Today’s horse owners are very supplement-oriented. When a new supplement hits the market, many horse owners are tempted to use it if they think it will somehow improve their horses’ health. The problem occurs when a well-intentioned owner oversupplements a horse’s diet either by not knowing the horse’s current intake of minerals, or by thinking if a little is good, then more must be better. In the case of selenium, more can be toxic.

The upper safe total intake of selenium per day, based on the most recent National Research Council publication Nutrient Requirements of Horses, is 20 mg for an average 1,000-pound horse. Many horses get enough selenium (or even too much) just from their regular diet; for these horses, adding even as little as 5 mg selenium per day via supplements can cause mild signs of toxicity. 

Conversely, a selenium deficiency can be just as detrimental to the horse. It’s a fine line to walk, so before adding any type of supplement to a horse’s diet, it is important to understand what the horse needs, how much he needs, how much selenium he’s already getting in his diet, and how selenium is metabolized in his system