We’re used to hearing precautions about feeding too much selenium, but most people know nothing more about it than too much of it can be toxic. Some know the horse needs it but not why or how much.
Selenium is beneficial and necessary. It’s a potent antioxidant, protecting the horse’s muscle from damage caused by end products of energy generation. This is why selenium is especially important for horses in heavy work and those with a history of tying up.
Selenium is also important for broodmares. Inadequate selenium intake in pregnant mares leads to a potentially fatal condition in the foals called white-muscle disease. Selenium deficiency is also associated with fertility problems, stillbirths and retained placenta.
Selenium is essential for the conversion of the largely inactive form of thyroid hormone, T4, into the metabolically functional form, T3, and is vital to the maintenance of a strong immune response.
Your horse’s hay may not include adequate selenium. Many areas of the United States are selenium-deficient, including the Pacific Northwest, the Great Lakes states, Eastern Seaboard, the Northeast and Florida. To learn about your area, consult your agricultural extension agent.
On the toxicity side of things, the National Research Council estimates that an intake in excess of 2 mg/kg dry matter of feed is unsafe for long-term feeding. For the average-size adult horse, this means feeding the horse 30 to 40 mg of selenium per day. Most vitamin-E-and-selenium supplements provide 1 mg of selenium per serving, while most grass hays provide 1 to 3 mg/day, if that.
The dose required to produce serious, acute toxicity in a one-time feeding is around 1,650 mg for a 1,100-lb. horse. Most equine diets come nowhere near what could be considered toxic and, on top of that, selenium is by and large poorly absorbed.
One study found that almost doubling the selenium content of the diet was unable to get the blood selenium level to budge. As a matter of fact, most equine diets are low in selenium, which can cause long-term health problems or at least a weakened immune response.
Because of the poor absorption of selenium from mineral supplements, the chance of giving too much is small. In fact, many people find that if they check the blood selenium level before and after supplementation there is often little improvement.
If your horse has a problem that may be selenium-deficiency-related, or you live in an area known to have selenium-deficient soils, don’t hesitate to supplement. You can safely supplement your horse’s diet at a rate of 1 mg per day of selenium, and many horses can benefit from 4 mg per day of supplemented selenium, depending upon their workload and their overall dietary level of selenium. Choose a supplement that also has vitamin E, an important cofactor. If you’re still worried, ask a veterinarian to look at your horse’s diet.