Selenium Overload - Look at Your Horse's Hooves and Hair!

By Juliet M. Getty, Ph.D.

Selenium – you know your horse needs it, but you may be overdoing it. It’s easy to do, with selenium added to just about every commercial feed or supplement. And how much is in the grass or hay that you’re feeding?  You can’t tell by looking at it -- selenium levels have no effect on the appearance, smell, or even the taste of the hay. 

Selenium content in soils varies according to region, alkalinity, and moisture conditions. In areas of drought, when the roots search deeper into the soil for water, they encounter more selenium. Certain parts of the US typically have more selenium than others.[i]  Maps like these can be useful, but be aware that even if your area is considered low or adequate, pockets of high-selenium soils may exist throughout the region. 

Problems arise when your forage is already adequate in selenium and you add a commercial feed or supplement that contains additional amounts of this mineral. Therefore, it is always advisable to have your hay and pasture tested. Your local county extension service may offer analysis services, or consider Equi-Analytical Laboratories for assistance (

Too much selenium affects hair and hooves

A consistent daily intake of elevated selenium levels will lead to a chronic condition known as alkali disease. Its signs are hair loss, a short tail or straggly mane. Hooves will also be affected – they become soft, chip easily, and cracks can develop around the coronary band. The reason? A change in keratin’s structure. Keratin, the protein found in hair and hoof tissue, relies on sulfur to maintain its integrity. High amounts of selenium can take sulfur’s place, leading to tissue breakdown.  

Selenium has a very narrow range of safety

Adequate selenium is needed for muscle, respiratory, and thyroid health. Too much, on the other hand, can lead to tissue damage; this can be avoided by maintaining a safe range. I recommend limiting total intake to no more than 0.6 ppm (mg/kg). Add up all the sources of selenium, including what naturally exists in your hay and/or pasture. A level between 1 and 3 mg per day is ideal for an 1100 lb (500 kg) adult horse that is not exercised heavily. Athletes or larger breeds, however, require slightly more -- between 3 and 5 mg.

Evaluate selenium when supplementing vitamin E

Vitamin E works with selenium as an “antioxidant team.” Selenium is part of the enzyme glutathione peroxidase, protecting the inside of the cell, while vitamin E guards the exterior cell membrane. Together, they neutralize damaging free radicals and therefore improve immune function. Therefore, many vitamin E supplements are packaged with selenium. Look closely at the dosing guidelines. For example, if your supplement offers 500 IU of vitamin E and 1 mg of selenium per ounce, and you want to supplement 2,000 IUs of vitamin E, you will also be feeding 4 mg of selenium! A better approach would be to first evaluate the selenium in your horse’s diet, and then decide on supplementation. You may find that all you need is a plain vitamin E supplement, with no selenium added.

Pay attention to labels and do the calculations

Most supplements or feeds that contain selenium will have its concentration shown on the label, making it easy to make the right choices for your horse. Generally, it will be listed as parts per million (ppm), which is the same as mg per kg (mg/kg) of feed. Here’s a simple way to calculate the number of mg of selenium you’re feeding:

Example #1: 4 lbs of feed that contains 0.6 ppm (mg/kg) of selenium.

  • Step 1: Convert 4 lbs to kg by dividing by 2.2:  4/2.2 = 1.82 kg
  • Step 2: Multiply 0.6 ppm (mg/kg) by kg of feed:  0.6 mg/kg X 1.82 kg = 1.09 mg of selenium

Example #2: 2 ounces of supplement contains 10 ppm (mg/kg) of selenium.   

  • Step 1: Convert ounces to kg by multiplying by .0284: 2 X .0284 = .057 kg
  • Step 2: Multiply 10 ppm (mg/kg) by kg of supplement: 10 mg/kg X .057 kg = 0.57 mg of selenium


 [i] Selenium in counties throughout the contiguous US:


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