DIGESTIBILITY OF STRUCTURAL CARBOHYDRATES IN COOL-SEASON GRASS AND LEGUME FORAGES

Structural carbohydrates of forages are not completely digestible by ruminants. The nature of the limitation and interrelationships among digestibility of structural carbohydrates are poorly understood. This field study was conducted to determine digestion kinetics of cell-wall carbohydrates to better understand these interrelationships. Leaves and stems of alfalfa (Medicago sativa L.), birdsfoot trefoil (Lotus corniculatus L.), smooth bromegrass (Bromus inermis Leyss.), and orchardgrass (Dactylis glomerata L.) were sampled in late May, and stems were again sampled 6 wk later. Plant samples were incubated in buffered rumen fluid for 0, 6, 12, 24, 48, and 72 h, and digestion kinetics were determined for neutral-detergent fiber (NDF) and cell-wall carbohydrates. Potentially digestible cellulose as a proportion of total cellulose decreased to a greater extent with age in grass (53%) than in legume (22%) stems. Hemicellulose often was more digestible than cellulose. However, cell-wall glucose, the principal component of cellulose, was more digestible than xylose, the principal component of hemicellulose. Conversely, the remaining neutral sugars associated with hemicellulose generally were more digestible than glucose. Xylose had the lowest digestibility of the neutral sugars, which was as low as 17% in legume stems. Arabinose digestibility was > 60%, even in highly lignified stems, suggesting that not more than a small portion is bonded to lignin. Across plant parts, a close relationship existed between glucose and xylose digestibilities with coefficients of determination of 0.94, suggesting that they may mutually limit each other's digestibility. With stem aging of grass, NDF rate of digestion was reduced from 0.076 to 0.042 h-1 by a decrease in hemicellulose rate of digestion from 0.146 to 0.060 h-1.