Q&A: Vitamin E in Horses — How Common Deficiency Really Is and Where Selenium Fits In

Question: I would be interested to learn the percentage of horses tested for vitamin E that are deficient, and what their diets typically look like. I supplement year-round.

Multiple studies show that a significant percentage of horses test deficient or borderline. The challenge is that vitamin E status depends heavily on management, forage type, season, and individual absorption, rather than feed labels or soil conditions alone.

Why is Vitamin E important?

Vitamin E is a critical antioxidant that helps protect nerve and muscle cells from oxidative damage. In horses, adequate vitamin E levels are especially important for normal neuromuscular function, coordination, and muscle strength. It plays a key role in supporting healthy nerve signaling and muscle repair, which is why deficiencies are often associated with muscle weakness, soreness, poor topline development, or abnormal neurologic signs.

Because horses do not efficiently store vitamin E, consistent daily supplementation is often necessary, particullary horses who can't have access to lush pasture. Providing appropriate vitamin E support helps create an optimal environment for nerve healing, muscle development, and overall performance.

Why do horses become vitamin E deficient?

1. Lack of fresh pasture

Vitamin E is not derived from soil. Plants synthesize vitamin E internally during photosynthesis, which means fresh green pasture is the richest natural source. Once forage is cut and stored as hay, vitamin E content declines rapidly. Horses relying primarily on hay, even high-quality hay, are therefore at significantly higher risk of deficiency

2. Seasonal effects

Vitamin E status often follows a seasonal pattern, with the lowest blood levels seen in late winter and early spring. This is when stored forage has been fed the longest and fresh pasture is unavailable, which explains why many deficiencies are detected at this time of year

3. Inadequate or ineffective supplementation

In the large field study testing assessing management practices and vitamin E levels reported many horses were receiving vitamin E supplements, but often at low doses (<500 IU/day). More importantly in the same study, over 20% of horses with seemingly adequate pasture access and supplementation still tested low, suggesting that intake consistency, supplement form, storage, or individual absorption can significantly affect blood levels.

4. Increased demand or reduced absorption

Horses in work, growing horses, horses with neurologic or muscle conditions, and horses experiencing illness or inflammation may have higher oxidative stress, increasing their vitamin E requirements. Gastrointestinal health and individual metabolism also influence absorption and utilization.

Where selenium fits in

Vitamin E and selenium function together as part of the body’s antioxidant defense system. Vitamin E protects cell membranes from oxidative damage, while selenium is required to neutralize free radicals after oxidative stress occurs. Selenium status plays a critical modifying role in how clinically significant a vitamin e deficiency becomes.

Unlike vitamin E, selenium is soil-dependent. Horses in selenium-deficient regions may have adequate vitamin E intake but still experience compromised antioxidant protection if selenium levels are low. Because deficiencies or imbalances in either nutrient increase oxidative stress in muscle and nervous tissue. vitamin E and selenium are best evaluated together, especially in horses with poor performance, muscle soreness, neurologic signs, or delayed recovery.

Why testing matters

The only way to know a horse’s vitamin E status is through blood testing, ideally measured as plasma alpha-tocopherol. Many veterinarians recommend testing in late winter or early spring, when deficiency is most likely to appear in hay-based systems.

A simple vitamin E & selenium decision tree for horse owners

1. Does your horse have regular access to fresh green pasture?•

• Yes, daily grazing for much of the year → Go to step 2

• No, hay-based or restricted grazing → High risk → Proceed to testing

2. Is your horse in work, growing, aging, or recovering from injury or illness?

• Yes → Increased demand → Testing recommended

• No → Go to step 3

3. Is your horse receiving vitamin E supplementation year-round?

• No → Supplementation recommended → Consider baseline testing

• Yes → Go to step 4

4. Has your horse’s vitamin E level been tested in the past 6–12 months?

• No → Test, especially late winter/early spring

• Yes → Adjust supplementation based on results

5. Selenium status known?

• No → Test selenium alongside vitamin E

• Yes → Ensure intake is appropriate, not excessive

Take-home message

Vitamin E deficiency is common, often silent, and strongly influenced by management and season. Because even well-managed horses can test low, targeted testing and individualized supplementation remain the most reliable way to support long-term muscle, nerve, and performance health.

If you’re unsure whether your horse’s current program is truly meeting their needs, a simple blood test can provide clarity and help prevent issues before they become performance-limiting or clinical.

Sources

1. Pitel, M. O., McKenzie, E. C., Johns, J. L., & Stuart, R. L. (2020). Influence of specific management practices on blood selenium, vitamin E, and beta-carotene concentrations in horses and risk of nutritional deficiency. Journal of Veterinary Internal Medicine, 34(5), 2132–2141.

   
   

2. Pagan, J. D., Kane, E., & Nash, D. (2005). Form and source of tocopherol affects vitamin E status in Thoroughbred horses. Pferdeheilkunde: Equine Medicine, 21, 101–102.

   
   

3. Fagan, M. M., Harris, P., Adams, A., Pazdro, R., Krotky, A., Call, J., & Duberstein, K. J. (2020). Form of vitamin E supplementation affects oxidative and inflammatory response in exercising horses. Journal of Equine Veterinary Science, 91, 103104.