Selenium: facts and myths

We are often cautioned about feeding our horses too much selenium, but we rarely hear about the benefits of this essential trace mineral. Here’s why your horse needs adequate selenium in his diet!

Selenium is an essential trace mineral. Essential, meaning it must be in the horse’s diet since his body is not capable of producing it. Trace, because it is required in very small amounts. But don’t let that fool you into thinking it has a small role. It’s a major player in many areas of the body, preventing cell damage, protecting the thyroid, and stabilizing immune function.

What are your horse’s requirements?

According to the National Research Council, horses require a minimum of 0.1 mg selenium per 1 kg of dry matter intake (0.1 ppm). This translates into 1 mg of selenium from 10 kg (22 lbs) of total feed intake per day. However, evidence suggests this level is not high enough to protect against oxidative stress, and the requirement may be closer to 0.3 ppm per day. Consequently, a safe range for selenium is between 1 and 3 mg per day for a full-sized horse at maintenance. Larger breeds require more, as do working or performing horses — generally up to 5 mg per day. The total amount of it in the daily diet should not exceed 0.6 mg/kg of feed.

Because of its importance in the diet, it is generally added to commercial feeds and supplements. This can be a double-edged sword, benefitting the horse by meeting physiological needs, while endangering him if dietary levels get too high. To help clarify this dichotomy, let’s take a closer look at what we know about this important nutrient, and what myths have surfaced over the years.

Oxidative stress protection

The enzyme family called glutathione peroxidase (GSH-Px) contains a molecule of selenium as part of its structure. GSH-Px scavenges damaging free radicals produced during times of stress. Endurance horses, for example, are subjected to prolonged periods of aerobic activity, making them especially prone to free-radical formation. But any form of stress, whether from travel, unfamiliar environments, loss of a buddy, stall confinement, or not being allowed to graze naturally, can induce an oxidative rampage on the body’s tissues, potentially leading to arthritis, allergies, or digestive disturbances. Stress can even damage the brain’s hypothalamus, potentiating the development of equine Cushing’s disease and leptin resistance.

Vitamin E teamwork

Selenium doesn’t protect against oxidative stress alone; instead it teams up with vitamin E. It also protects the inside of the cell, while vitamin E guards the polyunsaturated fatty acid component of the exterior cell membrane. Together, they boost overall immune function by neutralizing highly volatile free radicals.

Since many vitamin E supplements contain selenium, evaluate the content before adding more.

Proper thyroid function

Selenium and iodine work together to promote a healthy thyroid gland. It is a component of iodothyronine deiodinase enzymes, which are involved in the production of thyroid hormones, specifically the conversion of T4 to T3, of which iodine is a key component. If iodine intake is high while selenium is deficient, thyroid damage can result. Therefore, it is best to maintain them at similar levels. A safe iodine range is between 1 and 5 mg per day. Choose a salt with a guaranteed analysis. Iodized table salt, for example, contains 45 ppm iodine; one ounce (two tablespoons or 28.375 grams) provides 1.28 mg of iodine. Make sure the amount of iodine added to commercially fortified feeds is not excessive and that it is balanced with selenium.

Misconceptions about selenium

  • “If some is good, more is better.”
    • More than 0.6 ppm per day can be detrimental over time. Let’s suppose your horse consumes 30 lbs of feed each day (forage and concentrates combined). At 0.6 mg per kg of feed, that computes to a whopping 8.2 mg. Look for hair loss along the mane and tail and hoof cracks around the coronary band that can indicate toxicity, because excess selenium replaces the naturally existing sulfur found in hoof and hair protein (keratin).
  • “All horses require selenium supplementation.”
    • Your decision to add it to your horse’s diet should be based on facts. Selenium intake should be calculated from all sources. The only true way to know how much  is in your horse’s hay or pasture is to have it tested.
    • Feed companies typically add between 0.5 to 0.6 ppm of selenium. Five pounds of your chosen commercial feed, for example, might provide between 1.14 and 1.36 mg, which is well within the safe range. But if your horse is already consuming plenty of selenium from his hay, this additional amount may be dangerous.
  • “Insulin resistant horses may have too much selenium in their bodies.”
    • This has been well documented in human nutrition and we may find it to be true in horses as research progresses. Selenium tends to be elevated in patients with type II diabetes. The reasons for this are complex. If you have an insulin resistant horse, consider having his levels tested and adjust his diet accordingly.
  • “All selenium supplements are alike.”
    • Read ingredient labels carefully. It is often supplemented either as organic selenium yeast or inorganic sodium selenite. The yeast (Saccharomyces cerevisiae) is grown in the presence of selenium, biochemically creating selenoamino acids. Plants naturally contain selenium in this form. Selenium yeast is highly bioavailable and is better retained by the horse’s body than the inorganic form.Furthermore, the potential for toxicity is reduced with selenium yeast supplementation because it is bound to amino acids, thereby controlling excessive absorption from the small intestine into the bloodstream. In contrast, sodium selenite is passively absorbed, potentially leading to unregulated uptake of toxic amounts of selenium.
    • Natural, whole foods are great ways to add it to your horse’s diet. Two foods in particular are high in selenium:
      • Brazil nuts — five nuts contain approximately 0.5 mg.
      • Chia seeds — two ounces contain 0.3 mg.
  • “Selenium concentration in plants is consistent within a specific geographical region.”
    • Within the US, low selenium levels tend to exist in the northeast, the Ohio valley, Florida, and the northwest. Western and eastern provinces of Canada are likely to also be low. But pockets of high-selenium soils can exist anywhere. Mining and industrial waste can contaminate soils and water supplies. Washington State, for example, typically low in selenium, has pockets along the coastline that are high. When in doubt, test. If you can’t test your hay or pasture, have your horse’s blood tested.
    • It is also risky to assume that hay grown from the same field will always be similar in selenium content. Soil alkalinity and dry conditions can increase the plant’s uptake. In areas of drought, when the roots search deeper into the soil for water, they encounter more selenium.

 Bottom line

Selenium needs vary according to your horse’s health status and activity level. Supplementation is often necessary, keeping in mind that it has a narrow range of safety. Testing your horse’s forage as well as knowing his selenium status will remove the guesswork from planning his diet.

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Dr. Juliet M. Getty earned her Master of Science degree in Animal Nutrition at the University of Florida. She completed her doctoral coursework in Animal Nutrition at the University of Georgia, and continued her studies at the University of North Texas, where she earned her PhD. Winner of several teaching awards, Dr. Getty has taught comparative nutrition studies at the University of North Texas for 20 years. At the same time, she has been working in the field, consulting privately with horse owners to customize feeding plans that address a variety of health conditions. Recently retired from academia, she now resides in Denton, Texas, where she devotes herself full-time to equine nutrition. Through her consulting company, Getty Equine Nutrition, she provides consultations locally, nationally and internationally.