Fiber is commonly determined as the dry matter (DM), or preferably the organic matter (OM), insoluble after extraction with a neutral detergent solvent. The NDF consists of the structural carbohydrates cellulose and hemicelluloses and potentially indigestible entities such as lignin and lignified structural carbohydrates. Conceptually, NDF can be divided into PDF and IDF. Analytically, IDF is estimated by fitting kinetic models[1'2] that describe the disappearance of NDF over digestion time. The IDF is estimated as the undigested NDF remaining when no further disappearance of NDF is detectable by the kinetic model. Alternatively, IDF is analytically defined as the undigested NDF remaining after exposure to agents of digestion for a sufficient time (6 10 days) to approximate complete digestion of PDF. The PDF is analytically defined as the difference between NDF and IDF. Thus, by difference, the quantitative estimation and utility of both IDF and PDF are inseparable.
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