Q: One of my barn friends gives her horse vitamin E supplements throughout the late fall and winter. My horse gets a complete feed and has access to high-quality hay for most of the winter. Do I need to supplement with vitamin E during those months?

— Kathy P., Pennsylvania

A: Vitamin E is a fat-soluble antioxidant that plays a crucial role in supporting horses’ immune systems and other physiological processes. In its most active form, alpha-tocopherol, vitamin E is most potent as an antioxidant and serves to protect immune cells from oxidative stress, boost antibody production and reduce inflammation.

Oxidative stress arises when free radicals (unstable molecules generated through metabolism or environmental factors) accumulate and cause cellular damage. Vitamin E helps neutralize these free radicals, preventing the destruction of cell membranes and other essential cell structures.

Horses cannot synthesize vitamin E on their own, and therefore must obtain it from their diet, primarily through fresh forage. Ensuring adequate vitamin E intake through diet or supplementation is a priority for horse owners, especially considering its profound impact on the neuromuscular system.

Vitamin E deficiency often occurs when horses have limited access to fresh pasture, as dried forages like hay and processed feeds contain significantly lower levels of vitamin E. Primary clinical signs and conditions associated with vitamin E deficiency in horses include muscle loss, weakness, poor performance and, in some cases, neurologic disease.

Evaluating Your Horse’s Vitamin E Intake

Many variables need to be considered when evaluating your horse’s vitamin E intake. The amount of vitamin E provided by pasture grazing depends on factors such as location, season, plant species, soil type and the amount of time your horse spends on pasture.

It’s challenging to make even broad generalizations about what type of pasture and how much would provide adequate vitamin E for any given horse because there simply hasn’t been enough research into the amounts of vitamin E in different pasture types—and due to the variation in absorption rates between individual horses. So, for example, we can’t say that if your horse is on timothy pasture for X hours a day in the spring, then his vitamin E levels are probably OK. It’s best to have his vitamin E levels checked directly.

It’s also important to consider the amount of vitamin E your horse gets from his daily rations, including targeted supplements (You can find this information in the ingredients’ label.) Most equine feeds and ration balancers, however, contain synthetic vitamin E, which is not well absorbed in horses and therefore usually doesn’t provide adequate amounts of the vitamin.

The National Research Council recommends that horses consume 1-2 international units (IU) per kilogram body weight of vitamin E daily. This equates to 500-1,000 IU for a 1,100-pound horse. Higher levels may be recommended for horses in hard work or those with certain health issues.

When determining whether your horse needs vitamin E supplementation in the late fall and winter, it’s also important to keep in mind that horses store vitamin E in the liver and fat tissues. Depending on pasture access in the spring and summer months, they can rely on these stores for up to 18 months before problems related to dietary vitamin E deficiency may be observed. This is how wild horses maintain their vitamin E levels throughout the winter.

However, individual horses metabolize vitamin E at different rates, so the time they can go without consuming vitamin E can vary. Additionally, some horses are genetically predisposed to diseases caused by vitamin E deficiency such as equine neuroaxonal dystrophy/equine degenerative myeloencephalopathy, a neurologic disorder that can cause ataxia (incoordination), dragging the feet and weakness.

Establish a Vitamin E Baseline

Due to the many variables that affect individual vitamin E levels in horses, establishing a baseline blood vitamin E level is important to determine whether your horse is deficient in vitamin E. If he is, work with your veterinarian to determine the appropriate supplement for him and to recheck his vitamin E levels after supplementation to ensure they’re not too low or too high. From there, your veterinarian can advise you on making any necessary adjustments to his supplement dosage.

The timing on when to recheck his levels depends on the type of vitamin E supplement he’s getting. Water-dispersible vitamin E products are highly bioavailable; therefore, it’s generally recommended to recheck within two weeks of starting. For powders and pellets with natural vitamin E, the bioavailability is lower, so it takes two to three months to increase levels.

Once a reasonable level is achieved, it’s always wise to recheck your horse’s levels once or twice a year. This is especially true if his diet changes significantly or if you notice any changes in his health that could be related to vitamin E levels, such as muscle loss, decreased performance, incoordination or weakness. On the other end of the spectrum, excessive vitamin E can cause health issues such as increased bleeding risk. Vitamin E interferes with vitamin K metabolism, which can increase the risk of bleeding.

Regardless of your horse’s situation, always consult with your veterinarian before making significant changes to vitamin E in your horse’s diet.

For More:

To learn how vitamin E can help support your performance horse’s immune health, click here.

About Carrie Finno, DVM, DACVIM, PhD

Carrie Finno, DVM, ACVIM, PhD, is an equine internist who serves as associate professor in veterinary genetics and the Gregory L. Ferraro Endowed Director of the Center for Equine Health at the University of California, Davis. She received her veterinary degree from the University of Minnesota in 2004 and then completed a three-year residency in large animal internal medicine at UC-Davis. She elected to pursue a career in translational genetic research, with a strong focus on inherited neuromuscular diseases. Her research studies the interaction of vitamin E and neural development in a naturally occurring disease in the horse.