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Structural Neutral Sugars in Legume and Grass Stems in Relation to Digestibility¹

Composition of cell wall (CW) polysaccharides of forages can affect rate and extent of digestion by rumen microorganisms beyond the effects attributed to lignin, yet few direct comparisons of monosaccharides in CW of species and cultivars of legumes and grasses have been made. This study was conducted to determine concentration of neutral sugars in CW polysaccharides of immature and mature grass and legume stems, and to relate neutral sugar concentrations to in vitro digestibility of dry matter. In vitro true digestibility (IVTD); neutral-detergent fiber, acid-detergent fiber, permanganate lignin, and acid-detergent insoluble ash (used to estimate holocellulose concentrations); and neutral sugars in ethanol-insoluble residues were determined in 150-mm segments of stem bases of fieldgrown cultivars of alfalfa (Medicago sativa L.), birdsfoot trefoil (Lotus corniculatus L.), red clover (Trifolium pratense L.), smooth bromegrass (Bromus inermis Leyss.), and orchardgrass (Dactylis glomerata L.). Concentrations of total neutral sugars (TNS) and holocellulose were greater in mature than in immature stems of all species except smooth bromegrass. Glucose comprised approximately 62% of the TNS in the grasses and 67% in the legumes, whereas xylose comprised 30% in grasses and 20% in legumes. Glucose proportion of TNS increased and arabinose proportion decreased with maturity. Arabinose proportion was similar for grasses and legumes, whereas galactose, mannose, and rhamnose were present in larger amounts in TNS of legumes than in the grasses. Differences in proportions of these neutral sugars also were found among grasses and among legumes. Among grass and among legume species, IVTD concentration was closely related to lignin concentration. In spite of large differences in proportions of these neutral sugars, the only sugar associated with IVTD in dependent of lignin was mannose in grass stems.

Key Words: Cell-wall carbohydrates • Cellulose • Hemicellulose • Glucose • Xylose • Arabinose • Mannose • In vitro digestibility

² Plant physiologist, USDA-ARS, and professor, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011; associate professor, Dep. of Animal Science, Iowa State Univ., Ames, IA 5001l; and professor, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011.

Received for publication March 13, 1987.

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