Selenium is a micronutrient that is considered essential for both humans as well as animals. While this element is absorbed by plants from the soil, animals and humans derive selenium from plant sources. As selenium is required in very small amounts, deficiency of this trace mineral is very rare. However, it has been noticed that those who reside in places, where the soil is deficient in selenium, may develop selenium deficiency (as plants that grow in such soil will have almost no selenium). Even certain medical conditions may prevent the body from absorbing selenium. Though selenium deficiency is uncommon in humans, animals can be more prone to develop this condition. Selenium and Horses Though, the element selenium was initially noticed for its toxic effects, later, researches revealed that it is essential for various bodily functions in mammals. This trace mineral is found to play key roles in various bodily mechanisms, including the thyroid metabolism. It is also said to be vital for cell functions and for boosting immunity. It is said to be an excellent antioxidant that fights free radicals. Horses derive their daily dose of selenium from the food they eat. Whether it is grass, oats, barley or hay, the selenium content in the food may vary with the selenium content of the soil in which these plants grow. It has been estimated that horses need less than 0.1 milligram of selenium, per kilogram of the food they require for a day. According to FDA, an average horse requires 3 mg of selenium per day. While selenium deficiency may cause health problems in them, this trace mineral may cause serious side effects, if consumed in excess. Selenium Deficiency and Toxicity As mentioned above, horses that are exclusively fed with foods that are grown in selenium-deficient soil are prone to develop deficiency of the trace mineral. Horses with this condition are found to develop muscle cramps, which may lead to tying up or azoturia. They may also be prone to infections, and even wounds may take time to heal. Another complication that may develop from selenium deficiency in horses is white muscle disease or nutritional muscular dystrophy. This condition affects the heart and skeletal muscles in horses. In that case, the animal may have selenium deficiency symptoms like brownish urine, difficulty in swallowing and fast heart rate. Selenium toxicity is caused by excess intake of this mineral. This is often found to happen with intake of plants/other foods/water that are rich in this trace mineral. Administering large amounts of selenium supplements without any idea about the requirement of the animal may also result in selenium toxicity. In very large amounts, selenium can be fatal for the animal. In other cases, the affected horses may lose hair from the mane as well as tail. These animals may also develop excess salivation, lameness, cracking of hooves, joint erosion and respiratory problems. In severe cases of selenium toxicity, the horse may exhibit watery diarrhea, abnormal movements, dilated pupils, weak pulse and high body temperature. Selenium Supplements for Horses So, both high and low levels of selenium are unhealthy for horses. If you notice any such symptoms, you have to consult the vet, who will find a remedy for the condition. Though selenium supplements for horses are available in the market, it is not advisable to administer such products to these animals, unless as directed by the vet. You can detect selenium deficiency in the horse by conducting a simple blood test. If the horse is found to be deficient in this micronutrient, you have to enhance its selenium intake, but only as per the instructions of the vet. Before starting with the supplements, calculate the amount of selenium the animal is deriving from the diet and how much more is required. Such calculations can be done with the help of an equine nutritionist. It is always better to detect and rectify the condition as per your vet's instructions. As stated above, administering selenium supplements to horses, without studying the exact requirement, is not at all advisable.