Valerie De Clerck, Doctor of Veterinary Medicine

You would love to breed a foal from your mare this year, but are unable to get her pregnant? Mare infertility is a frustrating and complex problem for both the owner and the veterinarian, and probably also for the mare herself... The cause of infertility can vary, we list the main causes and solutions for you.

Dark brown horse grazing in the pasture with other horses

The most common reasons why YOUR mare may not become pregnant

1. The egg is not fertilised

The cause here may lie with the mare or with the stallion. The quality of the egg might have been poor, failure of ovulation or bad timing of insemination… Mares also have a seasonal cycle and are optimally fertile between May and September, the chance of successful ovulation is greatest during this period. The older the mare gets, the lower the quality of her eggs…

Another possibility is that the stallion's sperm is of insufficient quality to fertilize the egg. The quality of fresh semen is usually better than refrigerated or frozen semen. Furthermore, the quality varies depending on the stallion.

2. The fertilised egg fails to implant in the uterus.

For the implantation of the fertilised egg, the health of the mare's endometrium or uterine lining is very important. The endometrium produces the "uterine milk" that serves as nutrition for the fertilized egg (1). 

Diseases of the endometrium are a major cause of infertility in mares and the most common are 

1.  fibrosis (endometrosis), 
2.  damaged blood vessels, 
3.  inflammation (endometritis); and 
4.  deterioration of the endometrial glands (2).

The endometrium may also be affected by poor anatomical conformation. For example, Thoroughbred mares have a high incidence of aspiration of air due to poor conformation of the vagina and vulva. Saddlebred mares are more likely to suffer from fungal infections due to cervical incompetence (4). 

of clinically healthy mares haS some degree of fibrosis in the uterus ("endometrosis"), which reduces the chances of implantation and successful pregnancy. The severity of fibrosis increases with age (3).

3. The mare loses the embryo or fetus after implantation


When this occurs within the first two months, it is called early embryonic death. Early Embryonic Death occurs in 4 to 20% of the mares.


At a later time (> 40 days gestation) we speak of abortion, this occurs in 7 to 8% of the fruits. There are several known causes of abortion, which are usually divided into infectious and non-infectious causes. The most common infectious cause of abortion is Rhinopneumonia caused by the Equine herpesvirus (EHV-1 & 4). It is recommended to properly vaccinate your mare before and during pregnancy to protect her from this virus.


If your mare fails to get pregnant, the veterinarian will find out the cause of the problem and try to solve it. Important factors to consider when examining the infertile mare are listed below (4).

1. History of the mare

This includes the breeding and medical history of the mare.

2. Clinical examination

The mare's general condition, body weight, the presence of other conditions such as laminitis, pain, lameness or any other disease that might influence the mare's fertility are checked. For example, overweight mares are less fertile, the same applies for laminitis, pain and lameness. These mares often have difficulties clearing the uterus from debris after insemination due to lack of movement. 

The veterinarian will also have a close look at the reproductive system: Does the mare have a regular cycle? Is the conformation of the vulva and vagina normal? Does the cervix look normal?

3. Rectal ultrasound

The veterinarian will perform a rectal ultrasound to check the reproductive organs internally. Are there any cysts present on the ovaries? Is the endometrium in good health? Is there any fluid present in the uterus?

4. Management of the barn

How are the mares housed, nutrition, temperature, daylight, ...?

5. Additional examinations (if needed)

When an inflammation of the uterus is suspected a bacteriological examination is carried out. Sometimes the veterinarian will take an endometrial biopsy to check for microscopic abnormalities, such as fibrosis. In some cases, a camera is used to have a look inside the uterus (endoscopy).


1. Treat underlying problems affecting your mare's general health

If there is an underlying medical problem, this must be first addressed in consultation with the veterinarian. For example, an infection in the uterus will usually be treated with antibiotics. In case of an anatomical abnormality, an attempt will be made to correct it. In the case of an underlying disease such as laminitis, Equine Metabolic Syndrome or arthritis, the condition and pain need to be treated first.

2. Create a calming environment and provide the mare with sufficient exercise

Once the underlying causes have been ruled out or resolved, you can increase the chance of pregnancy by good (stable) management. Attention should be paid to nutrition, housing and exercise of the mare. The mare must not be too thin and certainly not too fat. The mare must not be exposed to too many stressful factors and she must have sufficient exercise. Preferably the mare also has contact with other horses.

3. Adapt the diet of your mare before, during and after pregnancy and support her with the right supplements

As in women, the nutrition of the pregnant mares is of great importance. The pregnant mare should have access to pasture for grazing. Grass contains important nutrients for the mare and the growing fetus. Roughage alone does not contain enough minerals, vitamin E and proteins. These proteins are essential for the growth of cells and body tissues, so the growing foal needs a lot of them! Especially in the last months of gestation and during lactation, sufficient protein is important.

This extra nutritional need does not mean that you have to start "fattening up" your mare as soon as your vet finds out that she is pregnant... If the mare is in good condition before gestation, then her normal diet is sufficient to cover her nutritional requirements during the first 8 months of pregnancy. 

It is only during the last three months that the nutritional needs increase due to a growth spurt of the foal. 

There are several good concentrates on the market which are adapted to the increased energy needs of the mare. It is also recommended to supplement the mare with vegetable oils, these are way healthier than a larger amount of sugar and provide more energy.

Brown mare with brown foal

Also, the following natural vitamins and ingredients can have a positive influence on the fertility of the mare:

•  NATURAL VITAMIN E: a deficiency of vitamin E harms fertility in the mare and even leads to deformity of the foal (muscle disease, white muscles) or abortion. It is also recommended to continue natural vitamin E supplementation during lactation. Vitamin E increases the antibodies in the milk, which gives an extra boost to the foal's immunity (5,6).

•  BROCCOLI is very rich in Indol-3-Carbinol and has a positive influence on hormone balance and endometrial health (7,8).

•  FOLIC ACID, VITAMIN B12, CHOLINE AND BETAINE not only have a positive influence on the fertility of the mare but also the development of the foal. This also applies to cysteine, a building block of the powerful antioxidant glutathion (9–12).

•  FLAXSEED balances hormones and has a positive influence on fertility in both mares and stallions (13).

...and now fingers crossed that nature gives you a little miracle this year.

Improve the overall health of your mare's endometrium and chance of successful pregnancy.


Extra energy and rich in the best plant-based omega-3 fatty acids (DHA from microalgae).



1. Thompson RE, Premanandan C, Pukazhenthi BS, Whitlock BK. A review of in vivo and in vitro studies of the mare endometrium. Anim Reprod Sci. 2020;222(February):106605. doi:10.1016/j.anireprosci.2020.106605

2. Schöniger S, Schoon H-A. The healthy and diseased equine endometrium: molecular analyses. Animals. 2020;10:625-648.

3. Lehmann J, Ellenberger C, Hoffmann C, et al. Morpho-functional studies regarding the fertility prognosis of mares suffering from equine endometrosis. Theriogenology. 2011;76(7):1326-1336. doi:10.1016/j.theriogenology.2011.06.001

4. LeBlanc MM. When to refer an infertile mare to a theriogenologist. Theriogenology. 2008;70(3):421-429. doi:10.1016/j.theriogenology.2008.04.021

5. Peugnet P, Robles M, Wimel L, Tarrade A, Chavatte-Palmer P. Management of the pregnant mare and long-term consequences on the offspring. Theriogenology. 2016;86(1):99-109. doi:10.1016/j.theriogenology.2016.01.028

6. Bondo T, Jensen SK. Administration of RRR-α-tocopherol to pregnant mares stimulates maternal IgG and IgM production in colostrum and enhances vitamin E and IgM status in foals. J Anim Physiol Anim Nutr (Berl). 2011;95(2):214-222. doi:10.1111/j.1439-0396.2010.01043.x

7. Fowke JH, Longcope C, Hebert JR. Brassica Vegetable Consumption Shifts Estrogen Metabolism in Healthy Postmenopausal Women. Vol 9.; 2000.

8. Valente Pereira FM, Rosa E, Fahey JW, Stephenson KK, Carvalho R, Aires A. Influence of temperature and ontogeny on the levels of glucosinolates in broccoli (Brassica oleracea var. italica) sprouts and their effect on the induction of mammalian phase 2 enzymes. J Agric Food Chem. 2002;50(21):6239-6244. doi:10.1021/jf020309x

9. Waterland RA, Travisano M, Tahiliani KG, Rached MT, Mirza S. Methyl donor supplementation prevents transgenerational amplification of obesity. Int J Obes. 2008;32(9):1373-1379. doi:10.1038/ijo.2008.100

10. Swegen A, Grupen CG, Gibb Z, et al. From Peptide Masses to Pregnancy Maintenance: A Comprehensive Proteomic Analysis of The Early Equine Embryo Secretome, Blastocoel Fluid, and Capsule. Proteomics. 2017;17(17-18):1-13. doi:10.1002/pmic.201600433

11. Ebisch IMW, Peters WHM, Thomas CMG, Wetzels AMM, Peer PGM, Steegers-Theunissen RPM. Homocysteine, glutathione and related thiols affect fertility parameters in the (sub)fertile couple. Hum Reprod. 2006;21(7):1725-1733. doi:10.1093/humrep/del081

12. Sinclair KD, Allegrucci C, Singh R, et al. DNA methylation, insulin resistance, and blood pressure in offspring determined by maternal periconceptional B vitamin and methionine status. Proc Natl Acad Sci U S A. 2007;104(49):19351-19356. doi:10.1073/pnas.0707258104

13. Samavat H, Kurzer M. Estrogen Metabolism and Breast Cancer Hamed. Cancer Lett. 2015;28(356):231-243. doi:10.1016/j.canlet.2014.04.018.Estrogen

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