The endurance horse is a distance athlete with high nutritional demands to support his work. Competitors can cover 25 to 100 miles in a single day. One health risk endurance can pose is exertional rhabdomyolysis (ER) or “tying-up.” This is a condition where a horse experiences some degree of muscle stiffness usually shortly after starting exercise. It might progress to severe muscle damage or necrosis, and renal (kidney) failure secondary to myoglobinuria, which is the presence of the broken-down muscle protein in urine.

Sporadic ER is a single or rare occurrence due to overwork or other confounding factors such as hot, humid weather. Veterinarians diagnose ER episodes using bloodwork. Chronic ER is a collection of heritable myopathies, including recurrent exertional rhabdomyolysis (RER), myofibrillar myopathy (MFM), and polysaccharide storage myopathy (PSSM) types 1 and 2. Veterinarians make definitive diagnoses of these conditions via muscle biopsy. In this article, I’ll provide nutritional recommendations to decrease episodes of sporadic ER. If you’re concerned your horse could be affected by a chronic ER condition, work closely with your veterinarian, who can make a diagnosis and management plan.

A Foundation of Forage

Providing good-quality forage through hay and pasture is going to be the base upon which you build your endurance horse’s dietary program. Typically, veterinarians and nutritionists recommend feeding the equivalent of 1.5-2% of the horse’s body weight daily in forage. The horse is a hindgut fermenter, which means the billions of bacteria living within the gastrointestinal (GI) tract take fiber and convert it into usable energy. Forage also holds water within the GI tract, which can help the horse maintain hydration during work. We typically don’t recommend feeding straight alfalfa because it provides high calcium levels that can lead to metabolic imbalances, as well as excess protein for the horse to excrete, leading to more work for the kidneys and potential for dehydration.

Meg Sleeper, DVM, Dipl. ACVIM, clinical professor and cardiology service chief at the University of Florida’s College of Veterinary Medicine, in Gainesville, has been involved in the endurance discipline for many years as both a competitor and an official. She said she finds regular testing of both pasture and hay to be helpful in managing her own horses’ diets and ensuring appropriate nutrient content and balance. In general, she recommends providing good-quality pasture and/or a grass or mixed grass-alfalfa hay.

Meeting Nutrient Requirements

Forage alone, however, does not usually provide an athletic horse sufficient caloric or vitamin/mineral intake. A concentrate, or “grain,” with moderate nonstructural carbohydrates (NSC) and protein will address their immediate demands, while a fat supplement can help add calories without “heat.” Vitamin E and selenium deficiencies can potentially increase a horse’s risk of developing ER.

Sleeper said she finds providing adequate vitamin E and other antioxidants to be beneficial to her endurance horses. Vitamin E is usually found in adequate supply in grass, hay, and rice bran, but it can be supplemented if lacking. Manufacturers typically include selenium in commercial feeds in adequate amounts, so read labels carefully to ensure you’re not feeding it in excess.

The horse’s body converts starches from grains such as corn, barley, and oats into readily available glucose and glycogen for energy, but too much can make a horse “hot” as well as increase ER risk.

As I noted earlier, protein is absolutely needed for maintenance requirements. Some excess can be converted to energy, but too much can lead to dehydration. Paul Szauter, PhD, a genetic researcher focusing on equine myopathies (muscle disorders), said adequate, balanced protein can help repair muscle breakdown and maintain fitness. The overall protein requirement for an endurance horse is typically 8-10% of total diet, so keep this in mind when selecting a commercial feed and combining it with hay.

Added fat is an excellent way to provide more calories with a lower volume of feed, because it has 2.25 times the digestible energy as most grains. It is palatable and easily digested, but palatability decreases if a diet is made up of more than 20% added fat.

Horses lose electrolytes, including salt, calcium, and potassium, in sweat, urine, and feces. At the very least, provide your horse with a free-choice salt and trace mineral block. Daily electrolyte supplementation as part of the regular diet, as well as during competitions, can help replace massive losses occurring on a long ride. Depending on the climate and ride conditions, which can result in excess losses, you might give electrolytes before, during, and after a competition. There are many commercial electrolyte products on the market, but experienced endurance riders often develop their own recipe that works for their horses over time.

Take-Home Message

These are general guidelines you can adjust based on an individual horse’s nutritional demands. Overall, maintaining good stores of fat and glycogen, providing sufficient forage, vitamins, and minerals, and encouraging adequate hydration are the premises on which maintaining these unique horses’ energy requirements and decreasing the chances of an ER episode are based.