Selenium is a trace mineral that plays an outsized role in your horse's health, primarily as a partner to vitamin E in the antioxidant defense system. Together, they protect cell membranes from oxidative damage caused by exercise, metabolism, and environmental stress. Selenium is a key component of the enzyme glutathione peroxidase, which neutralizes harmful peroxides before they can damage muscle fibers, red blood cells, and other tissues. What makes selenium uniquely challenging for horse owners is that soil selenium levels vary dramatically by region — large swaths of the United States, United Kingdom, New Zealand, and parts of Europe have selenium-deficient soils, meaning hay and pasture grown in those areas contain inadequate selenium. Conversely, parts of the western US have seleniferous soils where plants accumulate toxic levels. This regional variability means you genuinely cannot assume your horse is getting the right amount without knowing your local soil conditions or testing your hay. Selenium deficiency causes white muscle disease (nutritional myodegeneration), a painful and potentially fatal condition where skeletal and cardiac muscle fibers degenerate. It also impairs immune function and reproductive performance.
Your horse needs about 1 to 3 milligrams of selenium per day — roughly the weight of a few grains of sand. For a 500kg horse, the NRC recommends 1mg for maintenance and up to 3mg for working horses. Most commercial horse feeds include selenium, but if your horse is on a hay-and-pasture-only diet in a deficient region, a targeted supplement or selenium-fortified mineral block is essential. Have your hay tested or ask your vet about regional selenium status before supplementing.
0.0% of daily nutrient intake
Selenium makes up 0.0% of your horse's total daily nutritional requirements by weight. That's a tiny amount — but it matters.
Stiff gait or reluctance to move (white muscle disease), muscle soreness or wasting especially in the hindquarters and shoulders, poor immune response with frequent infections, reduced fertility in mares and stallions, weak or ill foals, poor hoof quality, and a generally dull coat. In foals, severe deficiency can cause sudden death from cardiac muscle degeneration.
Selenium toxicity is a real and serious concern — the margin between adequate and toxic is narrower for selenium than almost any other equine nutrient. Chronic excess causes alkali disease: hair loss from the mane and tail, cracking and separation of the hoof wall (horizontal rings and eventual sloughing), lameness, weight loss, and a garlic-like breath odor. Acute toxicity from accidental overdose causes blindness, difficulty breathing, and death. Never supplement selenium without knowing your horse's current intake.
| Life Stage | Size | Min | Max | Unit | Notes |
|---|---|---|---|---|---|
| Adult | — | 1 | 3 | mg | For a 500kg horse. Highly region-dependent — have hay tested or consult your vet about local selenium status. The toxic threshold is approximately 5mg/kg diet. |
| Pregnant / Nursing | — | 1.5 | 3 | mg | Pregnant mares need adequate selenium for foal health. Deficiency causes white muscle disease in newborn foals. Some breeding farms supplement mares in the last trimester. |
| Working / Active | — | 1.5 | 3 | mg | Working horses have increased antioxidant demands. Selenium works with vitamin E to protect muscles during exercise. |
Source: NRC 2007
Selenium and vitamin E form the cornerstone of the equine antioxidant defense system. Selenium-dependent glutathione peroxidase neutralizes peroxides inside cells, while vitamin E protects cell membranes from free radical damage. Together they prevent oxidative damage to muscles, red blood cells, and other tissues far more effectively than either mineral alone.
What this means: Always supplement selenium and vitamin E together rather than individually. Deficiency of either one increases the horse's susceptibility to muscle damage (tying up, white muscle disease). This is especially important for horses without pasture access, since fresh grass is the best natural source of both.
Excess dietary iron can impair selenium absorption and utilization. Since many equine forages are extremely high in iron, this antagonism can worsen selenium deficiency in horses already living in selenium-poor regions.
What this means: In regions with selenium-deficient soils, be especially vigilant about iron levels in your hay and water. High-iron water (common in well water) compounds the problem. If your hay is both low in selenium and high in iron, targeted selenium supplementation becomes even more important — consult your vet.
