Your vet has just confirmed that your mare is pregnant, and you can’t wait for that healthy foal to arrive!

When should you be changing her feed? How much weight does she need to gain? What additional nutrients does she require to grow a healthy baby?

All these questions are important to consider when planning a nutritional program for your expecting mare.

How you feed a mare while she is pregnant not only affects growth and development in utero but can also have lifelong impacts on the foal. Your mare’s diet affects the foal’s bone and tendon health, neurological development, immune status and more.

If your broodmare’s diet is lacking in key nutrients, she can have a higher risk of complications during gestation and longer recovery after birth.

Carefully designing how youfeed and manage the mare while she is pregnant will give your foal the best start and support the health and longevity of the dam.

Feeding the Pregnant Mare

Planning the feeding program for your broodmare should start even before you begin breeding her.

Sound nutrition helps to support fertility and to build your mare’s nutrient reserves to meet the high demands of late gestation and early lactation.

If a mare’s diet is deficient in a key nutrient, she will tap into these bodily reserves to provide the foal with whatever is lacking.

But significant gaps in the diet cannot be compensated for and can lead to an increased risk of:

  • Loss of pregnancy
  • Retained placenta
  • Loss of skeletal strength in the mare
  • Weakness in the foal
  • Difficulty suckling
  • Bone, tendon and muscle disorders in the foal
  • White muscle disease in the foal
  • and more…

What to Feed Pregnant Mares

Horses do best on a forage-based diet with plenty of fibre to support gut health. Hay alone will typically provide more than enough energy and protein during early gestation, depending on the quality of the forage.

Your mare will also need a supplemental source of vitamins and minerals to avoid common deficiencies in nutrients such as selenium, vitamin E, copper and calcium.

Later in pregnancy, your mare will require additional sources of protein and calories to keep up with the foal’s increasing requirements.

It is strongly recommended to submit a hay sample for analysis to determine whether free-choice access to forage is appropriate through all stages of gestation.

A hay analysis will also help you determine how much supplemental energy and protein is required to meet the needs of the gestating mare.

Example Diets for Gestating Mares

Below is an example diet for a mare that begins gestation at a body weight of 500 kg (1100 lb).

In these examples, the hay represents a typical moderate-quality mixed hay with 10% crude protein and 2 mcal / kg digestible energy on a dry matter basis.

Depending on the body condition of your mare, hay of this quality may need to be rationed during early gestation to avoid excessive weight gain.

For a personalized feeding program, submit your mare’s information online and our equine nutritionists can design a customized diet to best support the health of her and the foal.

Table 1. Sample diet for early, mid, and late gestation

Nutrient Early gestation
(1-6 months)
Mid-gestation
(7-8 months)
Late gestation
(9-11 months)
Feed
Hay Free-choice Free-choice Free-choice
Alfalfa
pellets
0.5 kg / 1 lb 0.5 kg / 1 lb
Roasted
soybeans
0.5 kg / 1 lb
Omneity
pellets
200 grams
(2 scoops)
200 grams
(2 scoops)
200 grams
(2 scoops)
Optimum
Digestive
Health
80 grams
(1 scoop)
80 grams
(1 scoop)
80 grams
(1 scoop)
w-3 oil 90 ml 150 ml
Salt 15 grams
(1 tbsp)
15 grams
(1 tbsp)
15 grams
(1 tbsp)
Nutrient Analysis1
Digestible Energy
(% of requirement)
119 111 103
Crude Protein
(% of requirement)
161 134 131
Lysine
(% of requirement)
172 144 148
NSC
(% of diet)
8.9 8.9 9.2
Fat
(% of diet)
2.8 3.7 4.9

1 National Research Council’s Nutrient Requirements of Horses (2007)

Each mare’s individual needs will differ depending on several factors including body condition, health concerns, breeding history and future breeding goals.

Consult with your veterinarian to ensure your mare is in good health through gestation.

Weight gain

Mares typically gain 12 to 16% of their initial body weight during pregnancy to support both the fetus and the placental tissues. [1] For example, a 1100 lb (500 kg) mare should gain 132 to 176 lb (60 to 80 kg) during gestation.

Mares should have a body condition score (BCS) of 5 to 6 on a 9-point scale during gestation.

Energy requirements increase significantly when lactation begins and some mares may lose weight during this period. For this reason, it is not advisable to have a mare below a BCS of 5 when she foals.

Additionally, mares in thin body condition (BCS < 5) are less likely to get pregnant. If rebreeding is a goal, keeping mares in good body condition is important. [2]

Nutrient Requirements

The nutrient requirements for pregnant mares laid out by the NRC (2007) are intended to provide sufficient nutrients for the mare and to support the developing fetus and placenta.

Most of a mare’s nutrient requirements do not increase significantly until the last trimester of gestation when 75% of fetal growth occurs.

During early pregnancy (< 5 months), most nutrient requirements are the same as a horse at maintenance.

Energy and protein requirements start to increase around month 5 of gestation (Table 2) and mineral requirements start to increase during month 7 (Table 3).

Vitamin E and A requirements increase as soon as a mare gets pregnant but do not change over the course of gestation (Table 3).

Table 2. Energy and protein requirements of a 500 kg (1100 lb) mare throughout gestation1

Month of gestation Digestible energy
(Mcal)
Crude protein
(grams)
< 5 16.7 630
5 17.1 685
6 17.4 704
7 17.9 729
8 18.5 759
9 19.2 797
10 20.2 841
11 21.4 893

1 From National Research Council’s Nutrient Requirements of Horses (2007)

Energy

It may be tempting to increase your mare’s feed as soon as you find out she is pregnant, but careful attention should be given to her body condition and weight.

Because her energy and protein needs don’t increase until mid-gestation, she could become over-conditioned if fed too much energy in early gestation. Over-feeding energy to mares during pregnancy could have detrimental effects on the foal.

Encouraging weight loss or excess weight gain during pregnancy is unlikely to change the birth weight of the foal but could have other implications for the baby.

Over-feeding grains to pregnant mares

Feeding mares excess calories resulted in foals with altered glucose and insulin dynamics, with some effects lasting 19 months after birth. [3][4][5] This may have implications for the foal’s risk of equine metabolic syndrome.

In these studies, most of the excess calories were coming from grains, such as barley or oats. It is unclear whether the excess calories or the starch and sugar (collectively termed non-structural carbohydrates, or NSC) differences caused the disruption of glucose and insulin dynamics in the foals.

Fat and fiber diets during gestation

One study fed two groups of pregnant mares the same amount of energy, but one group was fed energy in the form of fat and fiber and the other group was fed energy in the form of NSC.

Mares fed the fat and fiber-based diet had greater linoleic acid in milk and immunoglobins in colostrum compared to mares fed the high NSC diet. [6]

Linoleic acid is an omega-6 fatty acid that is a precursor to prostaglandins. Prostaglandins are fat-based molecules that are involved in tissue repair. They can provide protection against gastric ulcers.

As foals are prone to gastric ulcer development, milk that is higher in linoleic acid may have a protective effect against ulcer formation.

In another study, mares were fed grain to increase their calorie intake to 120% over their energy requirement and were compared to mares fed 100% of their energy requirement with pasture.

The excess grain diet resulted in poorer quality colostrum with less immunoglobulins. [7]

These results show that mares should receive adequate energy from fat and fibre sources during pregnancy and should avoid diets with high levels of NSC.

Protein

By the last month of gestation, the protein requirement for your mare will have increased by 40%.

Most forages contain sufficient protein to meet the requirements of a horse at maintenance or in early gestation, but you may need to add protein sources during late gestation to meet the increased requirement.

Adding high-quality protein sources, such as alfalfa or soybeans, will help provide her with the essential amino acids required for her developing foal.

Vitamin & Mineral Requirements

The following table shows the vitamin and mineral requirements for a 500 kg (1100 lb) mare during early, mid, and late gestation.

Table 3. Mineral and vitamin requirements1

Nutrient Early gestation
(1-6 months)
Mid-gestation
(7-8 months)
Late gestation
(9-11 months)
Macrominerals (grams)
Calcium 20 28 36
Phosphorus 14 20 26
Magnesium 7.5 7.6 7.7
Potassium 25 25 25.9
Sodium 10 10 11
Microminerals (milligrams)
Copper 100 100 125
Iodine 3.5 3.5 4.0
Iron 400 400 500
Manganese 400 400 400
Selenium 1 1 1
Zinc 400 400 400
Vitamins
Vitamin A
(kIU)
30 30 30
Vitamin E
(IU)
800 800 800

1 National Research Council’s Nutrient Requirements of Horses (2007)

Minerals

Minerals have critical roles in the body including supporting metabolic processes, maintaining energy balance and supporting tissue structure. All minerals have a role to play in supporting the growing foal and gestating mare. However, when balancing diets of gestating mares some of the most important ones to consider are:

  • Calcium
  • Phosphorus
  • Copper
  • Iodine
  • Selenium

Calcium and Phosphorus

Calcium and phosphorus are the main minerals involved in building the foal’s skeleton in utero. The mare’s diet should have adequate levels of these minerals and in an appropriate ratio to support proper development of the foal’s skeleton.

These minerals are also important for preserving the integrity of the mare’s bones. If your mare’s diet provides inadequate calcium, her body will take this mineral from her bones to supply a sufficient amount to the foal.

Calcium and phosphorus requirements increase throughout gestation (Table 2), at a calcium to phosphorus ratio of roughly 1.4 : 1.

One study found that feeding gestating mares 20% below the recommended amount of calcium resulted in foals born with thinner, weaker bones. During lactation, the mares also had reduced bone strength. [8]

Copper

Copper is involved in the development of collagen and connective tissue, which are proteins that are important for mobility and joint health.  

Copper has been studied for its role in preventing the development of developmental orthopedic disease (DOD) in foals. Providing sufficient copper to dams reduces the risk of the foal developing DOD.[9]

However, supplementing copper to foals or yearlings above their requirement has no effect on the incidence of DOD. [9][10] Ensuring adequate intake of copper during gestation will be the most beneficial strategy to reduce the risk of copper-deficiency-related DOD in the foal.

Iodine

Goitre (enlarged thyroid gland) can be apparent at birth in foals. This can be a result of either the mare being fed too much or not enough iodine.

The majority of forages contain less than 0.2 mg per kg dry matter of iodine, but some forages can contain up to 2 mg/kg DM depending on where it is grown. [11]

Iodine supplementation is often recommended to meet requirements, especially in iodine-deficient areas.

Over supplementation of iodine (>50 mg I/day) to mares also causes goitre in foals. [12] Iodine is concentrated in the placenta, so developing foals in utero are particularly sensitive to overfeeding of iodine.

Kelp, a member of the seaweed family, is sometimes used to provide iodine to horses. It can contain very high levels of this mineral, so care should be used when adding it to the diet.

There are other seaweeds and algae that contain considerably less iodine, and these may be more appropriate for increasing iodine intake in horses. [12][13]

Selenium

Selenium is an important antioxidant that helps support healthy reproductive function.

The forage content of this mineral varies geographically. In general, higher levels are found throughout the Mid-West and Southern US and low levels are found along the coasts. A forage analysis is the only way to determine your forage selenium content accurately.

Although the selenium requirement during gestation does not differ from horses at maintenance, it is important to ensure your mare is at least meeting her requirement for this trace mineral.

Selenium supplementation, along with vitamin E, during late gestation can reduce placental retention time and improve reproductive performance in mares. [22]

Selenium has also been shown to improve placental cell function [7], increase colostrum selenium content and increase muscle selenium levels in foals. This may be relevant to the prevention of white muscle disease in foals. [21]

Vitamins

Vitamin E is another important antioxidant in the equine diet. This vitamin also plays a role in both the mare and foal’s immunity from disease.

Ensuring the mare gets enough Vitamin E during pregnancy can enhance the passive transfer of immunoglobulins to the foal in the colostrum (the first milk produced by the mare).

Mares supplemented with 2,500 IU of vitamin E during the last 5 weeks of pregnancy produced colostrum with more vitamin E and immunoglobins. [14] Vitamin E supplied to the dam during pregnancy can enhance the immune function of the neonatal foal.

Additional Supplements

Omega-3’s

While not a required nutrient, supplementing pregnant mares with docosahexaenoic acid (DHA), a marine-derived omega-3 fatty acid, can have beneficial effects for mom and baby.

Mares fed DHA during the last 3 months of pregnancy had faster uterine involution rates after giving birth, and their foals stood and nursed more quickly. [15]

DHA supplementation is beneficial for visual and cognitive function in humans. Supplying DHA to a developing foal may also have beneficial neurological effects. [16]

One study found that foals from mares fed DHA during pregnancy have greater circulating concentrations of DHA, indicating that DHA can pass through the placenta to the developing foal. [17]

Mad Barn’s w-3 oil contains high levels of microalgal DHA and natural vitamin E. This is a great way to add calories to the pregnant mare’s diet in late lactation and support her reproductive health as well as the foal’s brain development.

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  • Palatable source of Omega-3's

Gut Support

Another beneficial supplement to consider during pregnancy is Optimum Digestive Health. It contains prebiotics, probiotics and yeast which can help maintain hindgut function through pregnancy.

Additionally, yeast supplementation to pregnant mares has been shown to increase the antibody content of mare colostrum to provide greater immune support to the foal. [18]

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  • Support hindgut development
  • Combats harmful toxins in feed
  • Complete GI tract coverage

Feeds to Avoid During Gestation

There are certain types of feeds you should avoid or limit when feeding your pregnant mare. Some feeds contain compounds that may affect the mare’s normal hormonal balance which could cause issues with pregnancy, birth, or lactation.

You should consult with an equine nutritionist before adding herbs to a pregnant mare’s diet. Common herbs to avoid during pregnancy include: [19]

Tall Fescue

Tall fescue is a species of grass that is prevalent in much of the southeastern United States.

It is often infected with an endophytic fungus called Neotyphodium coenophialum. This endophyte produces ergot alkaloids, which can cause issues for pregnant mares grazing this grass.

Mares consuming endophyte-infected tall fescue during the last 30-60 days of pregnancy may have longer gestation times, dystocia (issues with delivery of the foal), reduced or no milk production, and weak or stillborn foals. [20].

Not all tall fescue is infected with this endophyte; some tall fescue is endophyte-free or contains a novel endophyte that does not produce the toxic ergot alkaloids. The only way to tell if your tall fescue grass has the toxic endophyte is through testing.

If testing is not available, it is recommended to remove your mare from pastures containing tall fescue at least 60 to 90 days before her expected foaling date.

Summary

The nutrition your new foals receives while they are in in utero is critical for their development and long-term health.

Feeding your pregnant mare a carefully balanced diet will help to promote the well-being of both the mare and her foal during gestation, birth, nursing and for years after.

For personalized feeding recommendations, submit your mare’s information online to receive a free customized feeding program from our highly trained equine nutritionists.

Is Your Horse's Diet Missing Anything?

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References

  1. Lawrence, L.M. Feeding stallions and broodmares. Equine Applied and Clinical Nutrition: Health, Welfare, and Performance. 2013.
  2. Henneke, D. et al. Theriogenology. 1984.
  3. Dobbs, T.N. et al. Glucose and insulin dynamics in mares and their foals. Australasian Equine Science Symposium. Gold Coast, QLD, Australia. 2012.
  4. Ousey, J.C. et al. The effects of maternal health and body condition on the endocrine responses of neonatal foals. Eq Vet J. 2008
  5. 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.
  6. Hoffman, R.M. et al. Dietary carbohydrates and fat influence milk composition and fatty acid profile of mare’s milk. J Nutr. 1998.
  7. Thorson, J.F. et al. Effect of selenium supplementation and plane of nutrition on mares and their foals: foaling data. J Anim Sci. 2010.
  8. Glade, M.J. Effects of gestation, lactation, and maternal calcium intake on mechanical strength of equine bone. Journal of the American College of Nutrition. 1993.
  9. Pearce, S.G. et al. 1998. Effect of copper supplementation on the evidence of developmental orthopaedic disease in pasture‐fed New Zealand Thoroughbreds. Eq Vet J. 1998.
  10. Grace, N.D. et al. “Digestible energy intake, dry matter digestibility and mineral status of grazing New Zealand Thoroughbred yearlings.” New Zealand Veterinary Journal. 2002.
  11. National Research Council Nutrient Requirements of Horses: Sixth Revised Edition. 2007.
  12. Baker, H.J. and Lindsey J.R. Equine goiter due to excess dietary iodine. J Am Vet Med Assoc. 1968.
  13. Pagan, J.D. Micromineral requirements in horses. World Equine Veterinary. 2000.
  14. Bondo T. and Jensen S.K. Administration of RRR-alpha-tocopherol to pregnant mares stimulates maternal IgG and IgM production in colostrum and enhances vitamin E and IgM status in foals. J Anim Physio Anim Nutr. 2011.
  15. Adkin, A. Supplementation of broodmares with docosohexaenoic acid and its effects on reproductive performance and foal cognitive development. Univ FL Thesis. 2013.
  16. Brenna, J.T. and Carlson, S.E. Docosahexaenoic acid and human brain development: Evidence that a dietary supply is needed for optimal development. J Human Evol. 2014.
  17. Kouba, J.M. et al. Effect of dietary supplementation with long-chain n-3 fatty acids during late gestation and early lactation on mare and foal plasma fatty acid composition, milk fatty acid composition, and mare reproductive variables. Anim Repro Sci.
    2019.
  18. 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.
  19. Hess, H.M. and Miller, R.K. “Herbs during pregnancy.” In Drugs During Pregnancy and Lactation. Academic Press, 2015.
  20. Blodgett, D.J. Fescue Toxicosis. Vet Clin North Am Equine Pract. 2001.
  21. Thorson, J.F. et al. Effect of selenium supplementation and plane of nutrition on mares and their foals: selenium concentrations and glutathione peroxidase J Anim Sci. 2010.
  22. Ishii, Mitsuo. Effects of vitamin E and selenium administration on pregnant, heavy draft mares on placental retention time and reproductive performance and on white muscle disease in their foals. Journal of Equine Veterinary Science. 2002.