Breeding a mare at the right time in her cycle is imperative to ensure adequate pregnancy rates. Most pregnancy failures are due to inappropriate breeding management resulting in not only disappointment but also increased breeding costs, such as longer periods of boarding time at a breeding facility, veterinary services or repeated shipments of semen for artificial insemination. In order to breed successfully, it is important to understand the mare's reproductive system.

The period of time between two successful heats is termed the estrus cycle, which lasts for about 21 days: approximately one week in heat or estrus, when the mare is receptive to a stallion, and two weeks not in heat, termed diestrus. This cyclicity between estrus and diestrus is the result of a complex interaction between hormones secreted by the brain, ovaries and endometrium (lining of the uterus). Understanding this is fundamental, not only to breed mares at the right time of their cycle and hence optimize pregnancy rates, but also to allow manipulation of the cycle with administration of hormones in order to synchronize breeding timing.

Additionally, mares are considered a seasonal species because estrus cycles occur only at a particular time of the year, which we call the breeding season. Increased day length is the main cue for estrus cyclicity in mares, and therefore most mares cycle during the late spring and summer. This seasonal limitation puts more pressure into appropriate breeding management for optimal pregnancy rates.

Veterinarians and breeding managers must work as a team to manage broodmares and stallions, and to detect possible problems that may jeopardize pregnancy rates in a horse breeding program.

Basic Anatomy of The Reproductive System

The reproductive tract of the mare is located within the pelvic area and therefore is easily evaluated through the rectal wall by palpation or ultrasound, a tool commonly used in breeding management of mares.
        
The reproductive tract is composed of the following parts: vulva, vestibule, vestibulo-vaginal sphincter (tissue separating the vestibule and vagina), vagina, cervix, body of the uterus, uterine horns, oviducts (or Fallopian tubes) and ovaries.
        
The vulva, vestibulo-vaginal sphincter and cervix are important barriers for preventing contamination or infection of the uterus. The vagina and the uterus are sterile in the normal, reproductively sound mare. This is an important consideration, and whenever bridging the reproductive tract of the mare for an examination or for insemination, the vulva should be thoroughly scrubbed to avoid introducing an infection into the vagina or uterus. Furthermore, vaginal exams should always be avoided at any time during pregnancy.

The uterus of the mare is Y-shaped with a body and two uterine horns. During pregnancy, the embryo develops within one of the uterine horns. The entire uterus enlarges to fit the foal as pregnancy advances.

Mare ovaries have a typical kidney bean shape. Eggs develop within blister-like structures called follicles. These follicles can become very large before ovulation and bulge over the surface thus increasing the overall size of the ovary, making them easily felt (palpated) or seen on ultrasound exam through a rectal exam performed by a trained veterinarian.

During the breeding season, a mare ovulates one follicle (occasionally two) at each heat period. If the mare is bred during that heat, fertilization may occur within the oviduct and the mare may become pregnant. The oviducts or Fallopian tubes (one for each ovary) are responsible for transporting the fertilized egg into the uterine horn for further development into a fetus during pregnancy.