Follicle suppression of circulating follicle-stimulating hormone and luteinizing hormone before versus after emergence of the ovulatory wave in mares

The effect of the ovarian follicles on plasma concentrations of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) before versus after the expected emergence of the ovulatory follicular wave was studied on Days 0 to 18 (Day 0 = ovulation) in four groups of mares (n = 6/group). In addition to a control group, all follicles ≥6 mm in diameter were ablated on Days 0.5, 6.5, or 12.5 in a herd of mares with reported emergence at 6 mm of the future ovulatory follicle on mean Day 10.5. Concentrations of FSH were not different between the Day-0.5 or Day-6.5 ablation groups and the corresponding controls. However, ablation on Day 12.5 resulted in an immediate FSH increase (group-by-day interaction, P < 0.003). For LH, ablation on Day 0.5 resulted in an interaction (P < 0.02), partially from lower (P < 0.05) concentrations on each of Days 15.5 to 18.0 than that in the controls, whereas ablation on Days 6.5 or 12.5 did not result in a significant group effect or interaction. Testosterone concentration, but not progesterone or estradiol concentration, was lower (P < 0.04) on Day 2 in the Day-0.5 ablation group than that in the controls. We inferred that follicles did not contain adequate FSH suppressors on Days 0.5 and 6.5 and that they were present only in the Day-12.5 ablation group or after the expected emergence of the ovulatory wave. The hypothesis of an association between low postovulatory concentrations of an ovarian steroid and low concentrations of LH after Day 15 was supported.