Ever since an overdose of selenium killed the polo ponies in Wellington, I’ve seen a lot of posts on forums from people concerned about the selenium levels in their horse’s diets. Selenium is an important and necessary part of a balanced diet. In fact, here in the Northeast, where our soil is selenium deficient, many people supplement with selenium and vitamin E. According to an article in The Horse, Good/Bad Effects of Antioxidants,

Selenium deficiency in young horses causes stiffness, listlessness, lung edema, and an increase in heart rate, respiration rate, and salivation. A deficiency in adult horses also causes stiffness as well as a decreased immune response and lowered fertility.

So finding the right balance is important. One of the best discussions I’ve seen about this is on the “Ask The Nutritionist” forum hosted by Dr. Juliet Getty. This forum is a great resource for horse owners as Dr. Getty will answer your questions at no charge! Here’s what she says about selenium:

The correct dosage for equine athletes such as these polo ponies is between 3 and 5 mg. Most horses get much less than this when being fed a commercial feed. Commercial feeds typically contain .5 to .6 ppm of selenium. So, 5 lbs, for example, provides between 1.14 and 1.36 mg of this mineral, which is well within the safe range. I like to limit selenium intake to 3 mg per day for most horses.

Hay and pasture can also provide selenium though soils in some regions of the country are low, particularly the northeast, the Ohio valley, Florida, and the northwestern portions of the U.S. It is always advisable to have hay and pasture tested, especially if there are rumors of high concentrations in certain areas.

Chronic selenium conditions can occur when selenium is consumed at higher-than-normal levels for a period of time. A condition known as alkali disease will result. Alkali disease is characterized by hair loss along the mane and tail and the hooves will crack around the coronary band. When too much selenium is present, it replaces the naturally existing sulfur found in keratin resulting in hoof tissue breakdown.

In general, the total amount of selenium should not exceed .6 mg per kg of feed. This should translate into no more than 3 mg per day for the average horse, and 5 mg per day for the athlete that is working intensely.

Vitamin E needs to be provided along with selenium since they work together as an “antioxidant team.” 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.