The Role of Selenium in the Equine Diet

Happy New Year! This is part of Owl Equestrian's new series, Health Spotlight, featuring a number of topics related to equine health.

A trace mineral required in minuscule quantities, selenium is a powerful antioxidant, protecting tissues from the free radicals that are formed during the conversion of food to energy. Selenium is also found in a number of selenoproteins (selenium-containing proteins) in the body. While often toxic in doses exceeding 5 mg per day, selenium’s role in maintaining cellular and muscular integrity is imperative in functions related to respiratory, cardiac, thyroid and muscular health. It may also have a role in crib-biting behaviour in horses.

As an antioxidant, selenium contributes to the formation of an enzyme (glutathione peroxidase) that destroys free radicals (lipo- and hydrogen peroxides) by converting them into harmless alcohols, an antioxidant activity that protects the integrity of cell membranes. It is important to mention that selenium does not act alone in this process, rather it works in coordination with vitamin E. The relationship between this vitamin and mineral duo is essentially a balancing act, where they act in concert to protect cells from the damaging effects of oxidation. For example, vitamin E decreases the formation of lipid peroxides in cell membranes, while selenium in the intracellular fluid removes any lipid peroxides that do form. If there are insufficient amounts of either selenium or vitamin E, oxidation-induced cellular damage may occur, with effects similar to deficiency.

The simplest way to think of this balance is:

  • inadequate vitamin E = increased peroxide formation, requiring more selenium to remove peroxides, or
  • inadequate selenium = decreased removal of peroxides, requiring more vitamin E to decrease peroxide formation

Clinical Signs & Effects of Selenium Deficiency

Clinical signs of selenium deficiency vary with the degree of deficiency in a horse’s diet. In mild cases, a horse’s immune system may be depressed, resulting in increased susceptibility to disease. Fertility may also be reduced. Signs of severe selenium deficiency include impaired cardiac function, respiratory distress, cardiomyopathy, and white muscle disease. Horses may also engage in crib-biting when selenium is deficient.  More detail on these conditions is available here.

Maintaining a Proper Selenium Balance

Dietary Sources of Selenium

In an ideal situation, a horse will be able to attain appropriate levels of selenium from forage. However, as the map below illustrates, extensive areas of soil across North America are selenium deficient. Supplementation is recommended for horses that are kept solely on hay and/or pasture in selenium deficient areas. While a complete vitamin and mineral supplement may seem like a logical approach to ensuring horses are receiving adequate amounts of selenium, unlike other trace minerals, balancing selenium requires a great deal of caution. Too little can result in a deficiency and too much can be toxic.

Source: Pard, K. (2016)



While horses fed concentrates may receive adequate amounts of selenium through feed, organic selenium yeast is the best source of supplementation for horses consuming a diet consisting solely or primarily of selenium-deficient forage. It has a high level of bioavailability and is the most similar to the selenium found in plants. Selenate, a byproduct of the mining industry, is also an acceptable selenium supplement, however, in areas with high concentrations of iron or sulphur, it will not be effective because both iron and sulphur will oxidize the selenate before it can be absorbed by the body. In cases of dangerously low selenium levels, particularly if clinical signs are observed, horses should receive therapeutic doses of selenium and be retested after a few months to monitor improvements or adjust supplementation levels accordingly.

Prior to adding selenium to the diet, it is recommended to determine the amount of selenium the horse is getting from forage and feed to determine whether supplementation is necessary, and in what quantity. Other factors to consider include, age, activity level, and stage of life. For example, pregnant and nursing mares will require higher levels of selenium than a performance horse fed a high concentrate diet, which will also differ from a horse with low-to-moderate activity levels kept at pasture.  

The National Research Council recommends approximately 1 mg/day of selenium for a 500 kg horse (0.1 mg/kg dry matter), however, higher intake, in the range of 2-4 mg per day for an average-sized horse is thought to be optimal to not only prevent deficiencies, but to promote optimal immune function. Up to 5 mg per day may be safe for pregnant and nursing mares, but will poses a risk of toxicity for other horses. Horses should have their blood selenium levels tested once a year to monitor for deficiency and, where appropriate, toxicity.

Take Home Points

  1. Soils across extensive areas of North America are selenium deficient, with the exception of the central regions of the continent.
  2. When balancing the amount of selenium in the equine diet, there is a fine line between deficiency and toxicity.
  3. Even in areas with selenium deficient soils, it is advisable to learn exactly how much selenium is provided by forage (and concentrates if fed) prior to adding supplemental selenium to the diet.
  4. While pregnant mares may benefit from up to 5 mg of selenium per day, this level may be toxic for the average adult horse, for which daily intake should not exceed 2-4 mg.
  5. The most bioavailable form of selenium next to that contained in plant matter is organic selenium yeast.
  6. Selenate, a byproduct of the mining industry, should be avoided in areas with high concentrations of iron or sulphur because the iron and sulphur will block the bioavailability of the selenate.
  7. Early signs of selenium deficiency may include a depressed immune system resulting in increased susceptibility to disease, and reduced fertility.
  8. Severe selenium deficiency may lead to impaired cardiac function, respiratory distress, cardiomyopathy, white muscle disease, myodegeneration, and crib-biting.  


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