Structural Polysaccharides in Forages and Their Degradability

Plant cell walls are organized into complex matrices composed primarily of carbohydrates that represent a potential energy source for ruminants. An overview of the basic structural features of the major groups of polysaccharides that make up forage cell walls, metabolic changes during cell wall development, cell wall component interactions, and some aspects of cell wall degradation are presented. Polysaccharide components that compose the cell wall matrix can be disassociated and fractionated according to similar physio‐chemical properties. Plant structural polysaccharides vary in monosaccharide composition, substitution patterns, and glycosidic linkages between monomers. Chemical fractionation schemes typically separate structural polysaccharides into three major groups. These include pectic polysaccharides rich in galacturonic acid residues, hemicellulosic polysaccharides typically rich in xylose, and the cellulose fraction. Mature walls can be envisioned as a layered matrix with cellulose microfibrils embedded in an amorphous material composed of pectic and hemicellulosic polysaccharides. The amorphous regions of cell walls show the greatest variation during development. Interactions among individual components of the cell wall are key to the formation and structural integrity of the matrix. These include bonding interactions that range from weak hydrogen bonds to covalent cross‐links. During growth of plant cells there is a rearrangement of structural elements within the wall. The matrix undergoes reorganization by disruption of selected intra‐ and intermolecular bonds and de novo synthesis and insertion of new wall polymers. Individual polysaccharides may vary in rates and possibly extent of degradation by rumen microorganisms. These differences are due to the degree of hydrogen bonding of the individual polysaccharides, branching patterns, and associations with other wall constituents.