Selenium Sources and Utilization in Horses

Selenium is an important nutrient for horses and other animals. Along with other functions, selenium supports the development and maintenance of the immune system. Though large amounts of dietary selenium can be toxic, dietary deficiencies can also cause health problems.

The selenium content of grains and forage is determined largely by the selenium content of the soil on which the crop is grown. Selenium in alfalfa (lucerne) can range from .03 to .69 ppm, and corn may contain .03 to 1.0 ppm. Brewer’s yeast, wheat, and linseed products from the United States tend to have higher selenium levels than the same products from some parts of Europe because crops in the U.S. are raised in areas of the country where soils contain selenium.

Selenized salt is often recommended as a source of selenium. It could be a very useful source for animals grazing forages growing on low selenium soils. Such products containing 30 to 90 ppm selenium have been marketed. The voluntary salt intake of horses will vary depending on environmental temperature, work, and sweat production.

Form of selenium and interrelationships with other dietary ingredients such as copper, sulfur, mercury, and arsenic can influence selenium utilization. Copper does not appear to prevent selenium absorption, but rather enhances excretion. Arsenic could influence selenium metabolism in horses by increasing biliary excretion of selenium. Mercury and selenium form a complex that decreases the activity of both minerals. Sulfur has been shown to decrease selenium activity in sheep and presumably in horses. Silver and cadmium have been shown to influence selenium metabolism in other species but have not been tested in the horse. A low vitamin E content of the diet could influence the need for dietary selenium.

Few studies have compared the effectiveness of selenium compounds in horses. In a study conducted at Kentucky Equine Research, selenium utilization was measured in two diets containing 0.41 ppm selenium. In one diet, about 3/4 of the selenium was from sodium selenite and in the second diet it was from selenium-enriched yeast. The apparent digestibility of yeast selenium was greater than for selenite (57% vs 51%). Selenium retention was 25% greater for yeast selenium than for selenite. Exercise increased urinary excretion of selenium more in the selenite group than in the yeast-fed group and plasma selenium remained higher in the selenium yeast group than in the selenite group. More research is needed to quantify the selenium requirements of horses at various intensities of exercise and to determine the effect of form of selenium on antioxidant status.