The Harry M. Zweig Memorial Fund for Equine Research

Controlled Postponement of Ovulation in Mares

Principal Investigator: Robert Gilbert
Contact Information: Email:; Phone: 607-253-3435
Project Costs: $99,400
Project Period: 01/01/15-12/31/16

Dr. Robert Gilbert

The major goal of this research is to identify an effective means of delaying ovulation in mares in a controlled
way, without sacrificing fertility. Fertility in mares depends on well-timed insemination relative to ovulation.
This is not always easy to achieve. High demand Thoroughbred stallions with a full book and limited,
inflexible availability put pressure on veterinarians for achieving the best chance of pregnancy. Similarly,
semen for insemination is not always available when needed for reasons of pressure on the stallions, or
complications of shipping semen – for examples holidays and long weekends. Within limits, it is possible to
advance the time of ovulation, but no tool is available to predictably delay ovulation while maintaining fertility.
The fact that such a strategy is necessary is demonstrated by the observation that veterinarians resort to untested
strategies for delaying ovulation. We became aware of the use of progestagens to delay ovulation and when we
tested that approach found it both to be ineffective and to reduce fertility. More recently became aware of
veterinarians using flunixin (a nonsteroidal anti-inflammatory drug) for this purpose. Though the approach is
physiologically feasible, it has not been tested, and effects on fertility are unknown. In the first year of this
project we will test the hypothesis that strategic administration of flunixin before ovulation can postpone
ovulation without adverse effects on fertility. However, we are concerned that simply blocking rupture of the
follicle without affecting oocyte maturation may decrease fertility.

We therefore plan to test a novel approach during the second year of the project. Production of estradiol by the preovulatory follicle is important for exhibition of signs of estrus, maturation of the oocyte and initiation of the surge in luteinizing hormone (LH) that initiates ovulation. We postulate that temporary inhibition of estradiol production will place this development “on hold.” Once the inhibition of estradiol is stopped, the process can continue, with the delayed LH surge resulting in delayed ovulation, but in synchrony with oocyte development. We are optimistic that this approach will be more successful than previous attempts. Our second hypothesis is therefore that strategic treatment with an inhibitor of estradiol production (letrozole, an aromatase inhibitor) will postpone ovulation in a predictable way, without impairing fertility.

We anticipate testing one major hypothesis described above in each or the two years of this project. Successful completion of this project will provide veterinarians with a new and valuable tool in breeding management of mares. If either approach fails, or results in impaired fertility, the information gained will at
least deter veterinarians from use of ineffective or detrimental protocols. Unexpected results, such as recruitment of additional dominant follicles, would add to our understanding of reproductive physiology and open new avenues of investigation.