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Forage dry matter can be divided into two fractions on the basis of nutritional availability. The first fraction corresponds to the cellular contents and is composed of lipids, soluble carbohydrates, most protein and other water-soluble matter. This fraction is essentially available, but its digestibility appears incomplete because of the excretion of fecal non-cell-wall matter, which is principally of endogenous and bacterial origin. The second fraction corresponds to the plant cell wall, the availability of which is controlled by structural features that link cellulose, hemicellulose and lignin together. Of the cell-wall components, hemicellulose is a large and variable fraction which prevents crude fiber, cellulose, or lignin from being a good estimator of the content of the plant cell wall. The plant cell wall corresponds to what can be nutritionally denned as a total fiber fraction. The nutritive availability of the cell-wall fraction is not uniform among forages. The dual nutritive character of plant dry matter contraindicates the use of single factors to predict whole dry-matter digestibility.
P.J. Van Soest (Sun,) studied this question.