Selenium for Horses


Selenium was something I’d never really spent a tremendous amount of time thinking about until taking a couple of equine nutrition courses a few years ago.  And I still can’t say I spend a whole lot of time thinking about selenium. . .  but, it is something I feel is blog post-worthy, so here goes. . .

Selenium is quite possibly the trickiest component of your horse’s diet.  A trace mineral that acts as a powerful antioxidant important in many bodily functions, selenium is definitely needed by the horse, but only in very small amounts.  A deficiency or an excess of selenium can mean major problems for your horse.  It can even be deadly–remember the story from 2009 when 21 polo ponies’ deaths were blamed on a selenium overdose?

The National Research Council advises just 1 mg/ day of Selenium for a 1000 lb. horse, but working horses and pregnant mares will likely require more (up to 2-4 mg/day.)

Selenium is found in many types of grasses and hays and is also added to many commercial feeds and supplements, but it’s possible that those amounts still may not be enough.

Take a look at this map showing the naturally occurring levels of selenium in soils and forages across the U.S. (but keep in mind selenium-containing soil additives may drastically change these levels. . .)  In general, alkaline soils usually have adequate or high selenium levels whereas acidic soils tend to have lower levels.

So with all this confusion about selenium, how does one know if their horse is receiving adequate amounts of the trace mineral?  The best way to know is a ‘whole blood selenium test’ performed by your vet.  While many mineral levels aren’t reflected accurately by blood tests, selenium is.  Good to know, huh?

Selenium deficiencies are more likely than toxicities. But if a horse is fed a variety of feeds and/or supplements containing selenium on top of eating forage high in selenium, a toxicity can occur.  Here are some symptoms of both:


  • muscle weakness (especially in foals) or soreness
  • Performance loss
  • Poor appetite
  • Dull appearance
  • Difficulty swallowing
  • Below-normal immune function
  • Reproductive problems
  • Late-term abortion or foal death, retained placenta in pregnant mares

Chronic Toxicity:

  • Hair loss (especially mane and tail)
  • Rings in the hoof, just under coronary band
  • Garlic-smelling breath
  • Joint stiffness
  • Foot pain possibly followed by sloughing of entire hoof

Acute Toxicity:

  • Patchy sweating
  • Colic
  • Diarrhea
  • Increased heart and respiration rates

Remember, if you suspect a deficiency or toxicity of selenium in your horse (or if you’re just a worry wort!)–have your vet perform a whole blood selenium test.

Here are two good articles if you’re interested in reading more about selenium:

Safe Vitamin E and Selenium Intakes

A Necessary Poison





Hi! My name is Casie Bazay. I'm a mom, a freelance writer, and a certified equine acupressure practitioner.

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