Introduction

Hay is the foundation of many excellent horse diets.

It provides forage, fiber, chewing time, digestive support, and the steady intake pattern horses are designed for. For many horses, hay is not a compromise. It is essential management.

But hay is not the same as fresh pasture.

One of the biggest nutritional differences is Vitamin E. Fresh green forage is one of the horse’s most natural sources of Vitamin E. Once forage is cut, dried, baled, transported, and stored, its Vitamin E content begins to decline.

This matters because Vitamin E is a fat-soluble antioxidant involved in normal muscle function, nerve health, immune function, and protection of cell membranes from oxidative damage. Horses cannot manufacture enough Vitamin E on their own to meet their needs. It must come from the diet.

Key Takeaway: Hay can be high quality and still be low in Vitamin E. The issue is not that hay is “bad.” The issue is that Vitamin E is vulnerable to cutting, drying, storage, oxygen, light, heat, and time.

This article explains why hay loses Vitamin E, why the rate of loss varies, which horses are most likely to need additional support, and how owners can evaluate Vitamin E status without relying on guesswork.

Why Vitamin E Matters for Horses

Vitamin E is a fat-soluble antioxidant. In horses, the form most often discussed is alpha-tocopherol, because it is the primary form measured in blood and the form most commonly used to evaluate Vitamin E status.

Antioxidants help protect cells from oxidative damage. This is especially important in tissues with high metabolic demand, including muscles and nerves.

Vitamin E is commonly discussed in relation to:

• normal muscle function
• normal nerve function
• neuromuscular health
• immune function
• exercise recovery
• antioxidant balance
• support for horses with limited pasture access

Vitamin E does not work alone. Selenium, vitamin C, glutathione systems, enzymes, and other nutrients all contribute to antioxidant defense. But Vitamin E deserves special attention because fresh forage is such an important natural source, and many modern horses have little or no access to it.

Did You Know? Vitamin E is not produced in meaningful amounts by the horse. If fresh pasture is limited, the diet has to provide enough through hay, fortified feed, or supplementation.

For a deeper look at what can happen when status remains low, see Common Complications of Vitamin E Deficiency in Horses.

Pasture vs. Hay: Why the Difference Matters

Fresh green pasture is biologically active plant material. It contains natural Vitamin E, plant pigments, moisture, and fragile nutrients that are present while the plant is alive and growing.

Hay is preserved forage. To become hay, forage is cut, dried, cured, baled, moved, and stored. That process is useful because it lets us feed horses through winter, drought, travel, stall rest, dry-lot management, and seasonal pasture gaps. But preservation changes nutrient composition.

University extension guidance commonly notes that Vitamins A and E are found at high levels in fresh green forage and newly harvested hay, but that these vitamins decline as hay is stored.

Myth vs. Fact:

Myth: “Good hay gives a horse everything pasture does.”

Fact: Good hay is valuable, but it is not nutritionally identical to fresh pasture. Vitamin E is one of the nutrients most affected by conservation and storage.

What Happens to Vitamin E When Forage Is Cut?

Vitamin E is sensitive to oxidation. That is part of what makes it useful inside the body, but it also makes it vulnerable in harvested forage.

Once forage is cut, the plant is no longer maintaining itself as living tissue. It is exposed to oxygen, sunlight, heat, mechanical handling, drying, and time. Those factors reduce alpha-tocopherol content before the hay ever reaches the feed room.

Loss can occur during:

• mowing
• field drying
• tedding or raking
• baling
• transport
• storage
• long-term exposure to heat, oxygen, light, or moisture

This is why two hays that look similar can differ in Vitamin E content. The plant species matter, but so do harvest timing, weather, drying conditions, bale type, storage environment, and storage duration.

Key Takeaway: Vitamin E loss is not a single event. It is a process that can begin during haymaking and continues during storage.

Why Vitamin E Loss in Hay Varies

Horse owners often want one simple number: “How much Vitamin E does hay lose?”

The honest answer is that it varies.

Forage research shows that alpha-tocopherol content differs by plant species, maturity, cutting, field conditions, conservation method, drying time, bale type, storage environment, and length of storage. That means it is more accurate to think in terms of risk factors than a universal percentage.

Quick Tip: Instead of asking whether hay has “some” Vitamin E, ask whether your horse’s total diet is maintaining adequate blood alpha-tocopherol status.

How Fast Does Hay Lose Vitamin E?

Vitamin E loss can begin soon after cutting and continue during storage. Classic forage research in alfalfa hay documented tocopherol losses during storage, and later forage studies have reinforced that conserved forage vitamin content is variable.

Some popular summaries describe dramatic early losses in hay, but the exact percentage depends heavily on the forage and storage conditions. A more responsible way to state the science is this:

Key Takeaway: Hay commonly contains less Vitamin E than fresh pasture, and stored hay generally becomes less reliable as a Vitamin E source over time. The exact loss is not the same for every hay.

For horse owners, the practical conclusion is more important than the exact number. A hay-based diet should not be assumed to provide the same Vitamin E exposure as grazing fresh, green pasture.

This is especially true when hay is:

• stored for several months
• stored through winter
• older than one season
• sun-bleached
• rained on during curing
• stored in hot or humid conditions
• visibly faded, dusty, or weathered

None of these automatically means the hay is unsafe or poor quality. They simply make Vitamin E less predictable.

Fresh Hay vs. Old Hay

Newly harvested hay generally starts closer to the fresh forage it came from. Older hay has had more time for nutrient changes to occur.

This does not mean freshly baled hay should always be fed immediately. Hay also needs to be cured and stored safely. Poorly cured hay can heat, mold, or create serious health risks. Forage safety comes first.

But once hay is properly cured and stored, time still matters. A horse eating hay harvested two weeks ago may receive a different Vitamin E contribution than a horse eating hay harvested twelve months ago, even if both hays are the same species and from the same farm.

Practical Example: A horse on pasture from May through October may maintain Vitamin E status naturally during the grazing season. That same horse on stored hay from November through April may need a different plan.

Haylage, Silage, and Other Conserved Forages

Not all conserved forage behaves exactly like dry hay.

Haylage and silage are preserved at higher moisture levels than hay and rely on fermentation and anaerobic storage. Studies in conserved forages show that alpha-tocopherol and beta-carotene content can vary widely depending on conservation method, dry matter level, fermentation quality, plant species, bale type, and storage conditions.

This does not mean haylage or silage is automatically better or worse for Vitamin E. It means conserved forage quality is complex.

For horses, haylage and silage also introduce separate management questions, including fermentation quality, hygienic quality, botulism risk management, storage integrity, bale handling, and suitability for the individual horse.

Veterinarian Insight: If feeding haylage or silage to horses, forage hygiene and storage quality matter as much as nutrient content. Do not choose conserved forage based on Vitamin E alone.

Which Horses Are Most at Risk for Low Vitamin E on Hay?

The horses most likely to need Vitamin E attention are those with limited access to fresh green pasture.

This includes many normal, well-managed horses. Limited pasture does not mean poor care. Horses may be kept off grass for sound reasons: metabolic disease, laminitis risk, winter climate, drought, boarding limitations, mud management, stall rest, travel, rehabilitation, or dry-lot turnout.

Higher-risk situations include:

• hay-only diets
• winter feeding programs
• dry-lot management
• metabolic horses restricted from grass
• horses stalled for long periods
• horses in regular work with limited pasture
• older horses with muscle loss or weakness
• growing horses with limited pasture access
• broodmares and foals on hay-based programs
• horses with known low blood alpha-tocopherol
• horses with neurologic or neuromuscular concerns under veterinary care

Key Takeaway: The more your horse’s diet depends on hay instead of fresh green pasture, the more Vitamin E deserves attention.

For a broader practical decision guide, see Does My Horse Need Vitamin E? What Hay and Pasture Might Be Missing.

Can a Hay Analysis Tell You Vitamin E Content?

A standard hay analysis can be a useful tools in equine nutrition with caveats. It can help owners understand calories, protein, fiber, minerals, sugar, starch, and overall forage suitability of the tested sample.

But routine hay analyses do not always include Vitamin E. Many standard forage reports focus on energy, crude protein, fiber fractions, minerals, and sometimes nonstructural carbohydrates. Vitamin E testing may require a specialized assay and careful sample handling because of the rapid oxidation process.

Even when Vitamin E is measured, interpretation can be challenging because hay is not uniform. One flake may differ from another. One bale may differ from the next. Storage conditions can continue changing nutrient content after the test.

That does not make forage testing unhelpful. It means Vitamin E status is often better evaluated through the horse.

Quick Tip: Use hay analysis to understand the base diet. Use blood alpha-tocopherol testing to understand whether the horse’s Vitamin E status is adequate.

Testing Your Horse’s Vitamin E Status

The most practical way to evaluate Vitamin E status is a blood test measuring serum or plasma alpha-tocopherol.

UC Davis lists alpha-tocopherol concentrations greater than 2 micrograms per milliliter as adequate and less than 2 micrograms per milliliter as deficient. Your veterinarian may interpret results in context based on the horse’s clinical signs, diet, age, workload, pregnancy status, medical history, or neurologic concerns.

Testing is especially useful when:

• the horse has little or no pasture access
• the horse has been on stored hay for months
• the horse has unexplained muscle loss or weakness
• the horse has neurologic signs or incoordination
• the horse is in heavy work with limited pasture
• the horse has previously tested low
• you are supplementing but do not know whether the horse is responding
• you are deciding between maintenance supplementation and a more targeted protocol

Testing can also prevent unnecessary guesswork. Some horses on hay test low. Some test adequate because they receive enough through pasture, fortified feed, ration balancer, or supplementation. The blood result helps clarify which horse you are dealing with.

Veterinarian Insight: If a horse has neurologic signs, severe weakness, rapid muscle loss, difficulty rising, or abnormal gait, do not rely on supplementation alone. Call your veterinarian.

When Supplementation May Make Sense

Vitamin E supplementation may be appropriate when hay and fortified feed are unlikely to provide or are not providing enough Vitamin E to maintain adequate blood status.

Common scenarios include:

• a horse on hay with little or no fresh pasture
• a horse in winter management
• a metabolic horse restricted from grass
• a performance horse on a hay-based diet
• a horse with low serum or plasma alpha-tocopherol
• a horse with veterinary-managed neuromuscular concerns
• a horse not receiving the full serving of a fortified feed

The National Research Council minimum recommendation commonly cited for Vitamin E in horses is 1 to 2 IU per kilogram of body weight per day, depending on class and workload. For a 500 kg horse, that is often discussed as roughly 500 to 1,000 IU per day as a baseline requirement.

However, those recommendations do not fully answer the practical question for every hay-fed horse. They do not distinguish natural from synthetic Vitamin E, account for all delivery forms, or tell you whether a specific horse’s blood status is adequate.

Key Takeaway: Requirements are population estimates. Blood testing tells you how the individual horse is doing.

Choosing a Vitamin E Source for Hay-Fed Horses

When choosing Vitamin E for a hay-fed horse, the form matters.

Natural Vitamin E is typically listed as d-alpha-tocopherol, d-alpha-tocopheryl acetate, or RRR-alpha-tocopherol. Synthetic Vitamin E is typically listed as dl-alpha-tocopherol, dl-alpha-tocopheryl acetate, or all-rac-alpha-tocopherol.

Natural Vitamin E is generally considered more bioavailable per IU than synthetic Vitamin E. Delivery form also matters. Powders, oils, pellets, micellized liquids, and water-dispersible products may behave differently in the horse.

For more detail, read Natural vs. Synthetic Vitamin E for Horses and Vitamin E Delivery Forms for Horses.

Quick Tip: Do not choose Vitamin E by IU alone. Form, source, delivery system, storage, and blood response all matter.

Common Misunderstandings About Vitamin E in Hay

Misunderstanding 1: “If the hay is green, it has plenty of Vitamin E.”

Color can offer clues about freshness and curing, but it is not a reliable Vitamin E measurement. A green hay may retain more fragile nutrients than faded hay, but you cannot confirm Vitamin E status by appearance alone.

Misunderstanding 2: “A hay-based diet is automatically deficient.”

Not necessarily. Some hay-fed horses maintain adequate Vitamin E status through fortified feed, ration balancers, supplements, or partial pasture access. The point is not to assume deficiency. The point is to evaluate risk and test when appropriate.

Misunderstanding 3: “Any fortified grain covers Vitamin E.”

Fortified feeds can help, but only if fed at the intended serving rate. If a horse receives a small handful of grain as a carrier, the Vitamin E contribution may be much lower than owners expect.

Misunderstanding 4: “Synthetic and natural Vitamin E are interchangeable.”

They both provide Vitamin E activity, but they are not identical. Natural Vitamin E is generally more bioavailable per IU. When correcting low blood status, source and delivery form become more important.

Misunderstanding 5: “More Vitamin E is always better.”

The goal is adequate status, not unlimited supplementation. High-dose supplementation should be guided by need, form, total diet, and veterinary input when medical concerns are present.

Myth vs. Fact: Myth: “Hay-fed horses always need the same Vitamin E dose.” Fact: The right amount depends on pasture access, hay age, fortified feed intake, workload, health status, form of Vitamin E, and blood response.

Practical Checklist for Hay-Fed Horses

If your horse relies heavily on hay, use this checklist to decide whether Vitamin E deserves closer attention.

• Does your horse have daily access to fresh green pasture?
• Is your horse on hay-only or mostly hay management?
• Is your horse on dry lot, stall rest, or grass restriction?
• Is the hay newly harvested or stored for several months?
• Is your horse eating the full recommended amount of a fortified feed or ration balancer?
• Does your horse already receive Vitamin E from another supplement?
• Is that Vitamin E natural or synthetic?
• Does the supplement include selenium?
• Has your horse’s serum or plasma alpha-tocopherol ever been tested?
• Is your horse showing muscle loss, weakness, poor performance, or neurologic signs?

If several of these questions raise concern, Vitamin E status is worth discussing with your veterinarian or nutritionist.

Practical Example: A metabolic horse on dry lot may be well managed from a sugar and starch standpoint, but that same management removes fresh pasture as a Vitamin E source. The solution is not necessarily grass access. It may be targeted Vitamin E support.

Summary

Hay is a vital part of equine nutrition, but it is not nutritionally identical to fresh pasture.

Fresh green forage is one of the horse’s most natural sources of Vitamin E. Once forage is cut, dried, baled, and stored, Vitamin E begins to decline. The exact rate of loss varies by forage species, maturity, cutting, weather, drying time, storage environment, oxygen exposure, light, heat, moisture, and time.

For horses with little or no pasture access, hay may not provide enough Vitamin E to maintain adequate blood alpha-tocopherol status. This is especially relevant for hay-only diets, winter feeding, dry-lot management, grass-restricted metabolic horses, performance horses, growing horses, broodmares, older horses, and horses with neuromuscular concerns.

The most practical way to evaluate status is serum or plasma alpha-tocopherol testing. Supplementation may be appropriate, but the form matters. Natural Vitamin E is generally more bioavailable than synthetic Vitamin E, and different delivery forms may produce different responses.

Bottom Line

Hay can be excellent forage and still be an unreliable Vitamin E source. If your horse has limited pasture access, Vitamin E should be evaluated as part of the overall diet rather than assumed to be covered by hay alone.

The best approach is not fear-based. It is practical:

• recognize that hay loses Vitamin E after cutting and during storage
• review pasture access and total diet
• calculate what fortified feed or supplements already provide
• choose a Vitamin E form that matches the horse’s need
• test blood alpha-tocopherol when status matters
• work with your veterinarian if clinical signs are present

For many horses, Vitamin E supplementation is not about treating a problem. It is about replacing a nutrient that fresh pasture would normally provide.

FAQ: Vitamin E Loss in Hay for Horses

1. Does hay have Vitamin E?

Hay can contain some Vitamin E, especially when it is newly harvested, green, well cured, and stored properly. However, hay generally contains less Vitamin E than fresh pasture because Vitamin E declines after forage is cut, dried, baled, and stored. The amount remaining depends on forage species, maturity, harvest conditions, drying time, storage temperature, light, oxygen exposure, moisture, and age. For horse owners, the practical point is that hay should not be assumed to provide the same Vitamin E exposure as fresh green pasture, especially after months of storage.

2. How much Vitamin E does hay lose during storage?

There is no single percentage that applies to all hay. Research has shown that tocopherol levels can decline during storage, but the rate varies widely depending on forage type, moisture, harvest conditions, and storage environment. Some hays may retain more Vitamin E than others, especially when harvested and stored well. Older, sun-bleached, weathered, or poorly stored hay is less reliable. Rather than relying on a universal loss estimate, owners should evaluate risk factors and use blood alpha-tocopherol testing when they need to know whether the horse’s Vitamin E status is adequate.

3. Why does hay lose Vitamin E?

Vitamin E is sensitive to oxidation. Once forage is cut, the plant is no longer living tissue, and fragile nutrients are exposed to oxygen, light, heat, drying, mechanical handling, and time. Haymaking is necessary and useful, but the process changes nutrient composition. Vitamin E loss can occur during mowing, field drying, raking, baling, transport, and storage. Storage conditions matter as well. Heat, humidity, sunlight, air exposure, and long storage periods can make Vitamin E less reliable. This is why hay-fed horses may need additional Vitamin E even when their forage quality is otherwise good.

4. Can horses get enough Vitamin E from hay alone?

Some horses may maintain adequate Vitamin E status on hay-based diets if they also receive enough from fortified feed, ration balancer, supplements, or partial pasture access. But many hay-only diets are not reliable Vitamin E sources, especially when hay has been stored for months. The answer depends on the individual horse and the whole diet. A horse on fresh pasture is in a different situation than a horse on stored hay in winter. The best way to know is to measure serum or plasma alpha-tocopherol and review the diet with your veterinarian or nutritionist.

5. Do horses need Vitamin E in winter?

Many horses need closer Vitamin E attention in winter because fresh pasture is limited or unavailable and hay has often been stored for months. University extension guidance notes that Vitamins A and E decline during hay storage, which is one reason winter diets may need vitamin support. Not every horse requires the same amount. A horse eating a full serving of fortified feed may receive more Vitamin E than a horse eating only hay and a handful of carrier feed. Winter is a good time to review total intake and consider blood testing if status is uncertain.

6. Is fresh hay enough to replace pasture Vitamin E?

Freshly harvested hay may contain more Vitamin E than older stored hay, but it is still not the same as fresh pasture. Vitamin E can decline during cutting, curing, drying, and baling before storage even begins. Fresh hay quality also depends on plant species, maturity, weather, drying time, and storage conditions. Fresh hay may contribute useful Vitamin E, but it should not automatically be treated as equivalent to pasture. If a horse has no grass access, especially for months at a time, Vitamin E status is worth evaluating even when the hay looks green and smells good.

7. Can I test hay for Vitamin E?

Vitamin E can be measured in forage, but it is not included in many routine hay analyses. Standard hay tests commonly focus on crude protein, fiber, energy estimates, minerals, and sometimes sugar and starch. Vitamin E testing may require specialized handling and laboratory methods. Even then, one sample may not perfectly represent every bale, and nutrient levels may continue to change during storage. For practical horse management, hay analysis is excellent for understanding the base diet, but blood alpha-tocopherol testing is usually more useful for determining whether the horse’s Vitamin E status is adequate.

8. Which horses are most likely to need Vitamin E supplementation?

Horses most likely to need Vitamin E support are those with little or no fresh pasture access. This includes hay-only horses, winter-managed horses, dry-lot horses, metabolic horses restricted from grass, stalled horses, performance horses on hay-based diets, and horses with known low blood alpha-tocopherol. Growing horses, broodmares, older horses, and horses with neuromuscular concerns may also need closer attention. The goal is not to supplement every horse blindly. The goal is to recognize when hay is unlikely to provide enough Vitamin E and use testing or diet review to guide decisions.

9. What is the best Vitamin E for horses on hay?

The best Vitamin E source depends on the horse’s goal. For routine daily support in hay-fed horses, a clear, well-packaged natural Vitamin E product may be practical. Natural Vitamin E is generally more bioavailable per IU than synthetic Vitamin E. For horses with documented deficiency, neurologic concerns, or poor response to standard supplementation, a veterinarian may recommend a more bioavailable water-dispersible or micellized natural form. Owners should look beyond IU alone and consider source, delivery form, selenium content, storage, dose clarity, and whether blood levels improve after supplementation.

10. Should I choose Vitamin E with selenium?

Not automatically. Vitamin E and selenium both contribute to antioxidant systems, but selenium has a narrower safety margin than Vitamin E. Some horses need selenium, while others already receive enough through forage, fortified feed, ration balancers, or regional soil patterns. Adding selenium every time you add Vitamin E is not always appropriate. If the goal is specifically to replace Vitamin E lost from limited pasture or stored hay, a Vitamin E-only product may offer more control. Ask your veterinarian or nutritionist if selenium status is uncertain.

11. How do I know if my horse’s Vitamin E supplement is working?

The most reliable way to know is blood testing. Serum or plasma alpha-tocopherol can show whether your horse’s Vitamin E status is adequate, low, or improving. Testing is especially helpful for horses with no pasture access, known deficiency, neurologic signs, muscle loss, heavy work, or inconsistent response to supplementation. If blood levels do not improve as expected, review the form, dose, storage, feeding consistency, and total diet. Sometimes the issue is not simply the amount of Vitamin E, but the form or delivery system being used.

12. Is hay bad because it loses Vitamin E?

No. Hay is not bad. Hay is one of the most important tools in horse management and is often the foundation of a healthy diet. The point is that hay is preserved forage, and preserved forage is not nutritionally identical to fresh pasture. Vitamin E is one nutrient that can decline significantly after cutting and storage. A thoughtful hay-based diet can absolutely support a healthy horse, but it may need additional Vitamin E, minerals, or other nutrients depending on the horse and the forage. Good nutrition means understanding the limitations of each feed source.

Education-First CTA

If this article helped you realize that hay may not provide the same Vitamin E support as fresh pasture, the next step is to look at your horse’s actual management.

How much fresh pasture does your horse receive? How long has the hay been stored? Is your horse eating the full serving of a fortified feed or ration balancer? Has blood alpha-tocopherol ever been tested?

For owners looking for a straightforward daily natural Vitamin E option without added selenium, Daily Natural Vitamin E was created to make targeted Vitamin E supplementation easier to understand and easier to feed consistently.

If your horse has known deficiency, neurologic signs, muscle loss, weakness, or a complex medical history, work with your veterinarian to choose the appropriate form, dose, and monitoring plan.

References

1. Bruhn, J. C., & Oliver, J. C. (1978). Effect of storage on tocopherol and carotene concentrations in alfalfa hay. Journal of Dairy Science, 61(7), 980–982. DOI
2. Finno, C. J., & Valberg, S. J. (2012). A comparative review of vitamin E and associated equine disorders. Journal of Veterinary Internal Medicine, 26(6), 1251–1266. DOI
3. Higgins, J. K., Puschner, B., Kass, P. H., & Pusterla, N. (2008). Assessment of vitamin E concentrations in serum and cerebrospinal fluid of horses following oral administration of vitamin E. American Journal of Veterinary Research, 69(6), 785–790. DOI
4. Müller, C. E., Möller, J., Jensen, S. K., & Udén, P. (2007). Tocopherol and carotenoid levels in baled silage and haylage in relation to horse requirements. Animal Feed Science and Technology, 137(1–2), 182–197. DOI
5. National Research Council. (2007). Nutrient requirements of horses (6th rev. ed.). National Academies Press. DOI
6. UC Davis School of Veterinary Medicine, Finno Laboratory. (2019). Vitamin E in horses. University of California, Davis. Source
7. University of Minnesota Extension. (n.d.). Providing your horse vitamins and minerals. University of Minnesota Extension. Source