Diet plays a role in the health of the horse from birth through old age and is an often overlooked method of disease control. (Also see Nutrition: Horses.) In young horses, developmental orthopedic disease (see Lameness in Horses: Developmental Orthopedic Disease in Horses) is the result of rapid growth, trauma to articular cartilage or growth plates, genetic predisposition, and nutritional imbalances. Dietary management involves regulating energy intake to avoid excessive rates of growth and weight gain. The proper balance of protein, calcium, phosphorus, zinc, and copper is important in supporting healthy endochondral ossification and in stabilizing bone collagen and elastin synthesis. The amount of nutrients required in the diet for normal bone development are dictated by rate of growth. Excessive energy intake contributes to osteochondrosis by decreasing bone density and cortical thickness.

Deficiency of protein must be severe to interfere with endochondral ossification. Rapidly increasing protein intake may produce faster bone growth; however, if the diet lacks adequate minerals to support this increased growth, altered endochondral ossification can be seen. Calcium and phosphorus balance affect bone density, rate of growth, and cartilage thickness. Inadequate amounts of copper and zinc have been associated with an increased incidence of osteochondrosis and osteodysgenesis.

Some of the most common mistakes made when feeding young horses include feeding excessive grain and leafy legumes (eg, alfalfa, which results in too high an energy intake), feeding a diet with too little zinc or copper to support rate of growth, and feeding a diet with an improper calcium:phosphorus ratio. Cereal grains and grass forages are low in calcium, phosphorus, protein, and lysine. Excess energy from cereal grains may be more detrimental than excess energy from grass forages; one reason may be that energy from grain is derived from starch, whereas energy from grass forage comes from microbial production of volatile fatty acids. Starch, but not volatile fatty acids, stimulates insulin secretion, which has been implicated in stimulating hormone changes that contribute to osteochondrosis.

Older horses often have dental problems that compromise feed intake and mastication. Extruded or soft pelleted feeds are ideal. Hay should be good quality, leafy, and easy to chew.

Diet manipulation can help treat, control, and prevent other disease conditions. Horses with recurrent airway obstruction should be fed as dust-free a feed as possible. Adding water or oil to grains decreases dust. Hay should be thoroughly soaked and fed close to the ground. If complete pelleted feeds are fed, hay can be removed completely from the diet. On sandy soils, hay should be fed off the ground to reduce sand ingestion. Dietary management can be used to reduce the risk of gastric ulcers. Alfalfa hay, with its high calcium and protein concentration, acts as a buffering antacid and has a protective effect on the nonglandular squamous mucosa. Small hay meals fed frequently or access to pasture also reduces the risk of gastric ulceration.

Nutritional management for Quarter horses with hyperkalemic periodic paralysis is focused on decreasing dietary intake of potassium and increasing renal potassium losses. Dietary manipulation includes avoiding high potassium feeds such as alfalfa hay, brome grass, canola oil, soybean meal or oil, and sugar or beet molasses and replacing them with timothy or bermuda grass, beet pulp, and grains such as oats, corn, wheat, or barley. Affected horses should be exercised regularly and have access to pasture.

Heavily muscled breeds of horses including Quarter horses, draft horses, and Warmbloods are prone to myopathies associated with elevated muscle glycogen stores and polysaccharide storage inclusions in type II muscle fibers. Successful management of this condition, known as polysaccharide storage myopathy, focuses on increasing the fat content of the diet and eliminating or reducing grain intake.

Stall confinement, poor-quality or high-fiber feed, inadequate water intake, and ingestion of foreign material (eg, rubber fencing) predispose to intestinal impaction. Management practices to reduce the risk of impaction include ad lib access to fresh water (warm water may be preferred during cold weather), adequate exercise, good quality feed, and good dental care. If impaction has been a problem, poorly digestible feeds (eg, mature forages) should be placed with low-fiber, highly digestible forages (eg, growing grass or legume hays). A complete pelleted or extruded feed helps maintain soft feces.

There has long been an association between grazing lush pastures or consuming large amounts of legume hay and the development of laminitis. Anecdotal observations indicate that pasture-associated laminitis occurs at times of rapid grass growth (eg, spring and early summer and in the fall after rainfall) that favor accumulation of certain carbohydrates such as fructans, starches, and sugars. Some horses and ponies may be more susceptible to pasture-associated laminitis because of genetic predisposition and other metabolic factors, including obesity (regional adiposity), peripheral insulin resistance, and hyperinsulinemia. Strategies to reduce the risk of laminitis focus on limiting the intake of nonstructural carbohydrates such as fructans from pasture and other feedstuffs.

Horses and ponies with a history of recurrent laminitis should have limited pasture access during periods of rapid grass growth, such as spring and early summer. Nonstructural carbohydrate content also tends to increase during the morning, reaches maximal values in the afternoon, and then declines overnight. Therefore, a popular recommendation is to turn “susceptible” individuals out on pasture overnight or during the early morning and to remove them from pasture by mid-morning. Stemmy, mature pastures should be avoided because mature grasses may contain more fructans. Turning susceptible individuals onto pasture that has been exposed to low temperatures in conjunction with bright sunlight (eg, as in the fall after a flush of growth followed by cool sunny days) should be avoided, because colder temperatures reduce grass growth and result in concentration of the fructan.

If feeding forage, lush legume hays should be avoided because they tend to have higher nonstructural carbohydrate content. Soaking hay before feeding may help reduce the amount of fructans being fed. Grain and sweet feeds should be avoided. Supplemental concentrates include those with a low glycemic index.

Last full review/revision July 2011 by Wendy E. Vaala, VMD, DACVIM