THE MERCK VETERINARY MANUAL
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Feeding Practices in Horses

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Ideally, horses should be given free access to hay and/or pasture forages with salt and water ad lib. Horses should not be offered >0.5% of their body weight in high starch/sugar grain-based concentrates (eg, textured grain, pellets, or extruded feed) in a single feeding. More than this in a single meal reduces digestive efficiency and predisposes to problems such as gastric ulcers, insulin resistance, laminitis, and colic. If large amounts of grain-based concentrates are being fed (>0.4% body wt/day), the total amount offered daily should be divided into two or more feedings. Most horses fed good-quality forages require little to no concentrate supplementation. Exceptions are hard-working horses or those with limited access to good-quality forage (<2% body wt in feed dry matter).

It has been well documented that feeding >50% of the total ration in the form of grain-based concentrates increases the risk of colic and laminitis in adult horses. High starch/sugar intake also has been correlated to increased incidence of insulin resistance in both adult and young growing horses. Large (>0.25% body wt) meals of grain-based concentrates should not be offered <1 hr before strenuous exercise, transport, or other stress, or to exhausted horses with poor gut motility.

Because horses are particularly sensitive to toxins found in spoiled feeds, all grains and roughages offered should be of good quality and free of mold. Grains should be stored at a moisture content of <13%. In warm, humid areas, mold inhibitors may help reduce feed spoilage. In contrast, excessively dry, dusty feeds tend to initiate or aggravate respiratory problems. Dampening or soaking such feeds in water before feeding can help alleviate this problem.

Feeds

Good pasture provides both an excellent source of nutrients and the opportunity to exercise. The pasture should be kept as free of weeds as possible by regular mowing or clipping. A legume-grass mixture offers the advantages of good nutrient supply, persistence, and durability. Ideal mixes vary with region, and local recommendations from specialists should be followed. However, some forages should not be used for horse pastures. Alsike clover (Trifolium hybridum) and kleingrass (Panicum coloratum are potentially hepatotoxic to horses, and Johnson grass (Sorghum halepense) and Sudan grass (S sudanense) contain cyanogenic glycosides. Buffel (Cenchrus spp), panic (Panicum spp), pangola (Digitaria decumbens), kikuyu (Pennisetum spp), and Setaria spp grasses all contain potentially harmful concentrations of oxalates. None of these forage species should be used for horse pastures.

In sandy areas, horses should be provided with supplemental hay when pasture is short (ie, overgrazed) to prevent sand ingestion and subsequent colic. The hay should be offered in feeders or on a platform to reduce sand ingestion. The use of psyllium products to enhance the elimination of sand from the equine GI tract can be expensive, and efficacy has not been well documented. Supplemental hay is also recommended in any situation when the pasture is limited in quality (lots of weeds, undesirable weeds) or quantity to avoid weed ingestion and preserve the pasture cover.

Common types of hay used to feed horses include both grass hays, such as timothy, brome, coastal Bermuda, or orchard, and legumes such as alfalfa or clover. Legume-grass mixtures are generally high-yielding and contain considerably more protein, minerals, and vitamins than do grasses alone. However, they may be more difficult to cure in a warm, humid climate and more prone to mold. Coastal Bermuda grass has been associated with an increased risk of impaction colic, especially when harvested late. Alfalfa may be contaminated with blister beetles and also tends to be more allergenic than grass hays or clover. Oat hay has been used in some regions and, if properly harvested and baled, is roughly equivalent to good-quality grass hays. Teff grasses are used in some regions of the world but may be deficient in calcium and should be used with caution (or only if an accurate nutrient analysis is done), especially for growing, pregnant, or lactating horses.

The form in which harvested forages are provided to horses is of concern. High-moisture haylages and silages made from whole corn plants should be used with caution as forage sources for horses because of the risk of molds. Moldy corn silage especially can cause fatal leukoencephalomalacia. Feeding large round bales of hay in pastures can be economically advantageous if feeders that reduce waste are used, but it has been documented to increase the risk of botulism. Cubed or chopped forages are often recommended as substitutes for long-stem hay or pasture and for horses that have trouble chewing. Forage-based cubes may need to be soaked in water to decrease the risk of choke, at least initially.

Concentrates include all grains and byproduct feeds high in energy and/or protein (eg, wheat bran, soybean meal, rice bran). Processing grains before feeding is often desirable to improve nutrient availability. However, cracked or rolled grains are more susceptible to mold. Due to differences in density, grains should be measured by weight, not volume.

Oats, one of the most traditional grains for horses, may be fed whole, rolled, or crimped. Processing increases the bulk 20%–30% and improves digestibility by ~10%. “Hulled” or “naked” oats are more energy dense than regular oats and should be introduced slowly to reduce the risk of founder or colic.

Barley is a good grain for horses. It is higher in energy than regular oats but lower than corn. It may be fed as the only grain to horses that have a high energy need. Barley should be rolled or crimped to improve digestibility. Palatability, however, is not as high as that of oats or corn.

Corn (maize) is a high-energy feed, useful for horses that are working hard or in need of extra weight gain. However, the starch in corn is less digestible than that of oats and can more easily bypass small-intestinal digestion, resulting in colic and/or laminitis if suddenly fed in large amounts. To maximize digestibility, shelled corn may be cracked or rolled, but the moisture level should be low enough to avoid spoilage during storage. Moldy corn can cause fatal leukoencephalomalacia.

Sorghum grain (milo) and wheat should be fed with care. These grains must be cracked or rolled if fed to horses. They are not commonly used in horse rations.

Other concentrate sources of energy/protein used in various regions of the world include dried peas (Great Britain), sugar cane pulp (Brazil), fava beans (Middle East), and bread (note that additives such as sesame or chocolate might cause positive drug-test reactions).

Wheat bran and rice bran are byproduct supplements commonly fed to horses. However, both are very high in phosphorus (>1.2%), and the proper calcium:phosphorus ratio should be maintained when any form of bran is added to the diet. Wheat bran is not laxative, contrary to popular belief, but is extremely palatable to horses and often used as a wet “mash” to increase water intake or mask the flavor of other supplements. Because of its high phosphorus content, wheat bran is not recommended as a major or daily component of the ration unless the calcium intake is carefully balanced. Rice bran is a high-fat product added to rations of horses that need extra calories. Many rice bran products have added calcium to offset the high phosphorus content but still are designed to be fed in only limited (<1 kg/day) amounts.

Beet pulp, a byproduct of the sugar beet industry, is added to horse rations as both a source of calories and fiber. It contains moderate amounts of calcium and protein and can be safely fed on a daily basis in larger amounts than the bran products. Shredded beet pulp usually should be soaked in water before feeding to horses. Beet pulp pellets do not require presoaking and are often included in concentrate mixes.

Edible oils and fats may be added to rations to increase the energy density. Normal horse rations contain only 3%–4% fat, but horses can easily tolerate up to 10% fat if it is introduced slowly and they are given 3–4 wk to fully adapt to the change. Corn and vegetable oils are commonly used. Edible oils should be introduced slowly to the ration to avoid diarrhea. Although highly digestible, animal fat is not commonly used in horse rations.

Soybean meal is a palatable protein supplement with good amino acid balance for use in concentrate mixes. It may be fed when pastures or hay are low in protein and are of poor quality or when protein requirements are greatest, such as during early growth or lactation. Linseed meal or cottonseed meal should not be used as a protein supplement for young, growing horses because of their low lysine content, but they are adequate for adult horses.

Cane molasses is frequently added to grain mixtures (sweet feeds). It is highly palatable, minimizes separation of “fines,” and reduces dustiness of concentrate mixtures. It is also high in potassium. The readily fermentable carbohydrates and moisture that cane molasses contains may increase mold growth in hot weather and freeze solid in cold winter weather. High sugar/starch (>30% nonstructural carbohydrate) rations have been documented to induce relative insulin resistance in horses and are associated with increased incidence of vices such as wind sucking and wood chewing.

Limestone of a high grade (38% calcium) may be used as a supplemental source of calcium. When both supplemental calcium and phosphorus are needed, dicalcium phosphate, steamed bone meal, or defluorinated rock phosphate is recommended. Dicalcium phosphate is particularly good because the cost per unit of phosphorus is low, the elements are quite available, and it is fairly palatable.

Salt (NaCl) should be provided in a block or in granular form free choice to all horses. Salt content of forages in some regions (notably the southwestern USA) and many commercial feeds may reduce the need for supplemental salt but, because of variable losses in sweat, exact needs are hard to estimate. Horses, if given the opportunity, will voluntarily consume sufficient salt to meet their longterm needs for maintenance, even in hot climates. For maintenance, in horses that do not have high mineral needs, trace mineralized salt that contains added iodine, iron, copper, cobalt, manganese, zinc, and selenium is often used. The need for these additional minerals varies with the locality (see Minerals).

Exercise- and/or heat-induced sweat losses can cause acute sodium/chloride/potassium deficits that must be replaced more rapidly than voluntary homeostatic mechanisms can accommodate. It is common in these circumstances (eg, performance horses) to give oral electrolyte drenches before, during, and/or after strenuous activities.

Feeding Rates

Individual differences in the need for energy and nutrients and gross variations in nutrient contents of feedstuffs make it difficult to generalize about the amount of feed to provide. The amounts given in Table 5: Horses:Recommended Nutrient Concentrations in Rations for Horses and PoniesTables and Table 6: Horses:Concentrates Satisfactory for Use with Hays as Indicated inTables can be used as guidelines, but body condition should be monitored and amounts adjusted accordingly. The maximal dry matter intake in 24 hr is only 3%–3.5% of a horse's body wt, and many horses voluntarily consume <2.5% of their body wt in dry matter in 24 hr. Feed intake should therefore be monitored.

Table 5

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Table 6

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The need for concentrate supplementation while on pasture depends on pasture quality but is more important for young horses and lactating mares. If the pasture is of good to excellent quality, no supplementation other than water and salt are needed by most adult horses at maintenance or in light work. It is desirable to creep-feed nursing foals at the rate of 0.5%–1% body wt with concentrates formulated specifically for growth. Good-quality hay may be needed even when on pasture, especially in winter.

Forage-based total mixed rations and "complete" feeds,which may have concentrates added, have been developed for horses. These can be textured, pelleted, cubed, or extruded products. They have the advantage of uniform quality, complete control over nutrient intake, suitability for horses with bad teeth, less dustiness (which reduces respiratory problems), and reduced bulk for storage and transport. Disadvantages include an increased risk of choke and increased wood chewing, especially with the pelleted and extruded feeds fed as the sole source of nutrition. Most, however, are sufficiently high calorie that they need to be limit fed (<2.5% body wt/day). Wood chewing and boredom can be minimized by feeding long-stem hay with these products or by dividing the total daily allotment into multiple small feedings. Damage to stables and fences can be reduced by treating wood with foul-tasting substances or by covering or replacing wood with metal in vulnerable areas.

Last full review/revision June 2015 by Sarah L. Ralston, VMD, PhD, DACVN

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