Preparing your broodmare for breeding and pregnancy requires careful planning to ensure she is in optimal condition to carry and deliver a healthy foal.

A Breeding Soundness Exam (BSE) provides a health assessment of mares being considered for breeding. Some mares may require advanced reproductive testing to ensure they can conceive and carry a healthy foal to term.

Before breeding, your broodmare should have a healthy body condition score (BCS) of between 5 and 6 on the 9-point Henneke scale. Ensure your horse is up to date on deworming and vaccinations, and have your veterinarian perform a comprehensive exam to identify metabolic conditions.

Work with a nutritionist to formulate a feeding plan to support your broodmare and the developing foal. After the fifth month of gestation, nutrient requirements for energy and protein increase, and by the seventh month, mineral needs also increase.

Reproduction in Mares

Female horses are sexually mature at approximately 16 to 18 months of age and can produce offspring every year. However, their fertility peaks at the age of six to seven years. [1]

Mares can produce healthy foals into their late teenage years and 20s. Fertility begins to decrease between 10 to 15 years of age. [2][30]

Estrous (Heat) Cycles

Daylight activates the production of reproductive hormones from the mare’s brain. [3] The estrous cycle is primarily controlled by two sets of hormones, those from the pituitary gland in the brain, and those from the ovaries.

The reproductive hormones coordinate the estrous cycle: the length of time from one ovulation event to the next. [4] Mares ovulate approximately every 21 days when their estrous cycles are consistent. [5]

Mares have an estrous cycle that consists of two phases: [4]

  • Estrus phase: Lasts for approximately six days (referred to as being in heat)
  • Diestrous phase: Lasts for approximately 15 days (referred to as being out of heat)

The estrus stage of the estrous cycle is an ideal time for breeding to improve the chance of becoming pregnant.

Mares are typically seasonally polyestrous, meaning they have an estrous cycle when daylight hours are longer, as in spring and summer.

Most mares’ estrous cycles stop repeating consistently from September through March. This period is called anestrus. However, some mares can cycle all year.

When to Breed

Knowing when your mare is cycling normally is important to determine the optimal time for breeding. Mares are most likely to conceive within 12 hours of an egg being released from their ovary, during ovulation.

Ovulation can occur at any time during the estrus phase but is most likely to occur 24 to 48 hr before the end of the estrus period. [4]

Your veterinarian will need to monitor your mare’s estrous cycle to determine when she is ovulating and most likely to conceive.

Your veterinarian may perform rectal palpation or use transrectal ultrasound to assess the condition of the mare’s reproductive tract. [4] These methods are used to confirm the presence of an ovarian follicle that is ready to release an egg.

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Factors for Successfully Breeding Mares

A Healthy Reproductive Tract

A properly functioning reproductive tract is essential to optimizing the fertility of broodmares.

The mare’s reproductive tract is positioned horizontally within the abdominal and pelvic cavities. Reproductive organs include the vulva, vagina, cervix, uterus, oviducts, and ovaries.

Breeding problems can result from structural abnormalities, infection, and/or inflammation of the reproductive tract. [6]

Optimal Body Condition

Body condition has a significant impact on equine fertility. [7] Mares that have a body condition score between 5 and 6 on a 9-point scale (the ribs are covered without the presence of excess fat) are more likely to have a regular estrous cycle and to conceive compared to mares that are underweight or overweight. [30]

Managing your mare’s nutrition can help her reach an ideal body condition and should be considered well before breeding. It may several months of following an appropriate feeding plan for your mare to achieve an optimal body condition score.

According to human research, the metabolic status of mothers during pregnancy can influence the metabolic health of their children. [8] Research in horses has demonstrated that the foals delivered from mares fed excess calories have altered metabolic function than those born from mares of lighter to normal weights. [9][10][11]

Ideally, mares should maintain a body condition score of between 5 and 6 throughout their pregnancy. However, late gestation and lactation require higher energy, protein, and mineral levels to support the rapid growth of the foal.

Keeping mares in good body condition is important if planning to rebreed them. [12]

Absence of Disease

To optimize breeding success, mares should be healthy and free of infectious and systemic illnesses.

Mares with metabolic illnesses such as Equine Metabolic Syndrome (EMS) are more likely to have compromised fertility. [7]

EMS is associated with obesity, insulin resistance, increased inflammation, and elevated insulin. Alterations in hormones, including leptin and adiponectin, which are involved in regulating hunger and fat storage, may also be abnormal in horses with EMS. [31]

These alterations can potentially disrupt the normal development of follicles and impair fertility. [7]


Soundness and hoof health must also be considered, as they will impact pregnant mares’ ability to withstand the physical strain of carrying a foal.

Veterinary Care Before Breeding

If you intend to breed your mare, schedule a comprehensive veterinary exam to ensure they are healthy and suitable for breeding and pregnancy. Regular veterinary care can also help prevent potential problems during pregnancy and foaling.

Mares being bred should be up to date on their deworming and vaccinations. They should also be evaluated for metabolic disease or other illnesses that could negatively affect their reproductive health.

Breeding Soundness Exam (BSE)

Veterinarians typically perform a Breeding Soundness Exam on mares to assess their suitability for breeding. This exam focuses on reproductive health and includes assessing pregnancy history, a physical exam, and performing specific tests.

Testing on reproductive organs may include transrectal palpation and ultrasound, vaginal speculum exam, vaginal palpation, cell cultures, and biopsy. [13]

Additional tests such as uterine endoscopy, hormone tests, or genetic tests may be completed based on the results of the BSE.