The equine athlete relies on a healthy muscular system to provide the power to soar over jumps, make an intense sprint around barrels, or deliver the strength and precision required for dressage work. Selenium is an essential trace mineral that is found naturally in the soil and is required in small amounts in the horse’s diet in order to maintain healthy muscular function and decrease oxidative stress. It is also vital for proper thyroid and immune system function.

Muscular function and oxidative stress

During exercise, the horse’s muscles require massive amounts of oxygen. This provides the energy required for physical activity, but it comes at a price. The side-effect of the oxygenation process is the production of free radicals and peroxide formation, which causes damage to cells.

Selenium, being a powerful anti-oxidant, is capable of protecting cells from this damage, along with contributing to the muscle structure itself. In other words, it detoxifies the system by removing toxins that would otherwise be harmful and cause the destruction of cell membranes.

Vitamin E is also required in conjunction with selenium; together they work hand-in-hand to fight oxidative stress. Vitamin E helps prevent oxidative damage from happening in the first place by preventing peroxide formation, while selenium removes peroxides that have already formed. Their complementary roles make them both of high importance for the athletic horse.

How much selenium… and how much is too much?

The average 500-kg horse needs about 1 mg of selenium per day in its diet, which increases to 1.25 mg under strenuous work (NRC guidelines). However, it has been suggested that these amounts are the minimal requirements at which no signs of deficiency are present, but that 2 to 3 mg may be required to achieve optimal immune function (NRC guidelines, 500 kg horse, feeding rate of 0.5 mg/kg).

Although selenium has many beneficial properties, too much can cause chronic toxicity issues such as alkali disease, which is characterised by poor hoof quality and loss of mane and tail hairs. It is believed that selenium replaces sulphur in keratin, explaining why the hooves and hairs are affected. Keeping the horse’s total intake at under 6 mg per day is advisable in order to avoid toxicity issues.

An acute overdose is also possible and may even lead to death. Considering that supplements can contain fairly high levels of selenium and are meant to be fed in small portions, they can easily become a source of intoxication if horses gain access to an open storage bin. In the field, plants containing high levels of selenium such as goldenweed, milk vetch, poison vetch, and prince’s plume are not very palatable, therefore intoxication from pasture plants is fairly rare when plenty of good quality forage is available. A selenium soil concentration map (right) can help you evaluate the risk in your area.

Does your horse need a selenium supplement?

The first thing to consider is whether your hay and grain comes from a selenium-deficient area. If both were grown in selenium-deficient soils, they will not provide a significant source of selenium.

Next, you need to calculate how much selenium your horse is receiving through feed and other supplements. For example, if your horse is receiving 3 kg of a feed with 0.3 mg/kg of selenium, your horse is receiving roughly 0.9 mg of selenium. This calculation doesn’t take into consideration dry matter, which will change the value slightly, but it is a good rough estimate to get you started. An equine nutritionist can help you balance your rations.

Identifying and treating selenium deficiency

In order to diagnose selenium deficiency, a veterinarian can collect a blood sample for analysis. Testing for selenium is a specialised test that is not part of routine blood work. The Faculté de médecine vétérinaire de l’Université de Montréal uses a High Performance Liquid Chromatography (HPLC) test, which was developed in Egypt. Whole blood, plasma, or serum can be tested, along with any other type of tissue, including hairs. A fluorescent colouring which binds to selenium is utilized and allows for an extremely accurate result.

A calculation of total selenium intake per day, along with symptoms, will further support the diagnosis. Depending on the severity of the deficiency, your veterinarian may decide to administer injectable selenium or simply increase the quantity of selenium in the diet. Once a horse is severely deficient, achieving normal levels of selenium through diet alone can take several weeks.

As the muscular system is highly solicited in the athletic horse, selenium deficiency has a direct impact on performance. Adjusting your horse’s diet to ensure it is receiving the proper amounts of selenium and vitamin E throughout the year will help your equine partner stay at peak level.

Cribbing Behaviour

Suspecting a link between oxidative stress and cribbing, researchers recently investigated the possibility that a selenium deficiency could trigger the stereotypical behaviour of cribbing in horses. By testing selenium serum levels at different moments (while cribbing, at rest, etc.), it was determined that cribbers had a significantly lower selenium level than the control group. Interestingly, selenium levels were also at their lowest during cribbing behaviour. Several other parameters were also monitored, but selenium was the only variable that was significantly different between the control and cribbing groups, pointing to a direct link between low selenium levels and cribbing behaviour.


Many horses with slight-to-moderate deficiencies are asymptomatic. The more severe the deficiency and the more intense the horse’s training, the more noticeable the symptoms.

Common symptoms include:

  • Stiff gait
  • Sore, painful muscles
  • Poor performance
  •  Muscle spasms and/or trembling
  • Tying up (nutritional myopathy/rhabdomyolysis)

Long-term effects:

  • Muscle deterioration and white muscle disease
  • Difficulty chewing and swallowing (atrophy of the jaw muscles)
  • Heart failure (the heart is also a muscle!
  • Death


Symptoms of chronic toxicity:

  • Loss of mane and tail hairs
  • Poor hoof quality and horizontal cracks near the coronary band
  • Lameness and founder
  • Lethargy
  • Weight loss
  • Anaemia
  • Liver cirrhosis
  • Myocardial necrosis/scarring

Symptoms of acute poisoning:

  • Blindnes
  • Sweating
  • Colic/abdominal pain
  • Head pressing
  • Lethargy
  • Elevated heart and respiratory rates
  • Diarrhoea
  • Hoof sloughing
  • Sudden death

Organic vs. inorganic selenium

Organic selenium means that it’s derived from plants (living matter) whereas inorganic selenium is a mineral salt extracted from mining. Studies have shown that the organic form is better assimilated by horses. Reading the ingredient list on product labels will allow you to ensure your horse is receiving 100% organic selenium. Organic selenium will be listed as selenium yeast or organic selenium. Selenate and selenite are inorganic sources of selenium.