When it comes to caring for insulin-resistant (IR) horses, diet plays a very important role in managing insulin levels and preventing associated diseases such laminitis. Insulin resistance and hyperinsulinemia are key features of equine metabolic syndrome (EMS) and can also occur in horses with pituitary pars intermedia dysfunction (or equine Cushing's disease). These animals often need to lose weight and consume fewer carbs.

To that end, Ray Geor, BVSc, PhD, Dipl. ACVIM, a professor and researcher at Michigan State University's department of large animal clinical sciences, shared best feeding practices for IR horses at the 2013 International Equine Conference on Laminitis and Diseases of the Foot, held Nov. 1-3 in West Palm Beach, Fla. He first discussed weight loss, then carbohydrate consumption.

Managing Obesity

Geor explained that dietary restriction and exercise are two key components to curbing obesity in IR horses. "Dietary restriction is not rocket science," he said. "But it requires owner/trainer compliance and patience and sticking to the program (to be successful). Diet changes are likely a lifelong pursuit, especially for horses with EMS that are easy keepers and have a tendency to become obese."

Geor recommended owners of affected horses institute a weight loss program with the goal of improving the animal's metabolism to reduce his risk of developing associated laminitis. He offered the following guidelines for developing a program:

Base your horse's diet on forage or a forage substitute, and eliminate grain and calorie-dense feeds (e.g., sweet feeds) if possible. – Feed a lower-quality, low-energy forage, such as late-maturity hay, at between 60 and 80% of your horse's daily energy requirements based on his body weight.

When feeding a low-quality forage, add a ration balancer to help your horse meet his vitamin E, copper, zinc, and other requirements. "Alternatively, forage-based, low-calorie feeds that contain added vitamins and minerals are now available commercially," Geor said. "This type of feed offers convenience and may be used as a substitute for hay or fed as a component of the ration along with hay."

Although all weight loss programs need to be individually tailored, as a general guide start by restricting your horse's daily dry matter intake (DMI) to 1.5% of his body weight in total feed. The rate of weight loss will vary between horses but in general at least six to eight weeks of dietary restriction is needed for noticeable weight loss to occur, Geor said. If your horse's weight loss response is less than desired, you might need to restrict his DMI to 1.25% body weight. If he still doesn't lose weight after another six to eight weeks, you can restrict his DMI even further to 1% body weight, but Geor recommended never feeding lower amounts than that for health and behavior reasons. "Hay feeders and slow feeders (e.g., hay nets with multiple small holes) are one way to extend your horse's hay supply if you're not feeding much," he said.

Restrict or eliminate pasture grazing using a grazing muzzle or a drylot for turnout. "It should be noted that simply restricting the time allowed for grazing may not be an effective strategy for weight loss," Geor cautioned. "Ponies have been observed to consume up to 1% of body weight within three hours of pasture turnout."

And as with any diet changes, make feed changes gradually and avoiding withholding food from your horse for long periods. Divide rations into three to four small meals, and do not bed a dieting horse on straw or shavings to reduce the risk of him eating them and potentially suffering an impaction, Geor said.

Geor encouraged owners to use a number of simple tools to monitor their horses' weight loss, including body condition scoring and measurements of girth and abdominal (‘belly’) circumference. "Body condition score is not always a sensitive indicator of weight loss," he added. "However, girth and belly circumference measurements will decrease with weight loss, and I recommend recording these measurements at three- to four-week intervals."

But once your horse meets his targeted weight and condition, you can't just stop there: "Develop and continually update an appropriate weight maintenance program," Geor said. "It should include monthly assessment of body weight and BCS to ensure that the feeding program is appropriate for the current level of physical activity and other environmental influences on energy requirements."

Controlling Carbohydrates

Veterinarians and researchers have long associated laminitis with increased nonstructural carbohydrate (NSC) intake—especially in pastured horses. And because NSCs can contribute to exaggerated insulin responses, it's particularly important to restrict their intake in IR horses.

Geor offered suggestions for controlling NSC intake:

Feed a forage-based diet with a low NSC content (less than 12%). "Soaking hay can help lower NSC levels, but it's not a panacea," he said.

Eliminate grain and sweet feed (both high in NSCs) from the diet.

Restrict or eliminate pasture access. At certain times of year (e.g., during spring and early summer growth periods, after summer or fall rains, or after drought or frost) pasture forages' NSC content is quite high, so Geor advised keeping IR horses off pasture during these periods to reduce the risk of developing laminitis. He said a grazing muzzle might be the best way to safely restrict grass consumption (studies show it can decrease pasture intake by about 80%), as simply providing a smaller space to graze can lead to an overgrazed area, which will still have high NSC content due to stress. Also, "beware the 'Hoover' pony," Geor said. "They can adapt and consume around 40% of their daily DMI in just two to three hours."

In conclusion, Geor said correcting obesity and restricting NSCs are not a cure for insulin resistance but these strategies can help to prevent laminitis in affected horses.