top of page

Breeding the Mare: Factors That Can Influence Conception Rates


Adapted from the My Horse University Horse Breeding Short Course

A 65% conception (pregnancy) rate is considered average for a horse breeding farm. That means that 35% of all the mares bred are left open until the next breeding season. If your mare is one of the 35% open mares at the end of the breeding season, you may be asking the question “Why won’t my mare get pregnant?” This article discusses some major factors including fertility, breeding practices and nutrition that influence conception rates on the horse farm.

Stallion and Mare Fertility Both the mare and stallion should be evaluated for any potential fertility problems well before the breeding season begins. For stallions, semen can be collected and evaluated. Sperm count, motility, and viability are assessed as indicators of the stallion’s fertility. The stallion’s testes should also be physically evaluated. In addition, the stallion’s libido (responsiveness to mares) should also may be evaluated. It is important that a breeding stallion be physically sound in the hindquarters to mount mares or an artificial breeding dummy.

Stallion mounting a jump dummy so his semen can be collected

Figure 1. This stallion is mounting a phantom or "dummy" mare. In this process, sperm can be collected and tested for sperm count, motility, and viability.

For a mare, rectal palpation provides information regarding the condition of the reproductive tract. Further testing, including uterine ultrasound, uterine biopsy, and cultures may be recommended. In addition, the conformation of the vulva should be evaluated to determine if the mare will need to have Caslick sutures to prevent uterine infections.

Age is a primary factor in mare fertility, with an expected decline in fertility beginning at 12 years of age. As a mare ages, her uterus is more susceptible to infections due to the relaxation of the broad ligament that support the uterus in the pelvic cavity. In addition, as the older mare loses muscle tone, her vulva may sink, forming a shelf that traps manure and debris, increasing the chance of contaminating her reproductive tract.

A mare with desirable vulvar conformation

Figure 2. The vulva is the mare's first line of defense against infection reaching the uterus.

Mare Seasonality, Heat Detection and Breeding Mares are considered to be seasonal breeders, meaning their reproductive activity will change throughout the year as the seasons change, especially according to the length of daylight. A mare typically shows regular estrous cycles during long days (late spring and summer months). Her ovaries will produce follicles, and she will show behavioral signs of estrus (heat) such as frequent urination, “winking” her vulva, and being receptive to the stallion.

A horse's natural breeding season is from late April until mid September

Figure 3. Seasonal breeding pattern of mares in North America

Timing Breeding To determine the optimal time to breed, teasing is an essential tool. Mares should be teased every other day to detect estrus/heat. When teasing a mare, her behavioral signs should be recorded throughout the estrous cycles. As she begins to display signs of estrus while being teased, her ovaries should be examined.

A veterinarian should palpate the mare’s ovaries and/or use an ultrasound evaluation of them to monitor the growth and maturity of follicles. Monitoring follicular growth and keeping good records on the history of each mare can help indicate at what follicle size the mare will ovulate and the best time she should be bred. Each mare can vary according to the size of the follicle and when she will ovulate. By keeping accurate records of the growth of the follicle as well as past history this can help indicate at what size the follicle will ovulate. Some mares may ovulate when the follicle reaches a 35 mm or some may ovulate when the follicle is much larger such as 65 mm.

Optimal breeding is every other day while she is in estrus and before she ovulates. A mare will normally ovulate about 24 to 48 hours before the end of estrus. If you are shipping semen, you may want to use the hormone hCG (human chorion gonadotropin) to induce ovulation in order to maximize the chances of the mare’s conceiving. Your best chances to catch your mare (catch referring to conceiving) are inseminating her three days before the end of the estrus cycle to six hours after it. Keep in mind that most semen will live for approximately 48 hours. It is preferred to inseminate the mare at least 36 hours before ovulation.

Silent Heat Mares that exhibit no behavioral signs of estrus are called “silent heat” or “covert” mares. Usually, these mares have normal ovarian cyclic activity, but they lack the behavioral signs of being in estrus. In order to determine heat in these mares, palpation, cervical exams, or an ultrasound of the ovaries can all be used to ensure that these mares that are not displaying signs of estrus are in fact cycling.

Foal Heat Many breeders will breed their mares during foal heat. Foal heat occurs seven to nine days after foaling and lasts for approximately three to five days. Often, a mare will not display the typical behavioral signs of estrus such as frequent urination or winking of the vulva.

Some breeders may choose to wait a few months after foaling, especially if a mare had difficulties foaling, before rebreeding so that involution of the uterus can take place. Involution of the uterus is when the uterus returns to normal size post-partum. The mare’s uterus usually undergoes complete involution 3 – 4 weeks after foaling. Other factors that would caution against breeding at foal heat are a retained placenta, which subjects a mare to possible infection tears in the uterus lining, or a low body condition score (under 5).

Figure 4. Foal heat occurs seven to nine days after foaling.

Broodmare Nutrition Requirements Research has shown that thin mares (BC < 5) will have lower conception rates and higher incidents of fetal loss. Therefore, it is critical that a mare doesn’t move below a moderate condition score (BC = 5) during lactation. Mares with a BCS in this range tend to cycle earlier, have fewer cycles before conception, have a higher pregnancy rate, and are more likely to maintain their pregnancies. If a mare has a tendency to get thin during lactation, as is sometimes the case with heavy milkers, it is important that she start out the lactation period with a higher body condition score (between 6 to 7) to prevent her from becoming too thin.

Increasing the fat in mares’ diet during the latter part of gestation and early lactation is a good strategy for increasing energy without overfeeding starch. The manager of broodmares must be able to observe subtle changes in each mare’s condition and adjust diets to prevent her from becoming too fat or thin. The ability to condition score horses is a useful tool for all managers.

Figure 5. This mare in this photo is too thin with a body condition score of 3.

The main points of broodmare nutrition are summarized below:

  • Broodmares should not drop below a body condition score of 5 to optimize reproductive efficiency during breeding and lactation. However, there is no benefit in excessive conditioning. It could possibly lead to other metabolic problems.

  • Vitamins A and E should be fed in sufficient quantities to maximize a mare’s reproductive condition.

  • Pregnant mares have requirements similar to those of maintenance horses until they reach five months of gestation.

  • Broodmares have increasing energy and protein needs from the fifth month of gestation through the early stages of lactation.

  • Calcium, phosphorus, and microminerals copper and zinc should be supplied in adequate quantities and balanced to provide for optimum skeletal development of the foal.

  • The energy demand for broodmares almost doubles with milk production.

  • Fescue hay or pastures contaminated with the fungus endophyte should not be fed to broodmares in their third trimester of pregnancy or while lactating.

Conclusion Besides mare and stallion fertility, other factors including breeding management and nutrition play a major role in conception rates on the horse farm. Breeding success takes more than luck-it takes preparation of both the stallion and mare, careful record keeping and technical skill to ensure the mare has the best chance at conception.


bottom of page