EFFECT OF VARIOUS DOSES OF A GONADOTROPIN-RELEASING-HORMONE ANALOG ONINDUCTION OF OVULATION IN ANESTROUS MARES

One hundred anestrous mares (early February) were injected s.c. with i mplants containing 0,.9, 1.8, 3.6, or 5.4 mg of a GnRH analogue (goser elin acetate) in an attempt to induce ovarian cyclicity. Follicular ac tivity and concentrations of progesterone and LH were determined every 3 d, or daily during estrus. In treated mares that ovulated, the inte rval to the second ovulation of the season was compared to that for an additional group receiving 16 h/d of light beginning December 16 (pos itive controls). Of the mares that did not ovulate in 30 d, eight from each dose group were challenged on d 33 or 34 with an i.v. bolus of 9 50 mug of GnRH. Blood collected at -2, -1, and 0 h before GnRH and at 15, 30, 45, 60, and 90 min and 2, 3, and 4 h after injection was assay ed for serum LH. More mares (P < .05) ovulated when given 3.6- (n = 7) or 5.4-mg (n = 6) implants than when given .9-mg implants (n = 0) or placebo (n = 0). Mares with initial follicles I 15 mm in diameter were less (P < .05) likely to ovulate (10 of 88) than were those with foll icles 16 to 20 mm in diameter (5 of 12). Area under the curve (AUC) fo r LH was greater for mares receiving larger doses of GnRH. The AUC and peak LH were similar between ovulating and nonovulating mares. Lutein izing hormone peaked in all mares on approximately d 12. There was no difference (P > .05) in either peak LH or AUC among treatment groups i n response to the GnRH challenge. Ovulating mares had extended interov ulatory intervals (mean of 71.4 d) compared with positive control mare s (mean of 21.5 d). In summary, ovulation in response to GnRH depended on both GnRH dose administered and prior ovarian status.