Hay is almost the perfect food for horses—most varieties offer the right balance of protein, fiber, nutrients and energy to keep the average horse healthy.

The key word there is “almost.” One nutrient that hay may not provide in sufficient quantity is vitamin E. This essential nutrient is present in fresh pasture but begins to degrade as soon as grass and legume plants are harvested. And the longer the hay is stored before it is consumed, the more of its vitamin E is lost.

So for horses whose forage comes primarily from hay, with little or no grazing, vitamin E deficiency is a possibility. And it’s even more likely for horses who are in training with limited turnout because exertion increases the need for this valuable antioxidant. Vitamin E requirements are also higher for aging horses, those who are ill and those with certain health issues.

Vitamin E helps keep a horse’s muscles, nerves and all his internal workings functioning smoothly. And if he’s not getting it naturally in a green pasture, then you’ll need to find a way to add it to his diet. Here’s a look at what vitamin E does and what you can do to make sure your horse gets enough—but not too much.

Vitamin E in nature

“Vitamin E” is a collective name for a group of eight naturally occurring compounds that all have distinctive antioxidant activity. There are four tocopherols and four tocotrienols. Each is designated with an alpha-, beta-, gamma- or delta-.

“The most biologically available form is alpha- tocopherol, and this is why we can measure this one in the blood,” says Tania Cubitt, PhD, an equine nutritionist with Performance Horse Nutrition, a consulting firm in Middleburg, Virginia.

Vitamin E is fat-soluble, which means it’s handled quite differently by the body compared to water-soluble vitamins, such as vitamin C, which are not stored and are eliminated in urine if too much is consumed. Fat-soluble vitamins can be stored in the liver and the body’s fatty tissues. This means the horse can keep a supply of the nutrient when it is abundant, and access it when it isn’t. And that’s exactly what he does in nature. Vitamin E is abundant in fresh, green grass, but the amounts diminish as grass matures and dies. Horses who spend the winter foraging on dried grasses will draw on their stored vitamin E; then they will replenish their supplies of the nutrient when the green plants start growing in the spring.

All of which means that a domesticated horse’s lifestyle can work against his receiving enough vitamin E. “The normal horse under natural conditions is able to cope with seasonal fluctuations,” says Cubitt. “We have thwarted this, however, by putting our horses in an artificial environment. About 30 to 80 percent of the vitamin E in hay can be dissipated during the drying for harvest and during storage. If horses are stuck in a stall and not on pasture, they are relying on us to supply most of their vitamin E. I have seen a lot of horses that are actually deficient in vitamin E because of the way they are managed. So today we see a lot of horses being supplemented with vitamin E.”

What Vitamin E does for horses 

Vitamin E plays a role in many functions throughout the body, but it is known primarily as a potent antioxidant, meaning it binds with and limits the damage caused by free radicals, which are atoms or molecules with an odd number of electrons. Because they have an unstable electrical charge, free radicals tend to “steal” electrons from other molecules to become stable. But when the original molecule loses its electron, it becomes unstable and in turn tries to steal another electron from somewhere else. All this activity not only damages the molecules that have their electrons stolen, it may inhibit their ability to do their jobs within the body. If there are too many free radicals present in the tissue, this chain reaction can run out of control and injure cell walls, DNA and other vital structures.

Free radicals are a natural byproduct of the utilization of fats, carbohydrates and proteins as fuel. They do have beneficial functions; they can help neutralize bacterial or viral threats, for example. But when the number of free radicals in the tissues climbs too high—such as in the muscles after a horse exercises—the body deploys antioxidants to bind with them, breaking the cascade.

In the case of vitamin E, action centers on the fats that form the structure of cell membranes, where the nutrient remains ready to bind with free radicals that might otherwise damage the cell walls. “It helps protect the cells,” says Carey Williams, PhD, an equine extension specialist with Rutgers, the State University of New Jersey. “The vitamin E incorporates itself into cell membranes and protects them from oxidative damage.”

A shortage of vitamin E might mean more oxidative damage occurs in cells throughout the body, including those in muscles, nerves and immune cells. In the case of a hardworking horse, outward signs of oxidative injury might be muscle soreness and a slower-than-expected recovery from exercise. And so, when a horse’s workload increases, his need for antioxidants, including vitamin E, also goes up.

“When you damage muscle, for any reason, you have some oxidative stress,” says Paul Siciliano, PhD, of North Carolina State University. “In the cell, when metabolism is taking place, some pro-oxidants are produced. You could equate it to a campfire out in the woods. The fire is producing heat energy and it might send up sparks in the process. As long as the sparks get put out, things are fine. But if one of them starts another fire and it grows, you could have a problem. Vitamin E attaches to the cell membranes and quenches those little fires and keeps things working properly.”

Oxidative damage is most likely to occur in tissues in the immune system, nerves and muscles, because they are more highly metabolic—that is, they “burn” energy faster. “Thus they produce a greater proportion of these pro-oxidants just as a cost of doing business,” Siciliano says. “There is higher likelihood to have a problem in those areas if horses are short on vitamin E.”

Click here to read about five diseases linked to Vitamin E deficiency in horses.

How much vitamin E does a horse need?

A horse’s requirements for vitamin E have not been well established. “We have defined these requirements only because we know that horses consuming it at a certain level haven’t had any deficiency symptoms,” Siciliano says. Guidelines in the Nutrient Requirements of Horses from the National Research Council (NRC), last revised in 2007, recommend about 500 IU (international units) daily as a maintenance level for an 1,100-pound horse in light work.

“This isn’t a lot, so if a horse has access to pasture, that horse has plenty of vitamin E, since green pasture is the best source,” says Williams. “Just like your parents always told you to eat your vegetables—because they contain lots of vitamins—our horses that are out on pasture will have those vitamin needs met.”

The NRC recommendations are higher for hardworking horses and for breeding mares and stallions. “Working horses might need up to 1,000 IU per day,” says Williams. “These figures are the minimums, however. There have been many studies, including some I have done, that have shown that more vitamin E is even better. Most of the studies I did looked at supplementing 5,000 IU per day for the average-size horse on a hay diet and not on pasture. These horses had lower levels of muscle enzymes in their blood, which means less muscle membrane permeability or leakage of material into circulation.”

Higher levels of vitamin E may also be recommended for horses with certain health issues. “The categories that the NRC does not touch on, but which we have anecdotal evidence and research looking at, are guidelines for disease-state horses—horses with allergies, tying up, metabolic syndrome, or those that are suffering from or recovering from illness,” says Cubitt. “In horses with allergies, we know that potent antioxidants are effective. One guideline that has been suggested is about 5,000 IU per day. Horses who tie up and have muscle problems can also benefit from 5,000 IU per day. Horses with metabolic syndrome, insulin0 resistance and laminitis should also receive that higher level. Horses recovering from surgery, illness or stress may need 1,500 to 5,000 IU per day, depending on the severity of the illness/stress.”

Given that vitamin E is stored in fat, it’s not surprising that severely underweight horses may also have deficiencies. “These horses have no fat, so they can’t store it, so we have to feed them more vitamin E than the normal requirement; they need about 1,500 to 2,000 IU per day,” Cubitt says.

But it is possible to give a horse too much vitamin E. “A person needs to be careful with high doses of vitamin E, because vitamin E and beta carotene [the building block for vitamin A] have the same absorption pathway,” says Williams. “We found that high levels of vitamin E can actually decrease the level of beta carotene in the body. In one study, 10,000 IU of vitamin E was fed daily, and there was some interference with uptake of vitamin A. You are inhibiting one vitamin by overfeeding another.”

Horses on pasture would probably be getting adequate amounts of vitamin A, because beta carotene is also abundant in green grass. “If they are in stalls being fed hay, however, receiving too much vitamin E could become a problem,” Cubitt says.

Your horse’s vitamin E status

Your horse’s turnout schedule and activity level can provide clues to whether he’s taking in enough vitamin E, but a blood test is the best way to determine with some certainty.

“If your veterinarian tests plasma or serum concentrations for alpha-tocopherol, greater than 2 micrograms per milliliter is considered adequate, 1.5 to 2 micrograms would be considered marginal and less than 1.5 would be considered deficient,” Cubitt says. “If we were able to examine horses in the wild and measure their blood levels seasonally, by the end of winter they might be marginal, but that level would soon increase once the spring grass starts growing.”

If your veterinarian suggests that you increase the amount of vitamin E in your horse’s diet, you have several options. Obviously increasing his access to fresh grass will help—assuming this won’t put him at risk of laminitis or obesity. Grass contains somewhere between 30 to 100 IU of vitamin E per kilogram of dry matter.

Click here to learn about the differences between natural and synthetic Vitamin E. 

You may also want to see if you can get hay that has been cut earlier—grasses cut for hay while young and growing will have higher levels of vitamin E. Exactly how much of the vitamin hay loses, and how quickly, depends on several factors, including the conditions of harvest and the amount of sunlight it is exposed to when drying (sunlight denatures all vitamins). One study found that fresh alfalfa hay lost as much as 73 percent of its vitamin E after just 12 weeks in storage. In addition, some grains, such as corn, oats or barley, contain some naturally occurring vitamin E but only about 20 to 30 IU per kilogram of dry matter. Grains also lose some of their vitamin E over time in storage; dry, dark storage is best for all feedstuffs.

To avoid uncertainty and ensure that their horses receive what they need, many owners opt for supplements, balancer rations and commercial feeds formulated to provide the nutrients required by average horses or those at specific life stages or activity levels.

“Most commercial feed products are fortified with vitamins and minerals,” says Williams. “They usually provide about 100 to 150 IU of vitamin E per pound. So if a horse is eating two or three pounds of grain daily, this will be adequate if at maintenance or light work.” Vitamin E is included in a wide range of supplements, both as the primary ingredient and as an addition to products for joint health, digestive support and other formulas. However, if the goal is for a level of 5,000 IU per day, make sure you are feeding a sole concentrated source of vitamin E. Otherwise there is a risk of over- supplementing the other nutrients in the product to get to that level of E.

When it comes to keeping a horse healthy, often the best approach is the “natural” way—mimicking as closely as possible the way he would live in the wild—despite stalls, trailers and training schedules. When it comes to an essential nutrient like vitamin E, that means letting him graze as much pasture as possible during the warmer months. But when that’s not feasible, taking steps to make sure your horse gets enough of this essential nutrient will help to keep him healthy.

This article first appeared in EQUUS #379

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