Selenium - When and When Not to Supplement

By Juliet M. Getty, Ph.D.

Selenium, unlike most other minerals, has a very narrow range of safety. Too little, and your horse can develop muscle weakness, difficulty moving, and respiratory distress. If iodine is too high, along with low selenium intake, the thyroid gland can be damaged.

If selenium is consumed at a slightly high level for a period of time, the chronic condition known as alkali disease can occur. Alkali disease is characterized by hair loss along the mane and tail and the hooves will crack around the coronary band. This occurs because selenium replaces the naturally existing sulfur found in keratin, resulting in poor hair growth and hoof tissue breakdown.

In general, the total amount of selenium in the daily diet should be between 1 to 3 mg for the average sized horse at maintenance and up to 5 mg per day for the working horse.

Evaluate your pasture and/or hay

The first step in calculating your horse’s intake is to evaluate your pasture and/or hay. In general, low selenium levels exist in the northeast, the Ohio valley, Florida, the northwestern portions of the U.S., and parts of Canada. But pockets of high-selenium soils can exist throughout the country’s midsection. Therefore, it is always advisable to have your hay and pasture tested, especially if there is anecdotal evidence of high concentrations in your area. If at all possible, sending a sample in to a reputable lab such as Equi-Analytical Labs – will remove any uncertainty. 

Examine the amount of selenium in your horse’s feed and supplements

Add up the selenium content from all sources. If, after doing your calculations, you find that supplementation is needed, consider adding selenium to the diet. Selenium supplements are generally packaged with vitamin E. This is because selenium and vitamin E work together as an “antioxidant team.”

A word of caution…  if you are trying to increase vitamin E in your horse’s diet, be careful of vitamin E supplements that have added selenium, especially if your horse is already getting enough selenium from other sources. If you want to add more vitamin E to the diet, choose a supplement that only contains vitamin E.

Bottom line 

The more you know about your horse’s selenium intake, the better able you’ll be to make necessary adjustments and maintain his overall health.


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