Importance of Selenium
by H.F. Hintz

Selenium is the most interesting of all minerals. It was first known for its toxicity, then turned out to be an essential nutrient that is required in small amounts by humans, horses, and other animals. More recently the mineral has received much attention as a possible anticarcinogen for humans. Due to its antioxidant and anticarcinogen activity, selenium may have a protective role in at least 50 diseases of humans.

Toxicity
Chronic selenium toxicity of horses, also called "alkali disease," is characterized by hair loss from the mane and tail, sloughing of hooves, joint erosion, and lameness.

Excessive intake of selenium may result from consuming plant material raised in areas where the soil contains a high level of selenium. The areas with high selenium soils are west of the Mississippi River. Prior to the 1930s, when alkali disease was first proven to be caused by selenium toxicity and the high selenium areas were identified, many horses and cattle died of the disease. Dr. Merl Raisbeck of the Department of Veterinary Science at the University of Wyoming reported that each year before 1930 a vast number of animal deaths in Wyoming were attributed to alkali disease.

Although the number of cases of selenium toxicosis has greatly decreased since 1940, toxicity is still reported. During a recent three-year study in Wyoming Dr. Raisbeck found four substantiated cases of selenium toxicity of horses due to the ingestion of high selenium forage. A few recent cases have also been reported in Colorado and Iowa.

Cases of toxicity due to selenium in the water, excessive use of supplements, or environmental contamination are occasionally reported. The dangers of excessive selenium cannot be ignored.

Requirement
Selenium was first shown to be a required nutrient for laboratory animals in the 1950s. Shortly thereafter it was established as a help in preventing muscular degeneration in farm animals. Prior to 1950 white muscle disease was common in calves, lambs, and foals raised in areas where the soil is lacking in selenium. Selenium supplementation and injections have saved the lives of millions of animals.

Although severe selenium deficiency is usually much more common in young animals, it may also occur in older animals. Selenium deficiency may impair reproductive performance and decrease resistance to disease.

In the 1960s researchers discovered that selenium can help prevent tying-up disease in horses, but does not prevent all cases. Since selenium has been widely added to horse feeds, the number of tying-up cases that respond to selenium has been greatly reduced, and tying-up continues to be a problem.

Map: Relative Selenium Contents of Crops in Relation to Animal Requirements

Identifying Deficiency
If you live in an area where the soil is known to be lacking in selenium and you feed your horses only home grown feeds, you should use selenium supplementation.

Select a supplement that provides 1 to 3 mg of selenium per day or use a trace mineral salt that is fortified with selenium. Salt products designed for horses contain 30 to 90 ppm selenium. You may add the salt to the feed or offer it free choice.

Free choice feeding of selenized-salt has been widely used for sheep and cattle because supplementing grazing animals is often difficult. Fewer studies have been conducted with horses, but salt appears to be a reasonable source of selenium, particularly for horses that graze. If you use a commercial grain mixture fortified with selenium, you probably don't need a supplement.

If you are not sure whether or not your horses are getting enough selenium, and the horses have muscle problems, selenium may be involved. Your veterinarian can take a blood sample and have it analyzed for selenium or glutathione peroxidase, an enzyme that requires selenium.

Selenium is needed for a horse's normal muscle function and health. Three adages apply: An ounce of prevention is worth a pound of cure—selenium supplementation need not be expensive, but if selenium is needed and not provided, the cost can be great. Use moderation in all things, and good things come in small packages. The horse needs 1 to 3 mg of selenium per day, but 50 mg per day may cause toxicosis.

One mg is 0.0000022 lb. Thus you must exercise care when using selenium supplements. The amount of supplement you add must of course be greater than one mg, because the selenium in the supplement is diluted with carrier. Read the label carefully and add adequate but not excessive amounts to keep your horses healthy.

H.F. Hintz is a Professor of Animal Science at Cornell University in Ithaca, New York, where he teaches equine nutrition and conducts nutrition research. This article appeared in the Holiday 1999 issue of Rural Heritage.