THE MERCK VETERINARY MANUAL
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Nutrition of Horses

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During the last 20–30 yr, there has been an increased awareness of equine nutrition and its importance to the health of the horse. (Also see Horses in the Nutrition chapter). In every stage of life, nutrition is the foundation for equine health and longevity. The dietary needs of a horse change as it goes through each life stage. The great varieties of commercial feeds on the market attest to the recognition of different diets for different life stages. Horses must be provided with an adequate supply of energy, protein, vitamins, and minerals and have access to fresh, clean water.

Adequate fresh water intake is essential for every horse. Storage tanks, troughs, or pails should be placed so the horse can reach in comfortably. Because most horses are reluctant to put their head in a trough or pail below eye level, the water level should be kept high. If water levels get too low, many horses will refuse to drink. The optimal temperature for drinking water is 68°–78°F (20°–26°C). Horses will reduce water intake if the water temperature is too cold or too warm. When daytime temperatures exceed 100°F (38°C), water in exposed pipes or hoses will be dangerously hot and should not be used for drinking or bathing.

The nutrient energy requirement of the horse depends on its level of activity, the energy content of the diet, and the capacity of the animal's digestive system. The size and weight of the newborn foal is influenced by the nutrition of the pregnant mare. Horses fed for rapid body and skeletal growth may develop bone abnormalities or be more prone to lameness conditions. A balanced diet should be fed according to the desired rate of gain within the sound parameters of good health. The best measure of growth in a young horse is weight, and the best description of size of a horse is a combination of height and weight. A high correlation between the measure of the heart girth and body weight in the horse has been reported. In young growing horses, monthly measurements are helpful to monitor growth changes.

There are several reasons to know the weight of horses. Many feeding requirements are based on percentages of the horse's weight. Most feed recommendations are stated in the amount of feed (in pounds or kilograms) the horse should receive based on body weight. In young horses, "developmental orthopedic disease," although a multifactorial condition, can be related to rapid growth or imbalances in energy, protein, and minerals (see Developmental Orthopedic Disease in Horses). 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. The most variable dietary requirement for any horse is energy. A certain amount of energy is required for maintenance and daily activity. Metabolic demands are increased for such activities as growing, performance activity, or lactation. In some activities, such as racing, jumping, or polo, the energy requirement may be increased by as much as 100%.

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 (see 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 increased 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. (Also see Myopathies in Horses.)

Management practices to reduce the risk of impaction include ad lib access to fresh water, adequate exercise, good quality feed, and good dental care. If impaction has been a problem, poorly digestible feeds (eg, mature forages) should be replaced with low-fiber, highly digestible forages (eg, growing grass or legume hays). A complete pelleted or extruded feed helps maintain soft feces.

Grazing lush pastures or consuming large amounts of legume hay and the development of laminitis have long been associated. 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, 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.

The terms mature, senior, and geriatric refer to horses that have completed their growth cycle. However, because aging is a continuous process, there is no discrete age range for each category. Improved methods of overall care, management, and diet have enabled horses to live into their 20s or 30s. This increased longevity means they are used in a variety of leisure or competitive activities well into their teens or 20s. Good management of still-active older horses means recognition and evaluation of conditions that occur with age (eg, arthritis, hyperadrenocorticism, gastric ulcers, laminitis, navicular disease, kidney or liver dysfunction), followed by therapeutic management and special dietary considerations. Horses with arthritis require a balanced diet with adequate minerals. In horses prone to hyperadrenocorticism, the use of fat-supplemented feeds with highly digestible fiber and reduced starch and sugar intake is important. Horses with liver or kidney conditions should not be fed legume hays and fat-supplemented feeds. Horses confined to stalls most of the time are more prone to intestinal problems and development of undesirable behaviors.

Last full review/revision November 2014 by Thomas J. Lane, BS, DVM

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