day intervals) . During a pretreatment period (April 19 to May 29) and for 90 days af ter initiation of GnRH agonist treatment, follicular development and o vulation were monitored by transrectal ultrasonography of the reproduc tive tract at 2- to 3-day intervals. On each occasion a blood sample w as collected for determination of luteinizing hormone (LH) and progest erone. Estrous behavior was monitored by teasing of mares with a stall ion. Initiation of agonist treatment was random, relative to the stage of the estrous cycle, and all mares ovulated within 11 days before or after implantation. In 3 of 4 nontreated control mares, estrous cycle s were observed throughout the study, with interovulatory intervals ra nging from 18 to 26 days. In the remaining mare, concentration of prog esterone was high after asynchronous double ovulation during the pretr eatment period, suggestive of persistent corpus luteum. In group-2 mar es, ovulation occurred in all mares 7 days before and 2 days after ini tiation of treatment; however, the next anticipated ovulation was dela yed in 3 of 4 mares (interovulatory interval, 33 to 70 days). Estrous cycles were not disrupted in the remaining mare. At higher doses (grou ps 3-5), 1 mare each from groups 3 and 5 ovulated between days 0 and 2 of treatment initiation, but failed to ovulate during the remainder o f the study (anovulatory for > 88 days). Similarly, an additional 2 ma res of groups 2 and 3 ovulated within 2 days of GnRH agonist treatment . A second ovulation occurred in these mares 32 to 35 days later; ther eafter, both mares were anovulatory for the remainder of the study. in the remaining 8 mares, interovulatory intervals were either lengthene d (n = 6 mares, range, 32 to 82 days) or were unaffected (n = 2) by tr eatment. One group-6 mare had a lengthened interovulatory interval, 1 was anovulatory for > 90 days, and the remaining 2 mares were unaffect ed by treatment. During the 28-day treatment period, serum concentrati on of LH decreased (P < 0.05) only in mares of groups 3-5. In group-6 mares, concentration of LH was unchanged during each 28-day period aft er depot GnRH agonist administration. Thus, constant administration of a GnRH agonist to mares during the breeding season disrupted their es trous cycles. Anovulation or lengthening of the interovulatory interva l by GnRH agonist treatment was associated with persistence of a corpu s luteum or an extended follicular phase.