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.