It’s never bad to be aware of things that may be toxic to your horse, but precautions can be carried too far if you don’t know the details. Yes, selenium is a potentially toxic mineral (they all are) but deficiency is much more common.
USGS Map of Soil Selenium in USA – Light = Low
An owner contacted me because she had a hay selenium assay done which showed she was feeding about 3 mg per day from the hay but she was horrified to realize the horse had been receiving another 2 mg per day on top of that. The bare minimum requirement for an average size horse is 1 to 2 mg/day.
However, bare minimum requirements are a long way away from toxic and even the 5 mg/day is considerably below any intake that would be dangerous. It is estimated an average size horse would need to take in at least 20 mg/day to be at risk for chronic toxicity – i.e. toxicity that takes weeks to months to show up. Acute toxicity takes over 1500 mg/day.
Naturally occurring chronic toxicity can occur with hays having over 5 ppm selenium ( = 50 mg in 22 lbs of hay) but this is rare. Natural toxicity is more likely in animals grazing on very high selenium soils where wild selenium accumulator plants, which contain several hundred ppm selenium, are growing. The highest selenium soils in the USA occur in pockets of Wyoming and South Dakota. These soils are high saline shales with an alkaline pH.
Acute toxicity causes a neurological derangement called “blind staggers” and is fatal. Chronic toxicity, “alkali disease”, causes loss of mane and tail hair plus disrupted hoof growth resulting in separation of the hoof capsule at the coronary band. Another symptom of selenium toxicity is a DMSO or garlic-like odor on the breath. Recovery from chronic toxicosis takes up to 10 months if the hooves slough off. Again, both of these are rare.
Severe selenium deficiency causes white muscle disease, with degeneration of skeletal and heart muscle. This is usually seen in foals but can also affect adults. Adult deficiency has also been linked to atrophy of the cheek muscles and swallowing difficulty. A link to tying-up has not been confirmed but wild animals with deficiency develop extensive muscle breakdown when they are chased in attempts to capture them. Many studies have shown how selenium protects the muscles, lungs and red blood cells from damage caused by free radical generation during exercise. Selenium also plays critical roles in the normal functioning of the immune system and is needed for generation of the active form of thyroid hormone, T3.
It’s important to realize that the selenium dose in supplemented grains or vitamin/mineral supplements, which is regulated by law, is not going to cause toxicity even if intake from hay is already adequate. There is a lot more to worry about from deficiency than toxicity. If you have any concerns regarding whether your horse’s selenium intake could be toxic, consult your veterinarian. Selenium status is easily checked with a blood test.
Eleanor Kellon, VMD