Selenium is a mineral found in the soil. In some areas of North America, there is plenty of selenium left in the soil, while in many agricultural areas, the selenium in the soil is depleted. Horses ingest selenium when they drink water, graze on pasture and eat hay. The don't need large amounts, but it's a very necessary trace-mineral. A trace-mineral or micro-nutrient is only needed in very minute quantities, unlike the minerals like calcium or phosphorus, which are known to be macro-nutrients. So, it's important to get the right balance. Not all horses will need selenium added to their diets, and depending on where the horse lives, adding extra might be dangerous becuase the could be getting more than enough already.
Selenium has important functions in your horse's body. It is an anti-oxidant that in conjunction with Vitamin E, prevents free radicals from damaging otherwise healthy cells. Selenium is also important for healthy thyroid function.
Too Little Selenium
Too little selenium can lead to 'white muscle disease' which can damage all the muscles in the horse's body, including the heart which of course is made of muscle tissue. Streaks of white scar tissue form in the muscles, replacing healthy muscle tissue.
For horses living in the Great Lakes region, areas in the Pacific Northwest and the Eastern Seaboard, selenium deficiency may occur. Most affected will be foals and performance horses. Rapid muscle growth and repair requires the right balance of minerals, proteins, and fats. A deficit of selenium can impair the processes.
Remedying low selenium needs to be done with care to prevent toxicity. Because this mineral is needed in such small amounts, over and under supplementing are possible. This is why it is important to have your forage tested to confirm the levels of selenium available. It's never a good idea to guess at the amount of selenium your horse needs.
Too Much Selenium
High levels of selenium occur in the soils of New Mexico and Colorado and pockets of high selenium levels occur in the Dakotas, Wyoming, Montana, and Kansas. Certain weeds such as milk vetch, golden weed, purple locoweed and prince's plume grow in areas known to have high selenium levels in the soil. Horses may become poisoned from eating forage growing in high selenium soil or by drinking water with high levels of selenium. Most poisoning occurs in the spring and summer months when horses may be tempted to eat lush growths of weeds.
Toxicity can be sudden and 'acute', or it can be chronic. A horse that consumes a large amount of selenium over a short period may show nervousness and fear followed by depression, decreased appetite, diarrhea, fever, muscular weakness and respiratory distress—very similar to rabies symptoms. Death occurs within hours or days. Unfortunately, there is no treatment for acute or sudden selenium poisoning.
Chronic selenium poisoning, also called alkali disease or bobtail disease, occurs over a longer period of time. Symptoms of chronic, also known as sub-acute, selenium poisoning are thinning manes and tails, hoof cracks that may ooze, separation of the coronary band, lameness in all four legs, drooling and respiratory distress. Symptoms of severe poisoning include unsteady gait, blindness, respiratory distress or failure, twitching and an inability to stand.
Diagnosing and Selenium Poisoning
Tests of blood, hoof, hair and tissue samples can confirm a diagnosis of selenium poisoning. If selenium levels are too high, the horse must be fed a high protein diet and kept away from any high selenium water, feed or pasture. The horse may never recover completely from the poisoning. Veterinary and farrier care is important for the horse to become as sound as possible.
Preventing Selenium Imbalances
The FDA recommended daily intake of selenium for an average horse is 3mg. Most commercial horse feeds contain some selenium, so it's important to read the label to determine how much. If you are concerned that your horse is not getting the right balance of selenium, consult your veterinarian or an equine nutritionist.