On the other side of the barn are the horses that are prone to being too thin. “Horses that are underweight are not able to maintain body heat as well, and they may also be consuming inadequate levels of key minerals, vitamins and amino acids that can put them at greater risk of having poor hoof and coat quality, lowered immune system function, and so on,” said Clair Thunes, PhD, an independent equine nutritionist based in California (summit-equine.com). 

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There is a temptation for owners to want horses too fat. “Seeing a faint outline of ribs does not necessarily mean that a horse is underweight,” Thunes said. Being able to slightly see ribs can be better for specific horses (such as racehorses or top-level competitors), and proper weight often depends on the individual horse.

“A pregnant broodmare whose ribs are visible might be more of a concern than a gelding in light work whose ribs are visible,” Thunes explained. Similarly, an older horse with lameness issues might be more comfortable on the leaner side as less body weight reduces strain on arthritic joints.

“If, however, the ribs are prominent—for example, like your knuckles when you make a fist—and/or if the horse has a prominent back bone or hip bones, then the horse is likely too thin,” said Megan Shepherd, DVM, PhD, DACVN (American College of Veterinary Nutrition). She is a clinical assistant professor in nutrition at the Virginia-Maryland College of Veterinary Medicine (vetmed.vt.edu/vth/services/nutrition). 

Since weight loss or leanness can be a sign of a medical issue, the owner of a too-thin horse should consult with a veterinarian before just changing the horse’s feeding regimen. The thinness could be due to a medical problem that is easily remedied, or it might be caused by something serious.

“It’s important to know when the horse was last dewormed, when his teeth were last done and if the horse is able to eat the diet it is being fed,” Thunes said.

She reminded managers and owners that neglected, thin horses are not the same as hard keepers. “For horses that have been malnourished for months, people need to take a slow approach,” Thunes advised. “Feeding a starved horse too aggressively can be fatal.”

After major health problems are ruled out, the horse’s calorie intake needs to be increased. “This would be a time when feeding a higher-calorie hay that has a lower indigestible fiber content would be good,” Thunes said. Adding alfalfa, either in addition to or in place of current hay (up to 25% of the hay intake), will increase calorie intake.

A higher-calorie concentrate feed, such as one based on a super fiber such as beet pulp, generally offers a higher fat intake (8-12% fat) and significantly increases calorie intake. Feeding instructions printed on the bag should be followed. Owners should be discouraged from over-feeding grain to horses in order to avoid digestive upset.

Finding Middle Ground

There is no-one-size-fits-all solution to managing an overweight or underweight horse, and for many stables it’s also unrealistic to custom-order feed for every horse in the barn. Three or four carefully selected blends of feed offer enough variety to suit the majority of horses in any barn if the farm owner is supplying feed.

Katie Young, PhD, a consulting equine nutritionist with Kentucky Equine Research (KER; you can contact Young at kyoung@ker.com), suggested that boarding farms stock a ration balancer, a moderate-calorie feed and a high-calorie feed (that is high in fat and can be used to top-dress as needed). “With this kind of approach, you can maintain a pretty wide variety of horses,” she said.

Ration balancers are designed to provide a mature horse on a forage-only diet the needed vitamins and minerals that aren’t available in grass and hay. It’s important to follow the feeding instructions, so if an owner goes this route, make sure you read the label to ensure the horse isn’t getting over-fed with the balancer.

Restricting diets alone might not be effective for some easy keepers or horses with metabolic conditions. In those cases, specially formulated feeds that are high in fiber and low in calories might be the solution.

“When the feeding instructions are followed, some weight loss feeds allow for two full scoops of feed while helping a horse lose weight without increased exercise or limiting access to grazing or forage,” Young noted.

Moderate-calorie feeds are typically sufficient for the majority of horses in any barn that doesn’t have high-level competitors. A moderate-calorie feed is one with between 12-14% protein.

It’s important to remember that feeds are designed to meet the calorie, vitamin and mineral needs of a 1,000-pound horse at a specific feeding rate. If less than the recommended serving is fed to limit the calorie intake and reduce or maintain a horse’s weight, chances are he is not receiving an adequate amount of minerals and vitamins.

High-calorie feeds deliver high levels of fat and fiber that are necessary for promoting maintenance in very active horses or for weight gain in thin horses. Vegetable oils, flax seed and/or rice bran included as ingredients in formulated feeds or fed along with grain offer more calories than traditional feeds.

Learning More

Numerous resources are available online, in person or through a phone consultation if you’re interested in fully understanding the daily nutritional requirements of each horse in your barn and how nutritional needs vary by life stage, activity level, age, weight and body condition.

The National Research Council (NRC) provides specific recommendations based on the above-mentioned criteria.

Online ration calculators can help you determine the type and amount of feed a horse needs. Purina Mills offers one (purinamills.com/horse-feed/feeding-calculator) that allows users to calculate the correct amount of forage and feed in pounds based on the horse’s lifestyle and body weight.

Equine nutritionists also can be enlisted to create a custom ration for an individual horse. You can use independent nutritionists, feed specialists working for major feed brands (such as Purina) and nutritionists associated with universities.

Take-Home Message

Horses are individuals owned by individuals. In order to properly maintain a healthy weight in a horse, you will have to manage the animal’s diet as well as the owner’s wants and expectations.

Check out these equine nutrition books:

Introduction to Horse Nutrition
Equine Applied and Clinical Nutrition
Equine Nutrition and Feeding
Horse Health and Nutrition for Dummies
Understanding Equine Nutrition