Continuous administration of low-dose GnRH in mares II. Pituitary and ovarian responses to uninterrupted treatment beginning near the autumnal equinox and continuing throughout the anovulatory season.

The research study tested the effect of continuous low-dose GnRH treatment on mares during the seasonal anovulation period. Despite the trial, the treatment failed to prevent the occurrence or hasten the transition from the seasonal anovulation in mares.

Research Methodology

• The research involved 20 Quarter Horse mares which were divided based on age and body condition score into two groups – a control group which received saline (9 mares) and a treatment group which received Gonadotropin-releasing hormone (GnRH) (11 mares).
• The delivery of GnRH was done continuously via osmotic minipumps, with sham pumps placed in the control mares. The delivery rate was initially set at 2.5 microg/h for the first 60 days, increasing to 5.0 microg/h thereafter.
• The onset of the study was in late September or early October, prior to the horses’ natural anovulatory season, and continued until the following March.

Findings

• By November, 80% of mares had become anovulatory, and by December, 100% were anovulatory.
• Pituitary response, as indicated by serum concentrations of LH (luteinizing hormone) and total releasable pools of LH in March, showed no significant difference between the two groups.
• Mean concentrations of LH during the follicular phase and metestrus were higher in control mares that exhibited ovulatory cycles after the study began, as compared to GnRH treated mares. However, there was no significant change in the size of ovulatory follicles or interovulatory intervals between groups.
• Serum concentrations of FSH (follicle-stimulating hormone) were not affected by the treatment, but lowest from November through January.

Conclusion

• The continuous infusion of low-dose GnRH, beginning soon after the autumnal equinox and continuing until after the vernal equinox, did not prevent or hasten the transition from seasonal anovulation in mares.

Therefore, the hypothesis that continuous subcutaneous treatment with low-dose GnRH would prevent the development of seasonal anovulation was not supported by the results of this study.