Digestion of Soluble and Fibrous Carbohydrate of Forage by Horses

Abstract

The utilization of the soluble and fibrous carbohydrate fraction of the organic matter was compared among several species of forage in two 6×6 Latin square digestion experiments using horses. The major difference among forages in digestible organic matter and digestible energy was the result of different carbohydrate composition of the forages. The legume forages with the greater portion of soluble carbohydrate were the most digestible. Even the stemmy alfalfa hay of experiment I was more digestible than the grasses, because it contained less fibrous carbohydrate and more soluble carbohydrate. For forages in general, the cell wall constituents (CWC) analysis is a most meaningful separation of nutrients. CWC alone maintains a clear distinction between the soluble and fibrous fractions of the forages. The cellular content is almost completely digestible (Van Soest, 1967) and by definition exactly the inverse of the CWC. As CWC increases with maturity (Colburn and Evans, 1967), the more digestible nutrients decrease. CWC was shown to be inversely related to digestibility (Van Soest, 1967). Further partitioning the CWC into holocellulose and lignin makes a clear distinction between fibrous carbohydrates and the non-nutritive residue. The amount and digestibility of holocellulose is of major importance in assessing the nutritive value of a forage as the fibrous carbohydrates contribute substantially to the digestible energy obtained from the forage by herbivorous animals. One disadvantage of the CWC analysis is that fiber-bound nitrogen compounds and mineral matter are also included; CWC, therefore, is not simply holocellulose and lignin. Nutrients that are encased in or attached to the CWC may have reduced digestibility and represent the least digestible portion of these nutrients. This weakness may be an advantage in making a practical assessment of the digestibility of a forage. The separation of nutrients proposed in this paper shows that the higher soluble carbohydrate content along with the high apparent digestibility of soluble carbohydrate is the distinct advantage legumes have over grasses.