Forages: The Foundation of Equine Gastrointestinal Health

Horses have evolved over millions of years as grazers, with specialized digestive tracts adapted to digest and utilize diets containing high levels of plant fiber. They are capable of processing large quantities of forage to meet their nutrient demands. In an attempt to maximize growth or productivity, horses are often fed diets that also contain high levels of grains and supplements. Unfortunately, this type of grain supplementation often overshadows the significant contribution that forages make in satisfying the horse’s nutrient demands and can lead to serious gastrointestinal disturbances.

Digestive Function
Horses are classified anatomically as nonruminant herbivores or hindgut fermenters. The large intestine of the horse holds about 21 to 24 gallons (80-90 liters) of liquid and houses billions of bacteria and protozoa that produce enzymes which break down (ferment) plant fiber. These microbes are absolutely essential to the horse, because the horse cannot produce these enzymes without them. The by-products of this microbial fermentation provide the horse with a source of energy and micronutrients.

The equine digestive tract is designed in this way to allow the horse to ingest large quantities of forage in a continuous fashion. The small capacity of the upper part of the tract is not well-suited for large single meals, a fact that is often ignored by horsemen. Large single meals of grain overwhelm the digestive capacity of the stomach and small intestine, resulting in rapid fermentation of the grain carbohydrates by the microflora in the hindgut. This fermentation may result in a wide range of problems including colic and laminitis.

Forage Composition
Forages are composed of two components, cell contents and cell walls. Cell contents contain most of the protein and all of the starch, sugars, lipids, organic acids, and soluble ash found in the plant. These components are degraded by enzymes produced by the horse and are highly digestible. The cell wall contains the fibrous portion of the plant, which is resistant to digestive enzymes produced by the horse. The primary components of the cell wall are cellulose, hemicellulose, and lignin. The nutritive value of forages is determined by two factors: (1) fiber content (the proportion of the plant that is composed of cell wall), and (2) fiber quality (the degree of lignification).

These factors are important because the horse can digest practically all of the cell contents contained in forages, but bacterial fermentation can digest only 50% or less of most plant cell wall. The degree to which plant cell wall is digestible is largely dependent on the amount of lignin that it contains.

Factors aFFecting Forage Quality
Many factors affect the quality of forage. Most important of these are the species of plant, stage of maturity, location of where the plant was grown, and content of inhibitory substances. All of these factors should be considered when assessing the suitability of a particular forage for horses.

Species. Most plants that serve as forages for horses can be divided into two different categories, grasses and legumes. Grasses contain much structural matter in their leaves and leaf sheaths, and this can be as important as or more important than the stem in holding the plant erect. Examples of grass forages used for horses include temperate species such as timothy, orchard grass, brome grass, and fescue and tropical species like pangola, guinea, Bermuda, and kikuyu. Legumes, on the other hand, tend to be treelike, though on a miniature scale. Their leaves have very little structural function and form on the ends of woody stems. The primary legumes used as horse forage are alfalfa and clover.

At a similar stage of maturity, legumes tend to be higher in protein, energy, and calcium than grasses. ADF (acid detergent fiber; lignin + cellulose) does not vary that much between grasses and legumes at the same stage of maturity. NDF (neutral detergent fiber; lignin + cellulose + hemicellulose), however, is much higher in grasses than legumes. This is because grasses contain a great deal more hemicellulose than legumes. Therefore, evaluating the fiber content of forages based on ADF alone underestimates the total cell wall content and overestimates the total energy content of a grass. Remember, hemicellulose is typically only 50% digested in the horse, and cell solubles are almost completely digested. By only considering ADF, the assumption is that the rest of the forage (besides protein, fat and ash) is soluble sugar. This is truer in legumes, which contain only around 10% hemicellulose, than in grasses, which can have hemicellulose contents of 30% or more. The fiber in legumes tends to be less digestible than the fiber in grasses, largely because legumes tend to have higher lignin content per unit of total fiber. This means that the digestible fiber content of grasses is much higher than it is in legumes of similar maturity.

Because of these factors, legumes contain 20-25% more digestible energy than grasses at the same maturity. In certain instances, the amount of legume hay fed may be limited so that the horse doesn't get too fat. This can result in intakes of digestible fiber that are below optimal levels, particularly in extremely high-quality hays.

Stage of maturity. Generally, as plants mature they become less digestible because a greater proportion of their mass becomes structural. Legumes tend to mature by decreasing leafiness and increasing the stem-to-leaf ratio. Alfalfa leaves maintain the same level of digestibility throughout their growth. Their stems, however, decrease dramatically in digestibility as they mature because they become highly lignified to support the extra weight of the plant. The ultimate example of lignification for support is the oak tree. The wood of the oak tree is highly lignified and practically indigestible. When pulp wood is processed to make paper, the lignin is removed using harsh chemicals such as sulfuric acid (hence the sulfur smell around paper mills).

The leaves of grasses serve more of a structural function than in legumes. As they mature, these leaves become more lignified and less digestible. Since the stems of certain grasses serve a reserve function, they may actually be more digestible than the leaves of these grasses at a later stage of maturity. When forage is grazed as pasture, its nutrient quality is almost always higher than when it is harvested as hay unless the pasture is the dead aftermath left over from the previous growing season. New spring pasture can be quite low in fiber content and high in soluble carbohydrates.

Latitudinal effects. The digestibility of tropical forages is much lower than temperate forages. Plants that grow in the tropics have been genetically selected for a larger proportion of protective structures such as lignin to avoid predation. At the other extreme are the perennial plants in the far northern regions of the world. These plants have very short growing seasons and need to store energy in reserves as sugars and fructans rather than in irretrievable substances such as lignin and cellulose. Care should be taken when feeding high-fructan forages to horses since these compounds are poorly digested in the small intestine and may lead to colic or laminitis due to excess lactic acid fermentation in the hindgut.

Inhibitory substances. Besides lignin, a number of other substances in forages can reduce digestibility of fiber and minerals. Silica is used as a structural element that complements lignin to strengthen and add rigidity to cell walls. Alfalfa and other temperate legumes restrict absorption of silica and contain little in their tissues. Cereal straws are quite high in silica. This gives the straw a clean, glassy appearance, and it also reduces its digestibility.

There are also substances contained in forages that can inhibit mineral digestibility. Two that are particularly important are phytate and oxalate. Phytates contain phosphorus in a bound form that is unavailable to the horse. Phytate may also inhibit the digestibility of other minerals such as calcium, zinc, and iodine. Oxalates can reduce the digestibility of calcium in forages if the calcium-to-oxalate ratio in the forage is 0.5 or less on a weight-to-weight basis. This is a common problem in tropical forages that tend to be high in oxalates and low in calcium. Low calcium availability in tropical forages can lead to nutritional secondary hyperparathyroidism or "big head" disease. Therefore, when tropical forages are fed to horses, supplemental sources of calcium should be available.

Forage Intake
To accurately calculate the contribution that forage makes to the horse's overall feeding program, forage intake as well as composition must be known. Table 1 gives a range of forage and concentrate intakes for various classes of horses based on body weight. High forage intakes will occur where there is an abundance of forage available, such as with Kentucky pasture or Washington state alfalfa hay. Low forage intakes will occur where forage is sparse and of poorer quality such as in the tropics. These estimates illustrate how much forage quality and level of intake can affect a horse's overall feeding program. Not taking into account the contribution that forage makes to a horse's overall nutrient intake can result in some serious errors in feeding.

Article courtesy of Kentucky Equine Research.

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