Selenium (Se) is an antioxidant mineral that is necessary in the horse’s diet. Horses derive selenium by consuming hay or grasses that grow in soils that contain this trace mineral. However, if you live in a region with low selenium levels in the soil, your horse’s forage may be deficient in this essential mineral.
Not getting enough selenium in the diet can contribute to a wide range of health problems for your equine companion. It can impair muscle function, heart health, immune function, growth and energy levels in your horse.
So how common is low selenium in horses? If you live in the coastal regions of North America, near the Great Lakes, or in most parts of Canada, your soil levels are known to be low in this mineral. Selenium supplements or enriched feeds are necessary to support the health and well-being of your horse.
Selenium toxicity is also a concern for horses that get too much of this mineral. However, deficiency is a much more common problem and selenotoxicity can largely be avoided by supplementing with organic sources of this compound, rather than inorganic sources found in many feeds and supplements.
Mad Barn’s Omneity Equine Mineral and Vitamin Pellet contains 2.4 mg of organic selenium in a typical serving size. In addition to selenium, Omneity also provides all other essential minerals and vitamins that your horse needs in a science-backed formula designed to balance all different types of forages and diets.
Why Horses Need Selenium
Selenium is a micromineral that horses need to ensure optimal antioxidant defenses in the body. It is particularly important for horses during periods of growth or increased performance demands.
Racehorses, horses in heavy work and young foals are at the highest risk of insufficient levels of this mineral. 
Below are the top 7 reasons why horses need adequate selenium in their diet:
- Selenium is a key component of the antioxidant enzyme glutathione peroxidase. This enzyme helps to neutralize free radicals in the body and is involved in liver detoxification pathways. 
- It is necessary for the function of the acquired immune system, helping to protect against infection and disease.
- It plays a critical role in thyroid function and the type I iodothyronine 5-deiodinase enzyme which regulates the availability of T3 – the active form of thyroid hormone. Thyroid hormones control metabolism in the body. 
- Selenium is required for equine muscle development and growth. Along with Vitamin E, it is a component of proteins that prevent muscle conditions such as tying up (nutritional myopathy or exertional rhabdomyolysis). 
- This essential mineral is also important for reproductive health. Inadequate levels can lead to infertility and reproductive complications.
- It is known to prevent white muscle disease in which white scar tissue develops in the skeletal and cardiac (heart) muscles of horses. This can lead to rapid heart rates, difficulty swallowing, recumbency, weakness and urine discoloration. 
- Selenium is a component of the amino acid selenocysteine – an essential amino acid. It is required to synthesize 30-35 different selenoproteins with a range of cellular functions that continue to be researched in horses.
Signs of Selenium Deficiency
If your horse does not get enough of this nutrient in their diet, they may display some of the following signs. Horses that have a greater degree of deficiency and those engaged in more intense training display more serious symptoms. 
- Stiff gait
- Muscle soreness
- Diminished performance
- Muscle spasms, trembling or twitching
- Problems with chewing and swallowing
- Atrophy of muscle tissue
- Heart failure
Map of Selenium Deficient Soils in Canada & USA
Selenium concentrations in the soil vary widely in different parts of the world. The following map shows average soil levels in North America. The coastal and Northern areas have low concentrations while the Midwest generally has adequate levels in the soil.
However, even in regions with low concentrations of selenium, there may be pockets with high levels and vice versa. For this reason, it is best to have a hay analysis conducted to determine actual concentrations in your equine diet.
Regions considered low in this mineral are defined as those in which 80% of all forage and grain contain less than 0.10 ppm selenium. Medium regions have 50% forage containing greater than 0.10 ppm. Areas with adequate selenium are characterized as having 80% of all forages and grains with greater than 0.10 ppm. 
How Much Selenium does your Horse Need?
Selenium is a trace mineral or a micro-nutrient, meaning that it is only required in small amounts. According to the National Research Council (NRC), the requirement for a 500 kg horse is 1 mg per day. This amounts to 0.1 mg/kg of dry matter or 0.1 ppm (parts per million). 
Horses that are under strenuous work require a minimum of 1.25 mg per day. 
While this is the minimum amount of selenium that should be present in the diet to prevent deficiency, this amount may not be sufficient to support optimal health and performance.
This is why our Omneity Equine Mineral and Vitamin supplement has been formulated to contain 2.4 mg added selenium.
Unlike other trace minerals, the range of acceptable dosages for selenium is very narrow. Per the NRC, the upper tolerable intake level for this mineral is 2 mg/kg dry matter or 20 mg per day for a 500 kg horse. 
Using a supplement like Omneity is not likely to cause excessive intake of this mineral. Selenium status in horses can be evaluated using an inexpensive blood test.  Check with your veterinarian if you are interested in having a test performed.
The Best Selenium Supplements for Horses
There are several different forms of equine supplements available to buy that contain this mineral, but not all perform the same way in the body.
You may see products that contain inorganic forms of this compound such as selenate, selenide and selenite. These are mineral salts extracted from mining operations.
There are also supplements that contain organic forms derived naturally from living matter such as selenium-enriched yeast. Horse owners should consider selenized yeast as their best and safest choice. 
Selenium-enriched yeast is better utilized in the body than inorganic sources of selenium.  Organic selenium is considerably safer than inorganic sources because it is used more slowly by the body and stored safely in cells. Toxicity is less of a concern with organic Se than inorganic forms like selenite.
According to Alltech, Sel-Plex® is “European Union-approved and [the] only U.S. Food and Drug Administration-reviewed form of selenium-enriched yeast.” 
Risk of Selenium Toxicity in Horses
Nutritionists often hear concerns from horse owners about feeding supplemental selenium due to toxicity concerns. However, toxicity is highly unlikely when organic forms such as selenized yeast are used.
The upper tolerable limit for selenium is 20 mg per day for a 500 kg horse. A supplement such as Mad Barn’s Omneity provides 2.4 mg per day of added selenium, which means your horse is unlikely to reach toxic levels even if your forages have adequate Se.
If you are supplementing your horse’s diet with inorganic selenite, it is important not to provide too much of this mineral. Excess levels can contribute to selenium toxicity, which is marked by symptoms including:
- Mood disturbance
- Low appetite
- Digestive problems
- Muscle weakness
- Respiratory distress
In severe cases, consuming too much of this antioxidant can lead to selenium poisoning which can be fatal. Acute overdoses may result in muscle tremors, an unsteady gait, vision loss, difficulty breathing or fatality . However, acute overdoses are very rare particularly when organic sources are used.
It is more common for horses to be exposed to high levels of this mineral over longer periods of time. Chronic levels of high selenium in the diet can result in alkali disease or bobtail disease. Symptoms of chronically elevated levels can include:
- Thinning hair in the mane and tail
- Cracking of hooves
- Excessive salivation
- Respiratory problems
The best way to avoid selenium toxicity when using supplements is to have your horse’s diet reviewed by a professional equine nutritionist. You can submit your horse’s diet for our equine nutritionists to take a look at your horse’s nutrition program and we can help you determine what the right inclusion level for this mineral is.
If you believe your horse needs additional selenium, we recommend our Omneity Equine Mineral and Vitamin supplement which is designed to bring the majority of equine diets into balance and provide complete nutritional coverage.
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- Alltech®. Sel-Plex®.
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- Vet Services Wairarapa. Selenium Deficiency in Horses. May 15, 2015.
- Kentucky Equine Research. Selenium for Horses: How Important Is It?. November 9, 1999.
- Scott Cieslar. Natural E/Organic SE. Mad Barn.
- Beaudet, Angie. Could Selenium Deficiency be Affecting Your Horse? Horse Sport. August 5, 2019
- Thunes, Claire. Supplementing Selenium. The Horse. January 11, 2016