Technical reason: selenium is a key trace mineral in equine diets because it is a major component of glutathione peroxidase, which is an anti-oxidant enzyme, as well as several other enzymes.
- Selenium also has an important interaction with Vitamin E.
- Selenium and Vitamin E together are essential for both anti-oxidant benefits and for reducing the risk of certain muscle problems such as white muscle disease and exertional rhabdomyolysis syndrome.
Why you should care: Because selenium can also be toxic at higher intakes, it is also the only trace minerals regulated by the FDA. Most of the U.S. is selenium deficient in soils (and thus toxicity is not an issue in those areas), but there are some areas with high selenium in the soil where some plants accumulate unacceptable levels of selenium in forage and may cause chronic toxicity. If you aren’t sure about the levels where you live, ask area horse owners, farmers, or your local extension office.
How much is in the feed?
In the case of selenium, the label guarantee, which is “added” selenium, is listed in ppm = parts per million = milligrams per kilogram. The actual content will be slightly higher as there is naturally occurring selenium in the ingredients. The labeling requirement is based on added selenium per FDA guidelines.
- Premium horse feeds are commonly tagged at 0.6 ppm selenium, or 0.6 mg/kg of feed.
- This equals 0.273 mg per pound of feed. (0.6 mg/kg divided by 2.2 kg/lb. = 0.273 mg per pound of feed)
- Thus, if you feed 5 lbs per day of a 0.6 ppm selenium feed, you are providing 1.365 mg of selenium per day.
FDA requirement limits added selenium in feed to a maximum of 0.3 ppm concentration in total diet. This is why the maximum on complete feeds – feeds that include enough fiber to replace the hay/pasture portion of the diet – is 0.3 ppm.
On feeds which can be fed at a maximum of 50% of the diet (not that horse owners generally feed it that high), the limit is 0.6 ppm added selenium. Chronic selenium toxicity occurs at about 10X the FDA limit, so there is a pretty good safety margin.
How much does a horse actually need?
A horse’s basic daily requirement for selenium is 1-3 mg per day. Some chronic selenium symptoms might appear above 10-15 mg/head per day.
To figure out what your horse is consuming, have your hay/pasture supply tested, and then add the amount it is consuming from the hay, to the amount it is consuming from any feed & supplements, and you will know if you need to make any adjustments to your horse’s overall diet. Depending on your area, it may also be useful to consider intake from water sources.