With deficiency Overt disease is rarely caused by Se deficiency in adult horses. However subclinical effects probably occur in many tissues and reproduction, immune function, and the ability to train and exercise is enhanced with adequate Se / vitamin E status.

White muscle disease of foals is the best characterized disease caused by direct selenium deficiency. All of the diseases associated with low vitamin E status are exacerbated by low dietary selenium, ...more. If there is inadequate vitamin E, more peroxides are formed and, therefore, more selenium is needed; conversely, if there is inadequate selenium, fewer peroxides can be removed and, therefore, more vitamin E is needed to prevent their formation. Optimum amounts of both are necessary to minimize oxidation-induced tissue damage.

Selenium supplementation must be done carefully as in excess it can be toxic in amounts not much higher than recommended. Selenium toxicity can also occur without supplementation, on soils that are rich in selenium and some plants take up and concentrate the selenium in the soil. Very high doses can result in acute muscle damage and death.

More common is chronic toxicity with smaller but excessive doses. The selenium accumulates overtime. The primary clinical sign of chronic selenium toxicosis is one of generalized poor doing, more.