Feeding a Metabolic Horse: A Close Look at Ration Components

Horses with equine metabolic syndrome (EMS) cannot effectively metabolize sugars because of a reduced sensitivity to insulin, the hormone responsible for moving glucose from blood into the body’s cells for energy. This often leads to obesity and an increased risk of hyperinsulinemia-associated laminitis due to the high circulating blood insulin disrupting the laminae of the hoof wall. To reduce the risk of hyperinsulinemia-associated laminitis, the focus of feeding programs for EMS horses should be to reduce body weight and minimize the insulin response while providing adequate protein, vitamin, and mineral intake.

The current recommendations for horses with EMS are to provide a forage-based diet and a ration balancer or vitamin and mineral supplement to ensure adequate nutrient intake. Pasture access should be restricted, particularly in the spring months when cool-season grasses accumulate high levels of sugars and fructans and warm-season grasses accumulate starch. Cool-season grasses have a greater ability to accumulate fructans than warm-season grasses do for starch, making cool-season grasses a greater risk for a hyperinsulinemic response in horses with EMS.

If pasture access is provided, horses should be turned out early in the morning and brought in by 10:00 a.m., as nonstructural carbohydrates (NSCs), which are simple sugars, fructans, and starch, accumulate in the plant throughout the day as it photosynthesizes. Caution should be taken after colder nights, as the plants will slow down the metabolizing of NSCs for growth, meaning fructan and starch levels are already high in the morning and accumulate further throughout the day. Grazing muzzles also restrict grass access, but turning out in a drylot with conserved forage is the safest option to keep insulin responses low.

Conserved forage in the form of mature hay is preferable, ideally with an NSC content below 10% on a dry matter basis. Feedstuffs with high levels of NSCs often lead to a hyperinsulinemic response in horses with EMS. Warm-season grass hays, such as bermudagrass or teff, typically have lower NSC content than cool-season grass hays, such as orchardgrass or timothy. Maturity of the plant at harvest also affects NSC with later-cut (mature) grass hays typically having a lower NSC content.

To visually assess the maturity of your hay, pay attention to the stems and leaves. Mature hay is usually coarse and stemmy, whereas immature hay is typically softer with a higher leaf-to-stem ratio. Additional factors such as plant species and environmental conditions at harvest can affect the NSC content of hay. Nutritionists recommend that forage is tested to ensure the NSC content is below the threshold. If low-NSC forage is not accessible, soaking hay is an option to decrease the sugar content. Because soaking has variable results, another option may be to replace some high-NSC long-stem forage with a guaranteed low-NSC forage product such as a hay stretcher/replacer.

Forage-based diets often do not supply adequate vitamins and minerals, particularly trace minerals. To ensure micronutrient requirements are met, concentrated vitamin mineral products or ration balancers are recommended. The more concentrated products are preferable for horses that are overweight with low energy requirements, as feed intake can be minimized.

Not all horses with EMS are obese and some may require additional energy for exercise or weight maintenance. In these cases, a concentrate feed may be desirable to meet the energy demand. However, certain ingredients should be avoided to prevent a spike in insulin. To avoid a high NSC intake, concentrate feeds containing grains should be avoided. Common grains in equine concentrates include oats, corn, and barley. These ingredients have a high glycemic index, indicating a greater rate of carbohydrate absorption. Kentucky Equine Research previously characterized the glycemic index of sweet feed, oats, and cracked corn to be 129, 100, and 90 respectively, compared to rinsed beet pulp at 34, rice bran at 47, and orchard grass hay at 49.^* Grains that have been steam-flaked, micronized, or extruded should be avoided, as this heating process increases the digestibility and insulin response to the feed.

To safely increase energy, digestible fiber sources, such as soaked beet pulp and soaked soy hulls, or fat sources, such as soybean or canola oil and stabilized rice bran, can be added to the diet due to their low glycemic response. Fish oil is beneficial for glucose tolerance with marine-derived omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) improving insulin sensitivity in insulin-resistant horses. One of the best sources of EPA and DHA for horses is EO-3. Kentucky Equine Research found that adding EO-3 to the diets of horses with induced glucose intolerance significantly improved glucose clearance from the blood. While some marine-derived oils are often unpalatable to horses, EO-3 undergoes additional distillation, which removes specific compounds that can increase odors over time with exposure to heat without removing or altering the important beneficial fatty acids. In addition to increased processing, EO-3 contains a minty apple flavor preferred by horses in multiple taste-test studies.

Providing sufficient but not excessive protein is another important consideration. Recent research has shown that feeding ration balancers, with variable crude protein content, at their recommended rates did not alter insulin response significantly.^** This further confirms that ration balancers are suitable to meet the nutrient requirements of horses with EMS. For EMS horses who are not overweight, a small amount of alfalfa hay could be added to the grass hay diet to increase crude protein intake without adding excessive NSCs. However, alfalfa hay usually has more digestible energy than grass hay, meaning it should be avoided if weight loss is the goal.

Assembling a diet for a horse with a metabolic condition can be overwhelming for some horse owners. Working with a knowledgeable equine nutritionist can help ease the burden of coming up with an appropriate diet. With the additional guidance of a veterinarian, the horse’s owner can monitor how a horse responds to the diet and make adjustments as needed.

^*Huntington, P.J., and Pagan, J.D., 2008. Nutritional management of equine metabolic disorders. The Australian Equine Veterinarian 2008(3):40-44.

^**Macon, E.L., P. Harris, S. Bailey, V.D. Barker, and A. Adams, 2022. Postprandial insulin responses to various feedstuffs differ in insulin dysregulated horses compared with non‐insulin dysregulated controls. Equine Veterinary Journal 54(3):574-583.

This article was written by Iona Robinson, a year-long intern at Kentucky Equine Research. As part of the intern program, participants are asked to contribute to Equinews occasionally. Learn more about the internship program.