While there is no such thing as “too much” of some vitamins and minerals (meaning the horse excretes out what he doesn’t need), selenium is not one of them: even a little more than needed can be toxic to horses.
By Sarah E. Coleman
An essential trace mineral important for respiratory, immune system, muscle and thyroid health in horses, selenium has a narrow margin of safety, meaning more isn’t always better. The FDA recommends that an average, 1,000-pound horse receive just 3 mg of selenium per day. Horses can get selenium from the grass, hay and commercial feed they ingest.
While it’s easy to see the amount of selenium in commercial feeds and supplements that have clearly labeled tags, what your horse ingests from grass and hay can be more difficult to determine. Many areas of the country have soils that are selenium deficient, meaning that the crops grown on them are also low in selenium, so supplementation to be sure a horse is getting the adequate amount may be needed. The majority of horses in the United States don’t get enough selenium from forage along.
However, the converse can also be true: There are also areas of the country where selenium content in soil (and therefore hay) is quite high. Specific types of plants grown on this soil retain amounts of selenium that are toxic to horses. Thankfully, given adequate pasture, the majority of horses will avoid eating these plants. The amount of selenium in an area depends on what type of rock formed the soil.
So how can you know how much selenium your horse is eating? You can get your pastures analyzed for selenium content by a county extension agent. Once you know if your soils are high in selenium or deficient in selenium, as well as how much selenium he is receiving from a commercial feed (if he eats one at the recommended feeding rate), you can determine if you need to supplement his selenium intake or not.
You can also pull blood or serum to test your horse’s selenium levels. It’s important to note that the selenium concentrations change rapidly with selenium intake when testing serum, but a whole-blood test can remain elevated for up to 9 months after supplementation has ended.
A map to the selenium content in soil by county can be found here: https://mrdata.usgs.gov/geochem/doc/averages/se/usa.html
How Does Selenium Toxicity Present?
Many of us remember the deaths of 21 polo ponies in 2009 that resulted from an overdose of selenium in the compounded medication they were receiving; while this is an extreme case, there are certain things that can indicate a horse is receiving too much selenium:
Acute selenium poisoning:
- Labored breathing
- Muscle tremors
- Gait abnormalities
- Garlic-smelling breath
Chronic selenium toxicity is more common and can take place over weeks or months. This type of toxicity can result in:
- Excessive salivation
- Abdominal pain
How Can I Tell If My Horse Has a Selenium Deficiency?
A selenium deficiency can be difficult to determine. Many horses with selenium deficiencies have:
- Poor hair coat
- Work intolerance
- Muscle inflammation
Selenium deficiency may also cause white muscle disease, cataracts, retained placentas and stunt growth.
While only necessary in small amounts, selenium is integral to a horse’s health and wellbeing.