Q: I am looking for a good map detailing selenium soil levels in MN. I have clients ask about selenium supplementation and I worry about over supplementation.
A: We do not have a current map of MN soil selenium (Se) content. However, in general, the eastern half of the state has a bit more (about 80% of the feedstuffs contain somewhat greater than 0.1 ppm) Se than the western half (only 50% of the feedstuffs contain somewhat greater than 0.1 ppm).
Because the amounts of Se present in MN grown feedstuffs do not reliably meet nutritional requirements, adding Se from a mineral source, such as a ration balancer, is recommended. Most ration balancers for horses contain sufficient Se to meet the horses requirements without contribution from the hay, if fed at recommended levels. The maximum tolerance level for Se is three times the amount of Se recommended, or in the case of an 1,100 pound horse, 3 mg of Se per day. However, Se toxicity can be a real issue if that 'safe range' is exceeded.
This can easily occur if multiple Se containing supplements are used. Since the Se isn't why people usually purchase the supplement, they fail to account for total Se in the equine diet. Consequently, we tend to see more toxicity due to over supplementation than horse lacking Se. Signs of Se toxicity include dullness, roughness of coat, loss of hair, hoof soreness, stiffness and lameness
Specifically, some 'immune or energy booster' type supplements contain very high levels of Se. When multiple supplements are combined with a ration balancer, the result can be Se toxicity. This is especially the case with some owners who have the philosophy that if a little supplement is good, more is even better.
The take home message is to carefully read all supplement ingredient lists, especially if feeding multiple supplements, and calculate total daily Se intake, which should include forage, grain, supplement, and ration balancer Se concentrations. Work with an equine nutritionist if you are uncomfortable with these calculations.
By Marcia Hathaway, PhD, University of Minnesota