Pregnant Mares and Supplements

Eleven months is a very long time to wait for a foal. I know from personal experience that the further along the mare is, the more your hopes and dreams for your "perfect foal" grow. It doesn't matter if the foal is bred to gallop to the wire or jump to the moon, or whether you are dreaming about lots of markings or a perfect blanket, everyone wants one thing--a healthy foal. So, it's no wonder that breeders want to do everything possible to ensure the good health and well-being of the pregnant mare, maximizing the chances for a healthy newborn. One of the most frequently asked questions in our practice is: What supplement(s) should I give my mare to keep her healthy and pregnant? I wish there was a magic pill to ensure that every mare would deliver a perfectly healthy, full-term foal, but there isn't. This article will discuss the actual, basic nutritional requirements for a mare during her pregnancy and early lactation to help ensure a healthy mare and foal.

What a Girl Needs

It's no wonder I get asked about supplements so frequently. All you have to do is open any horse-oriented magazine to be bombarded with glossy advertisements for a vast array of supplements, not to mention new medications. So, what is a concerned horse breeder to do? You can't afford to buy all the supplements that are advertised. And even if you could, are they all necessary to ensure a healthy foal?

The bottom line for pregnant mares is good, basic nutrition. The majority of pregnant mares do not need any supplements when they have access to good-quality hay, grass, and a mineral block. Good nutrition (not over-nutrition) is the key for allowing pregnant mares to provide the essential nutrients to their unborn foals. This is true with two exceptions--vitamin E and selenium. These two nutrients are very important to the mare and the unborn foal, and they are in scarce supply in some parts of our world. So how do you know when to supplement? Read on.

Why Vitamin E and Selenium?

Although selenium is a mineral required by horses in very small quantities, the lack of it during pregnancy can have some serious consequences. In the northeastern United States, the soil is nearly devoid of selenium. Therefore, horses need supplementation. Selenium is required for normal musculoskeletal development and works in tandem with vitamin E. Without these two vital nutrients, foals can be born with or later develop white muscle disease.

White muscle disease (WMD, which also can occur in cows, sheep, and goats) results in weakness/stiffness in young animals. WMD can be so severe the animal cannot stand. This disease can affect the skeletal muscles or the heart muscles; foals affected with the skeletal muscle form of the disease often have rock-hard muscles and have difficulty standing on their own. Foals affected with the cardiac form might be found dead or suffer from heart failure. Usually fast-growing foals are primarily affected.

Treatment for the skeletal muscle form of the disease is rest, intravenous fluids, and supplementation of selenium and vitamin E. These animals often require hospitalization. Treatment of the heart or cardiac form of the disease is often not successful.

Prevention is much better than treatment for this disease, as is often the case. However, some areas of our country have adequate or even high levels of selenium and supplementation can be harmful as the difference between optimal levels and toxicity is not much. So, blindly supplementing selenium in the diet is not recommended. Also, many commercial grains have a small amount of selenium added--read the label, it will tell you how much selenium is in the grain. If you're unsure, a blood test can be performed by your veterinarian to determine if your mare is deficient.

If you're in a deficient area and need to supplement, the recommended dose is no more than 1 mg of selenium per day. Oral supplementation of selenium is easy, and by supplementing the mare, the foal will also benefit as selenium is excreted in the milk. If you're feeding multiple supplements, read the labels to see how many contain selenium. Too much can result in toxicity, which results in loss of hair from the mane and tail and sometimes cracked, deformed hooves that can slough off.

Removing the selenium from the diet will end the toxicity, but horses can take a while to look normal again. If you are unsure of selenium levels in your area, talk to your veterinarian or cooperative extension agent.

Vitamin E

Vitamin E in combination with selenium can have many added benefits to the pregnant mare and her unborn foal. Vitamin E is one of the most important, yet often overlooked vitamins, and it is present in good quantity in green grass and well-cured hay. Grains often have added vitamin E, but as they sit in storage, the vitamin E is lost.

Horses which do not have access to green grass or good-quality hay are at risk of a vitamin E deficiency. This deficiency has been implicated in causing a variety of disorders such as rhabdomyolysis (tying-up syndrome), equine lower motor neuron disease, white muscle disease (with low selenium levels), and degenerative myelopathy (a neurologic disease in young horses). Horses with normal levels of vitamin E also have been shown to have better immune responses to vaccines than horses with lower vitamin E levels. Oral supplementation of vitamin E is safe, as there have never been reports of toxicity from vitamin E.

The most important fact to know about supplementation is the amount: Normal horses should receive between 1,000-2,000 international units (IU) per day of the alpha-tocopherol form of vitamin E. You must read the labels on your supplements, because many don't have enough vitamin E per one-ounce scoop to meet the horse's daily requirements. Also, make sure the label states the form of vitamin E that the product contains. The alpha-tocopherol form is the most biologically active (available) for horses to utilize.

One easy way to supplement is to purchase human vitamin E capsules (the alpha-tocopherol form) and give one or two of the 1,000 IU capsules in their grain. Most horses gobble them right up. If not, there are several good commercial combination vitamin E/selenium supplements available.

Over-Supplementation of Iodine

Iodine is little thought about today. Now that mineral and salt blocks contain iodized salt, iodine deficiency in mares (and humans for that matter) has greatly decreased. However, a problem can arise when too much iodine is given. Supplements heavy in kelp (seaweed) can cause iodine toxicity, resulting in goiter (enlargement of the thyroid gland), poor muscle development in foals, and/or abortion in mares. Iodine is easily passed to the newborn foal through the placenta, and to nursing foals through the mare's milk. Remember that 50 mg of iodine is the maximum limit per day for adult horses.

Because you can purchase so many supplements over the counter without a prescription, you should consult with your veterinarian before supplementing in order to prevent possible toxicity and to get recommendations on what to supplement for your situation.

Protein Supplement--Is it Needed?

Remember that the basics for broodmares are good-quality hay and grain. This diet must maintain a good body condition, especially during late gestation and lactation; the protein level in the diet is very important so that the mare can produce ample milk for her foal. However, I find that the most common mistake when feeding broodmares is overfeeding protein.

According to Harold Hintz, PhD, MS, professor of animal nutrition at Cornell University, in his chapter titled "Nutrition of the Broodmare" in the textbook Equine Reproduction, pregnant mares require 9-10% protein in their diet (between hay and grain). When lactating, their protein requirement increases to 12%, then decreases to 10% in late lactation. Furthermore, the amount of hay and grain fed depends on the protein content of the grain and hay combined. Protein levels in grain are right on the label. However, the protein content of your hay can only be determined by core samples. These are taken from the middle of several of your hay bales and analyzed at a laboratory. This testing is a great idea to more accurately determine how much protein is needed in the broodmare's diet and can help you decide if and when you need to supplement protein.

For example, if your grass hay has a protein level of 8%, then your mare will only need a grain with a protein level of 10% to hit the overall target of 9% during maintenance gestation, but the grain would need to have a protein level of 22% during lactation to meet the 12% requirement (if you are feeding a 70:30 ratio of hay to grain). One option is to feed more grain, but too much grain can lead to digestive problems in your mare or create the need for more than three feedings a day. A better option than pouring on the grain is to feed an alfalfa or alfalfa-mix hay to increase the protein level in the diet.

On the other hand, if your alfalfa hay is 14% protein, you will need only a grain with 10% protein. As with any feed changes, make the switch in hay and/or increase in grain slowly over time to allow the mare's digestive system to adjust.

A protein deficiency in the diet has been known to cause low birth weight and/or slow growth in foals On the other hand, a protein level that is too high has been implicated in several developmental orthopedic diseases, such as club foot, flexural deformities, and osteochondrosis. So, you can see how finding a balance between the protein in hay and grain is very important. Guessing the protein content of your hay based on color is not a good idea--the small investment in core samples can save you hundreds, if not thousands, of dollars in treating the ensuing problems if the protein levels aren't optimal.

As you can see, feeding pregnant mares can be complicated, and there is a fine line between not enough and too much. Education is the key, and a little information can go a long way to helping you and your veterinarian decide what is needed for your mare and what supplements--if any--are really necessary. 

For more on basic broodmare nutrition, see "Broodmare Diet Basics" on page 87.


Hintz, H. Nutrition of the Broodmare. Equine Reproduction Eds. McKinnon and Voss. Philadelphia: Lea and Febiger, 631-639, 1993.

Mass, J.; Parrish, S.; Hodgson, D.; Valberg, S. Nutritional Myopathies. Large Animal Medicine 2nd edition. Ed. Smith, B. Philadelphia: 1513-1518, 1996.

About the Author

Christina S. Cable, DVM, Dipl. ACVS

Christina S. Cable, DVM, Dipl. ACVS, owns Early Winter Equine in Lansing, New York. The practice focuses on primary care of mares and foals and performance horse problems.

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