Breeding Basics: The Mare's Reproductive Cycle

Horse breeding is a business full of rewards and disappointments. Breeders eagerly await the birth of a healthy foal, but they also understand the risk of abortion or of a mare not conceiving.

To better help owners/breeders and veterinarians work together to achieve a successful breeding outcome, Christine Aurich, DVM, PhD, Dipl. ECAR, professor at the Centre for Artificial Insemination and Embryo Transfer and Graf Lehndorff Institute for Equine Science at the University of Veterinary Sciences in Vienna, Austria, recently composed a comprehensive report about mares' reproductive cycles--information first-time and experienced breeders alike can use in their programs.

The mare's reproductive cycle comprises the recurring physiologic changes prompted by reproductive hormones. When the mare is "in heat" she's in estrus, which means she's sexually receptive and easier to impregnate. She is in anestrus when the sexual cycle is not active; the mare is thus difficult or impossible to impregnate. The latter is induced by season (mares usually are in anestrus through the winter and early spring), pregnancy, and age (very young and older mares will likely remain in a period of anestrus), among other factors.

Once a mare is in estrus, the breeder should solicit the help of a reproduction veterinarian to monitor the mare closely for ovulation. Aurich explained that the foundation of a good reproductive program starts with a "veterinarian that has good knowledge of equine reproductive physiology and is experienced in managing mares during estrus and early pregnancy to achieve optimal results.

"In contrast to other domestic animal species, the reproductive stage of an individual mare can be easily determined and is an important prerequisite for successful manipulation of the mare's estrous cycle," Aurich continued, "but it requires repeated gynecological examination of the mare including transrectal ultrasonography for detecting the optimal time for mating."

Many veterinarians suggest the mare be bred 24-48 hours before ovulation so the sperm can penetrate the follicle before it ruptures. After breeding and ovulation mares might be checked for pregnancy initially around Day 16 or 17. The veterinarian should conduct pregnancy screens throughout the pregnancy to ensure continued fetal development, suggested Aurich.

She said the most successful reproductive program combines an insightful understanding of the mare's reproductive physiology together with these in-depth and frequent veterinary examinations of the individual mare.

The review, "Reproductive cycles of horses," appeared in the February 2011 issue of Animal Reproduction Science. The abstract is available on PubMed.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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