Selenium for Horses

The American Association of Equine Practitioners helps you decide whether your horse needs selenium supplements.

The American Association of Equine Practitioners helps you decide whether your horse needs selenium supplements.


I live in the Southeast and am worried about white muscle disease. How much selenium do my horses need to stay healthy?


Selenium is a trace mineral that is essential for cellular function in a horse’s body. Fortunately, large doses of selenium causing acute toxicity and death are uncommon in the horse. In fact, more often the opposite situation is problematic. Many areas of the United States produce selenium-deficient forage, including parts of the Pacific Northwest, the Great Lakes and down the Eastern Seaboard into Florida. The clinical syndrome that results from selenium (and Vitamin E) deficiency is called white muscle disease.

White muscle disease is a degenerative disease that affects skeletal and cardiac muscle in foals and other farm animals. Young, fast-growing animals nursing from dams fed a diet low in selenium and Vitamin E are commonly affected. The primary signs in young animals with white muscle disease are recumbency, fast heart rates, failure to suckle, difficulty swallowing and discolored (red to brown tinged) urine. Laboratory tests are available to diagnose selenium deficiency.

Learn more critical information about keeping your horse healthy with AQHA’s Common Horse Health Issues report. Download your copy today.

Selenium toxicity is more often a chronic condition. Certain “indicator” plants may reveal high levels of soil-based selenium (such as locoweed), and are common in areas such as Colorado and New Mexico. As previously mentioned, Florida is typically considered selenium deficient or adequate soil, depending on the region. The chronic signs of selenium toxicity are characterized by hair loss of the mane and tail, cracking of the hooves, and often signs of lameness, excess salivation and respiratory failure. Severe overdose of selenium can lead to death. In these severe cases, the signs of overdose may include a staggering gait, blindness, labored breathing, respiratory failure, collapse and muscle tremors. Selenium status in horses can be measured using serum, plasma or whole blood selenium levels.

If you are concerned about selenium levels, consult your local veterinarian for additional information on testing.

The FDA has set a daily recommended level of selenium for an “average” horse at a total of 3 mg per day. Many different types of feeds and supplements contain selenium. Take the time to read the labels and calculate how much, if any, selenium is contributing to your horse’s diet. Know what part of the country your hay comes from and test it on a regular basis. Consult often with your veterinarian or nutritionist when making changes to your horse’s diet.

By Dr. Amanda House, member of the American Association of Equine Practitioners

4 thoughts on “Selenium for Horses”

  1. We are among the “unfortunate” who have seen the terrible results of selenium poisoning. Several years ago we noticed a thin band of white forming just above the hoof on one of our mares. A short time later the same thing began to show up on two other mares. Soon the horses couldn’t walk and were in terrible pain. After many tests (and dollars) it was diagnosed. We had to have one of the mares put down when she lost all four hoofs. The two others were on medication and confined to small stalls to prevent moving about for several months. We were told the horses had developed an addicition for a type of milkweed (loco weed) found in one of our pastures. THe weed was “loaded” with selenium. Needless to say we get very jumpy when the weed is spotted or someone mentions selenium.

  2. Hi Melvin, white lines, as you describe, are also seen when grazing high protein pasture, eg fresh spring/autumn pasture which can reach 35% protein. One problem is that the a percentage of the protein is toxic or junk protein, the end result in severe cases is the hoof drops off.
    Fresh green pasture will contain from 4 to 5.5% nitrogen, protein is nitrogen x 6.25. High N results in a blood ammonia – increased histamines – restriction of arteries around the heart – blood pooling in the feet – inflamed sore feet – foundering. Other issues are – weight loss – bad temperament – abortion – foals weak or dead at birth. Always balance off excess protein.

    Bryan L McLeod

  3. My older horse, 28 yrs, lost most of his mane and tail hair. I had bought hay from a new source located around Oxford, CO. and been feeding it for around 6 weeks. I had the hay tested for selenium and it was 7.2; above 5 being toxic to horses. A month after stopping the feed, I had the vet take a blood sample. It showed the selenium level at 48.5 ug/dl; 110 ug/dl being the toxic level. The soil and water around Oxford, CO is notorious for high selenium. This I found out after the fact. My vet does not believe the high intake of selenium caused the mane and tail hair to fall out. He believes it is a bad fungus. My question is, would the selenium level fall within the month to an acceptable level? The horse is having no other signs of a fungus, no rubbing and no other patches of hair coming out.

  4. The water I used for my horses was contaminated probably by selenium. So far, two foals were born without genitalia and five were born with deformed legs. The mares have previously produced healthy babies, no deformed legs in 8 generations. Two different stallions and two different mares produced the colts that had no sheath, penis, scrotum or testicles. Has anyone else had birth defects due to selenium? Of course the water department does not want to reimburse me for the loss of the two stud colts. One colt was a metallic silver buckskin (TB) and the other colt was a black warmblood boy. Need to find as many people that have had birth defects caused by selenium. Of course, I have read Dick Francis’ book “Banker” that is why I thought it might be selenium. Or is there something else that causes birth defects? The ranch next to me had 10 babies born with birth defects. The only thing that we have in common is the water.

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