Effect of various doses of a gonadotropin-releasing hormone analogue on induction of ovulation in anestrous mares.

One hundred anestrous mares (early February) were injected s.c. with implants containing 0, .9, 1.8, 3.6, or 5.4 mg of a GnRH analogue (goserelin acetate) in an attempt to induce ovarian cyclicity. Follicular activity and concentrations of progesterone and LH were determined every 3 d, or daily during estrus. In treated mares that ovulated, the interval to the second ovulation of the season was compared to that for an additional group receiving 16 h/d of light beginning December 16 (positive 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 950 micrograms 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 assayed 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 < or = 15 mm in diameter were less (P < .05) likely to ovulate (10 of 88) than were those with follicles 16 to 20 mm in diameter (5 of 12). Area under the curve (AUC) for LH was greater for mares receiving larger doses of GnRH. The AUC and peak LH were similar between ovulating and nonovulating mares. Luteinizing hormone peaked in all mares on approximately d 12. There was no difference (P > .05) in either peak LH or AUC among treatment groups in response to the GnRH challenge.(ABSTRACT TRUNCATED AT 250 WORDS)