dietary fiber; that portion of ingested foodstuffs that cannot be broken down by intestinal enzymes and juices of monogastric animals and, therefore, passes through the small intestine and colon undigested. It is composed of cellulose, which is the 'skeleton' of plants, hemicellulose, gums, lignin, pectin and other carbohydrates indigestible by members of animal species other than Equidae and ruminants.
Dietary fiber is not to be confused with 'crude fiber', which is the term used in tables listing the composition of foods. Crude fiber is mainly lignin and cellulose and is the residue remaining after a food has been subjected to a standardized treatment with dilute acid and alkali. Crude fiber measurements usually underestimate actual total dietary fiber by at least 50%.
Plant fiber is essential in the diet of herbivores. Too much causes impaction of horse intestines and cattle rumens. Too little causes faulty ruminal function including ruminal tympany and ruminal lactic acidosis; a deficiency also causes an unacceptably low level of fat in cow's milk. See also impaction colic, ileocecal valve impaction, ruminal tympany, low-fat milk, phytobezoar.
acid-detergent fiber2 (ADF)
a modification of the procedure used to estimate crude fiber in animal feeds; it omits the alkali step, reducing loss of carbohydrates and giving a more accurate estimation. Estimates the proportion of dry matter which is insoluble in acid detergent, i.e. the cellulose and lignin in the sample. The normal range in farm animal feeds is 250-450 g/kg DM. The target for high quality grass silage is 350 g ADF/kg DM.
a laboratory estimate of the cellulose, hemicellulose and lignin content of a feed. It gives no indication of the digestibility of the feed. The sample is boiled gently in weak acid, then in weak alkali and the residue washed.
has little water-holding capacity and is not degraded in the intestine; includes celluloses, some hemicelluloses and lignin. Wheat bran and other cereal grains, and vegetables are sources.
neutral-detergent fiber2 (NDF)
a measure of fiber after digestion in a non-acidic, non-alkaline detergent as an aid in determining quality of forages. Contains the fibers in acid-detergent fiber, plus hemicellulose. soluble f. forms viscous solutions in water because of its great water-holding capacity and it is easily degraded by intestinal bacteria; includes pectin, vegetable gums and some hemicelluloses.