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

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.

Physical Exam

As part of a BSE, mares should always have a general physical exam before breeding to evaluate their overall health. This exam will assess her body condition as it can affect her estrous cycle and ability to conceive.

Another key component of the general exam is to assess orthopedic soundness. Lameness can affect the body condition of the mare and her ability to manage pregnancy.

External Reproductive Exam

As part of a BSE, an external exam of the mare’s anus, perineum, vulva, clitoris, and mammary glands is performed to identify potential problems before breeding.

Mares with a large vulvar opening may require a Caslick’s operation to reduce its size. [14][15] If the normal vulval seal is compromised, the risk of infectious agents entering the uterus and causing infection, fertility problems, and abortion increases. [14]

Vaccinations

Mares being bred should be up to date on their vaccinations to help prevent them from contracting infectious diseases that could potentially cause abortions. [16][17]

Vaccinations are commonly administered to protect against Eastern and Western encephalomyelitis, West Nile virus, influenza, and tetanus.

Deworming

Before breeding, mares should be on a regular deworming schedule to optimize their health. Consult with your veterinarian to determine an optimal deworming strategy for your mare.

Breeding Your Mare

Breeding Strategies

Methods of breeding mares include live cover and artificial insemination using fresh, cooled, or frozen semen.

Fertility rates in mares artificially inseminated depend on the quality of semen and if any processing methods are used to preserve it. [32] The success rates of live cover breeding depend on the fertility of both the mare and stallion.

Determining Pregnancy

Once your mare has been bred, your veterinarian can confirm pregnancy using ultrasound 14 to 18 days after ovulation. [18]

A fertilized egg (zygote) moves through the fallopian tubes and into the uterus on or near day six. [19] On approximately day 12 or 13, a multi-cellular embryo develops from the unicellular zygote. At this stage, it is usually large enough to be detected by ultrasound. [20]

The embryo will adhere to the uterus wall on or about day 17. [19] By day 26, a transrectal ultrasound can be used to confirm a heartbeat. [19]

If the mare did not become pregnant after breeding, your veterinarian can identify and address potential reasons for the outcome before trying to breed her again.

Gestation Period in Mares

Equine gestation refers to the period from conception to birth. Various environmental and nutrition factors can influence the length of a mare’s pregnancy. [21][22][23]

The average duration of pregnancy in mares typically ranges between 338 and 343 days. However, gestation can be a shorter or longer duration.

Multiple research studies have investigated the gestation lengths in mares. A study of gestation lengths of Standardbred mares at two commercial farms over a two-year period determined the average gestation time of 349 days for 594 foals. [24] This study also noted an average gestation length of 347 days for fillies and 350 days for colts. [24]

Mares foaling during months of the year with longer daylight hours usually have a shorter gestation length than those foaling in other months. Exposing bred mares to artificial light in late gestation has been shown to shorten gestation length. [21][22][23]

Caring for Your Mare During Pregnancy

Good broodmare management is essential for a healthy pregnancy.

Avoid Exposure to Stress

The effects of stress on pregnant mares are not fully understood. However, it’s best to avoid exposing your mare to stressful situations. Management strategies that aim to reduce stress have been found to correlate with reduced pregnancy loss and improved reproductive function. [33]

Stress can be caused by many factors, including social grouping, trailering, a lack of access to forage or water, environmental conditions, boredom, or negative reinforcement training.

Avoid Long-Distance Transport

Mares in late pregnancy should be transported at least 30 days before the foal is due to allow her to adapt to the new environment.

Research shows that transport induces the release of cortisol, a stress hormone. [25] One study concluded that transport-induced cortisol release in mares may advance the onset of foaling in mares in the late stage of pregnancy. [25]

Reduce the Risk of Disease and Injury

Be cautious when exposing your mare to other horses to avoid the risk of injury or disease transmission. Broodmares should be housed separately from transient horse populations.

Ensure your mare is at her foaling site four to six weeks before her due date. Mares need time to settle in a new environment and to develop immunity to local organisms.

Vaccinations and Deworming

Pregnant mares should be current on their vaccines and deworming. Consult your veterinarian for recommendations regarding specific vaccinations and deworming protocols during pregnancy.

Vaccinations:

Mares may receive a booster vaccine (for vaccines that require annual boosters) four to six weeks before foaling to increase the level of antibodies in their first milk (colostrum) to help protect their newborn foal from disease. [17]

Your veterinarian may recommend that your mare is vaccinated for equine rhinopneumonitis (commonly called rhino or virus abortion) during the gestation period (at five, seven, and nine months). [17] Other vaccines for rabies, rotavirus, and botulism may also be recommended.

Deworming:

Follow your veterinarian’s recommendations regarding a deworming protocol for your pregnancy mare.

Although deworming (anthelmintic) medications, including Praziquantel and Ivermectin, have been deemed safe to use in pregnant mares, your vet may recommend the administration of certain deworming medications at specific times. [26]

Deworming medications should not be administered during the first two months of pregnancy and during the last few weeks before foaling. [20]

It is important to deworm mares within several weeks of foaling as the mare will be the main source of parasites that could infect the foal. [16]

Medications

Do not administer hormones or other drugs to a pregnant mare without consulting your veterinarian.

Exercise

During the first seven months of pregnancy, your mare can engage in moderate exercise. In the last four months of the mare’s pregnancy, exercise should be light to moderate. Vigorous exercise should be avoided.

Optimal Nutrition for Pregnant Mares

Proper nutrition is important in preparation for breeding mares and becomes even more critical during gestation.

Mares typically gain 12 to 16% of their initial body weight during pregnancy to support the fetus and the placental tissues. [27]

Feeding your pregnant mare a carefully balanced diet will support the well-being of the dam and her foal during gestation, birth, nursing, and the first years of life.

Detailed information on formulating a diet plan for your broodmare can be found in this guide: How to Feed a Pregnant Mare

Early Pregnancy (< 5 months)

During early pregnancy, broodmares with a good body condition score should not be overfed. According to the National Research Council’s Nutrient Requirements of Horses, mares do not require increased caloric intake until 5 months of pregnancy. [34]

The ration provided in early pregnancy should primarily consist of high-quality forage provided at approximately the same amount as before pregnancy.

Monitoring your mare’s body condition using a weight tape or scale is ideal to track changes in weight and assess whether dietary adjustments are needed.

Mid and Late Pregnancy (> 5 months)

Nutrient requirements for broodmares do not increase significantly until the last 3 months of gestation, when 75% of fetal growth occurs.

Nutritional requirements for energy and protein increase in the fifth month of gestation. From the eighth month of gestation until the time of foaling, the requirements for many macro- and trace minerals increase.

Developing foals grow quickly in the last few months of pregnancy. The mare’s energy needs will increase to support this growth.

High-quality forage should still comprise most of the expectant mare’s diet throughout pregnancy. However, concentrated feeds may also be added to her diet to increase her energy intake in late gestation (8-11 months).

Consult an equine nutritionist to ensure your mare’s nutrition is optimized before breeding and throughout pregnancy.

Vitamins and Minerals

During pregnancy, your mare needs a supplemental source of vitamins and minerals to avoid common nutrient deficiencies such as selenium, vitamin E, copper, and calcium.

Feed a balanced vitamin and mineral supplement, such as Mad Barn’s Omneity, to ensure your mare meets all her vitamin and mineral requirements.

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Omega-3 Fatty Acids

Supplementing pregnant mares with docosahexaenoic acid (DHA), a marine-derived omega-3 fatty acid, can have beneficial effects for both the dam and her foal.

Mares fed DHA during the last three months of pregnancy had faster recovery of the uterus to its pre-pregnancy condition (uterine involution) after giving birth, and their foals stood and nursed more quickly. [28]

Supplementing DHA during gestation also benefits cognitive development in the foal. [28]

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Digestive Support

Consider providing your mare with digestive support during pregnancy to optimize nutrient absorption. Mad Barn’s Optimum Digestive Health contains prebiotics, probiotics, and yeast which can help maintain hindgut function through pregnancy.

Specifically, yeast supplementation to pregnant mares has been shown to increase the antibody content of mare colostrum to provide greater immune support to the foal. [29]

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Summary

Have your veterinarian evaluate your mare’s health before breeding. Ensuring your broodmare’s overall health before breeding and pregnancy can improve her ability to produce a healthy foal.

Consult with an equine nutritionist to formulate a balanced feeding program that meets all her nutrition requirements before breeding and throughout pregnancy.

Is Your Horse's Diet Missing Anything?

Identify gaps in your horse's nutrition program to optimize their well-being.

References

  1. Masko, M. et al. Breeding management of mares in late reproductive age considering improvement of welfare. A review. Journal of Animal and Feed Sciences. 2018.
  2. McCue, PM. Breeding the older mare. Journal of Equine Veterinary Science. 1991.
  3. Dawson, FL. Recent advances in equine reproduction. Equine Vet J. 1977. View Summary
  4. Anatomy, Physiology and Reproduction in the Mare. OMAFRA. Accessed 01/19/23.
  5. Sertich, P. Management of Reproduction in Horses. Merck Veterinary Manual. 2022.
  6. Satué, K. and Gardon, J.C. “Infection and Infertility in Mares”. Genital Infections and Infertility, edited by Atef Darwish. IntechOpen. 2016.
  7. Sessions-Bresnahan, DR and Carnevale, EM. The effect of equine metabolic syndrome on the ovarian follicular environment. J Anim Sci. 2014. View Summary
  8. Grev JE. Et al. Infant of a Diabetic Mother. S D Med. 2020.
  9. Dobbs, T.N. et al. Glucose and insulin dynamics in mares and their foals. Australasian Equine Science Symposium. Gold Coast, QLD, Australia. 2012.
  10. Ousey, J.C. et al. The effects of maternal health and body condition on the endocrine responses of neonatal foals. Eq Vet J. 2008. View Summary
  11. Robles, M. et al. Maternal nutrition during pregnancy affects testicular and bone development, glucose metabolism and response to overnutrition in weaned horses up to two years. PloS One. 2017.View Summary
  12. Henneke, D. et al. Body condition during pregnancy and lactation and reproductive efficiency of mares. Theriogenology. 1984.
  13. Pinto, C. Reproductive Evaluation of the Mare. Manual of Clinical Procedures in the Horse, First Edition. 2017.
  14. Burger, D. et al. Managing a mare for breeding and sport. Pferdeheilkunde. 2008.
  15. van Ittersum, AR and van Buiten, A. Het voorkomen van pneumovagina en het effect van de Caslickoperatie op de fertiliteit: een retrospectief onderzoek [The prevention of pneumovagina and the effect of the Caslick operation on fertility: a retrospective study]. Tijdschr Diergeneeskd. 1999. View Summary
  16. Espy, B. Expectant Mare: Assuring the Health and Well-Being of the Pregnant Mare. AAEP. 2022.
  17. Sertich, P. Vaccination in Breeding Horses. Merck Veterinary Manual. 2021.
  18. Sertich, P. Pregnancy Determination in Horses. Merck Veterinary Manual. 2021.
  19. Espy, B. Equine Reproduction from Conception to Birth. AAEP. 2016.
  20. Espy, B. Ultrasound and Your Horse. AAEP. 2016.
  21. Nolan, MB. Et al. Artificially extended photoperiod administered to pre-partum mares via blue light to a single eye: Observations on gestation length, foal birth weight and foal hair coat at birth. Theriogenology. 2017.
  22. Moraes, S.S. et al. Climatic factors affecting gestational length in mares under subtropical climate. Journal of Equine Veterinary Science. 2021. View Summary
  23. Ferris, R.A. and McCue, P.M.. Gestation Length, Duration of Labor and Foal Survival. Journal of Equine Veterinary Science. 2011.
  24. Dicken, M. et al. Gestation length and occurrence of daytime foaling of Standardbred mares on two stud farms in New Zealand. New Zealand Veterinary Journal. 2012. View Summary
  25. Nagel, C. Road Transport of Late-Pregnant Mares Advances the Onset of Foaling. Journal of Equine Veterinary Science. 2020.View Summary
  26. Mercier P. et al. Evaluation of the safety of ivermectin-praziquantel administered orally to pregnant mares. Am J Vet Res. 2003. View Summary
  27. Lawrence, L.M. Feeding stallions and broodmares. Equine Applied and Clinical Nutrition: Health, Welfare, and Performance. 2013.
  28. Adkin, A. Supplementation of broodmares with docosohexaenoic acid and its effects on reproductive performance and foal cognitive development. Univ FL Thesis. 2013.
  29. Ayad, M.A. et al. Effect of supplementing arabian and barbe pregnant mares with Saccharomyces Cerevisiae on Colostrum IgG1 Concentration in Algerian Breed. J Appl Environ Biol Sci. 2017.
  30. Morley, S.A. and Murray, J-A. Effects of Body Condition Score on the Reproductive Physiology of the Broodmare: A Review. J Equine Vet Sci. 2014.
  31. Martin-Gimenez, T. et al. Endocrine, morphometric, and ultrasonographic characterization of neck adiposity in Andalusian horses. Domest Anim Endocrinol. 2016. View Summary
  32. Samper, J.C. et al. Relationship between the fertility of fresh and frozen stallion semen and semen quality. Journal of Reproduction and fertility. Supplement. 1991. View Summary
  33. Malschitzky, E. et al. Management Strategies Aiming to Improve Horse Welfare Reduce Embryonic Death Rates in Mares. Reprod Domest Anim. 2015. View Summary
  34. National Research Council Nutrient Requirements of Horses: Sixth Revised Edition. 2007.