Dietary Fiber

In contrast to the primarily nonstructural, nonfiber carbohydrates whose monosaccharides are linked by alpha bonds which animals are able to digest, no animal produces the digestive enzymes necessary to break the beta bond linking the monosaccharides that make up fibers. Only bacteria have the enzymes necessary to break these beta bonds and, therefore, digest and utilize fiber directly. However, by having these bacteria in their gastrointestinal tracts, and by being able to utilize the products of bacterial digestion of fiber, animals are able to utilize plant fiber with varying degrees of efficiency. In using plant fiber for their own functions, bacteria convert the fiber to short chain, or volatile, fatty acids (acetic, propionic, lactic, isobutyric, butyric, isovaleric, and valeric). These volatile fatty acids, or VFAs, are absorbed and provide 30 to 70% of the horse’s, and 70 to 80% of the ruminant’s, total digestible energy needs, with the amount provided being at the upper end of these ranges on high-fiber (high-forage) diets, and the lower end of these ranges on high-nonfiber carbohydrate or starch (high-grain) diets. Bacterial fermentation of fiber occurs primarily in the rumen of ruminant herbivores, such as cattle and sheep; the cecum and colon of nonruminant herbivores, such as horses, rabbits, and rats; and in the large intestine of omnivores, such as people, pigs, and dogs, and carnivores, such as cats.

What is analyzed in a feed as dietary fiber consists of not only polysaccharides composed of monosaccharides linked by beta bonds, but also lignin and, in overheated feeds, starch that has been rendered undigestible because of heat damage. Lignin is completely resistant to digestion and fermentation by both animals and bacteria. The betabond polysaccharides or fiber consist of the insoluble plant structural fibers cellulose and hemicellulose, and of soluble fibers. Soluble fibers are the parenchymatous portions and secretions of plants, such as sap, resin, gums, pectin, and mucilages present primarily in fruits, vegetables, cereal grains, beans, and lentils. Insoluble fibers, on the other hand, are highest in nonseed and nonfruit portions of plants, such as the leaves, stems, hulls, and wood.

Total dietary fiber, therefore, is the sum of soluble and insoluble fiber, plus small amounts of lignin and, in heat damaged feed, undigestible starch. Although the amount of each in a feed can be determined by enzymatic procedures, this is unnecessary for feeding purposes and is rarely done. Nonenzymatic fiber analysis excludes soluble fibers, which are, therefore, included with the calculated amount of nonfiber carbohydrates, or NFC, in the feed. This is of little consequence because in feeds commonly fed the horse, the amount of soluble fiber is small compared to the amount of nonfiber carbohydrate, and because both are fairly equally utilized and provide the same amount of dietary energy. In contrast, insoluble fiber is the most poorly utilized potential source of dietary energy. Thus, the higher the insoluble fiber content of feed, the lower the amount of usable dietary energy that feed will provide. Undigested feed, which for most feeds is primarily undigested insoluble fiber, helps maintain normal gastrointestinal motility and function, and helps prevent too rapid an intake of the readily digested nonfiber carbohydrates. Excessive intake of readily digested nonfiber carbohydrates at one feeding may result in diarrhea, colic, and laminitis or founder.

All animals use nearly all of the soluble fiber ingested. However, for ingested insoluble fiber, the sooner its beta bonds are broken, the greater the percent digested and utilized by the animal. Thus, insoluble fiber utilization is greatest for ruminants, less but still quite high for horses, and least but still significant for nonherbivores. People, for example, are able to utilize 30 to 85% of the insoluble fiber in commonly eaten foods. Horses and cattle are equally effective in digesting low-fiber feeds. But cattle are better able to digest high-fiber feeds. Fiber digestion by horses is only 65 to 75% of that by cattle and sheep, whereas it is 115 to 120% for llamas, 65 to 75% for elephants, 50 to 55% for swine, and 40 to 45% for rabbits and hamsters. In all species, however, as the fiber content of the diet increases, the digestibility of diet organic matter generally decreases, but the reduction in digestibility is greater for horses than it is for cattle. The horse compensates for the lower energy content and energy digestibility in diets higher in fiber content through greater consumption of that diet. Donkeys can eat more and tend to digest fiber the most effectively; they therefore can get by better on high-fiber diets than can ponies and horses.

There are several methods used for determining the amount of insoluble fiber in feed. The crude fiber method is the oldest and most commonly used method. It is the only fiber value available for many feeds for the horse and is, therefore, used here. Unfortunately, crude fiber analysis is quite inaccurate; as much as 85% of the hemicellulose and 50% of the cellulose may be lost, resulting in an unpredictable underestimation of the fiber content of the feed and, therefore, an overestimation of the nonfiber carbohydrate content of the feed. Since nonfiber carbohydrate is more digestible and utilizable than fiber, overestimating a feed’s nonfiber carbohydrate content may result in an overestimation of its caloric content and, therefore, its feeding value. Other fiber analyses occasionally used are neutral and acid detergent fiber (NDF and ADF) methods. Although NDF includes nearly all cellulose and over 50% of the hemicellulose, it also erroneously includes a high amount of amylase digestible starch. Although acid treatment removes the starch, it also removes most of the hemicellulose.

Because most of the hemicellulose is utilizable by the horse, its loss in the ADF analysis results in an underestimation of that feed’s insoluble fiber content and, therefore, overestimation of the feed’s energy content and feeding value. However, ADF is the most accurate indication of a feed’s poorly utilizable fiber content that is generally available. Although amylase treatment of NDF is a more accurate indication of a feed’s insoluble fiber content than is ADF, because amylase removes only digestible starch, its amount in feeds for horses is available even less frequently than is ADF content.

This article is from "Feeding and Care of the Horse", second edition, by Lon D. Lewis, Lippincott Williams & Wilkins, 1995. Reprinted with permission from the publisher.