Q: I recently had my horse’s vitamin E levels checked because he’s getting older and doesn’t have access to fresh grass during the winter, but I’m not sure how to interpret the results. Is it better for his numbers to be higher on the scale or in the normal range, and at what range should I consider supplementing with vitamin E?

— Melissa F., New York

A: Vitamin E is a critical component of the equine diet as a potent antioxidant with roles in combatting oxidative stress, supporting the neuromuscular system and enhancing immune system responsiveness. The daily requirement for horses can vary significantly based on weight, diet, workload and life stage. For reference, an 1,100-pound horse at maintenance should consume approximately 500 IU of vitamin E per day. That same horse’s requirement increases to over 1,000 IU per day if he’s in some type of exercise program.

Green pasture is the best and most abundant source of vitamin E. A horse with access to adequate pasture (quality pasture for approximately 12 hours per day) will naturally maintain a serum vitamin E level of 3-6 µg/ml (which is ideal). Hay is another story since the nutrient is rapidly depleted post-baling. It’s safe to assume that horses not receiving adequate pasture intake will need to rely almost entirely on what’s in their feed and/or supplements.

When to Test

It’s highly recommended that horses who receive little or no pasture are tested annually for serum vitamin E status. This is also a good practice as vitamin E is a sensitive compound and not always particularly stable in supplements and/or feed. While most feed and supplement companies do a great job offering a stable form of the nutrient, I’ve come across many cases of horses receiving far beyond their requirements yet still testing as deficient in my consulting practice. Thus, it’s important to make sure the dietary source of the nutrient is viable, and this can be achieved through a simple blood draw from your veterinarian.

For horses on quality pasture who also receive adequate dietary intake of vitamin E from their grain ration or supplements, regular testing is not as critical unless there are observed symptoms associated with deficiency or if you’re curious. It’s also important to note that horses receiving the exact same diets can have some individual variation in vitamin E metabolism. Thus, it is best to test the full herd if there is suspicion of deficiency.

Interpreting Test Results

When you receive your horse’s test results, it’s important to know how to interpret their serum value. Adequate serum concentration is ≥ 2 µg/mL (micrograms per milliliter). While 1.5-2 µg/mL is considered marginal, and < 1.5 µg/mL is deficient. A horse that scores ≤ 2 µg/mL should undergo a vitamin E supplementation protocol to return serum levels to ideal. It’s best to consult with your veterinarian or an equine nutritionist to determine the most appropriate product and amount to supplement. Because horses on abundant pasture will naturally maintain serum at 3-6 µg/mL, this is a great target for “ideal.”

Vitamin E test results can be confusing for owners as the scale goes up to 10 µg/mL (or 1000 µg/dL depending on the lab your veterinarian sends the sample to). Many owners believe their horses should be on the high end of the scale for best results in performance, health, etc. Based on the current data available, there isn’t a reason to believe that “high normal” (7-10 µg/mL) is better than the suggested 3-6 µg/mL range for serum.

Thus, if your horse doesn’t come back with a “high normal” value but is still within 3-6, there isn’t support for additional vitamin E supplementation. If you have further questions and want to know the ideal form or product for your horse, your best bet is to connect with your veterinarian or a nutritionist.

About Rachel Mottet, MS, PhD

Rachel Mottet, MS, PhD, is an independent equine nutritionist under her business Legacy Equine Nutrition. She earned her B.S. in Equine Science from the University of Wisconsin River Falls, her M.S. in Animal Science from North Dakota State University and her PhD in Nutrition and Animal Science from the University of Minnesota.

Professionally, Mottet has worked with horse owners all over the world as an invited lecturer, university professor and consulting equine specialist for numerous equine feed and supplement companies. She has worked with FEI and Olympic competitors in numerous disciplines and has successfully formulated diets for thousands of horses. She actively competes in hunter/jumpers and owns a farm outside of Ocala, Florida, where she lives with her two dogs and three horses.

For More on Vitamin E and Equine Health:

• Learn about vitamin E’s essential role in supporting your horse’s immune health here.
• Does your horse need vitamin E supplementation during the winter? Find out more here.
• A veterinarian explains the differences between natural and synthetic vitamin E and how to determine which is best for your horse here.