Selenium for Horses - Safe Amounts & Signs of Deficiencies

“I just bought my first horse and he came with a SmartPak that has a multi-vitamin, vitamin E and selenium pellets, a hoof supplement and a joint supplement in it. I moved him into a new barn when I bought him and my barn owner is worried that I am giving my horse too many supplements. She said I have to be really careful especially with how much selenium I give because of selenium toxicity. She doesn’t want to feed him his SmartPak and now I am worried that continuing on his supplements may not be the best idea. My vet didn’t seem to know too much about supplements, so I’m not really sure what to do. Should I be worried?” – CF, Florida

Dear CF,
You’ve come to the right place! I get a LOT of selenium questions (and by “a LOT” I mean practically one a day) so can definitely provide some background on this trace mineral as well as some feeding recommendations that should have you, your barn manager and your veterinarian sleeping like babes.

As background, the body requires selenium for three main purposes: as an antioxidant, for immune function, and in thyroid hormone metabolism. Deficiencies have been reported in 46 of 50 states, mainly because the soil across much of the US is deficient in this mineral and therefore plants grown on it such as pasture and hays are also deficient. Signs of selenium deficiency include weakness, impaired movement, difficulty swallowing, respiratory distress, impaired cardiac function, and other nutritional-based muscle conditions such as white muscle disease in foals and tying up (rhabomyolysis) in adults.

Your veterinarian or county extension agent can confirm whether or not you live in a selenium-deficient area and therefore if your pasture or hay might be low in this mineral. If you really want to be exact, you could send in your hay for analysis and get back an actual number, knowing that it’s only accurate for that particular batch of hay. But unless you live in the Rocky Mountain or Great Plains region or get your hay from there, assume your horse’s forage is not providing excessive amounts of this nutrient. Your horse is probably getting more selenium from the combination of a multi-vitamin/mineral supplement and the Vitamin E & Selenium Pellets by SmartPak, so absolutely add up these two amounts. Since most joint and hoof supplements don’t contain selenium though, neither of these products should be adding to the total amount of this nutrient your horse is receiving from all his different feeds on a daily basis.

The sixth edition of the National Research Council (NRC) Nutrient Requirements for Horses estimates the horse’s daily selenium requirement at 0.1mg/kg of the total ration, which, when you do the math for an 1100 pound horse, comes out to about 1mg of selenium per day from all feedstuffs combined. However, keep in mind this is the minimum level required to prevent obvious signs of deficiency such as described above. One team of researchers showed that 3mg of selenium per day may provide benefits beyond those necessary to support life, such as improved immune status. More than 5mg of selenium per day is probably unnecessary since there appear to be no additional benefits but it is still well within the margin of safety. According to the NRC, the maximum tolerable concentration of selenium in the diet is 5 mg per kg of feed, or about 50 mg per day for an average-sized adult horse. Without excessive amounts of selenium in the hay or pasture though, this level would be very hard to reach even if you fed your horse several selenium-enriched grain and supplements.