Cell Wall Carbohydrates of Flaccidgrass Plant Parts. II. Neutral Sugar Relationships to In Vitro Disappearance and Fiber Fractions

The relationship between cell wall (CW) structural composition and in vitro dry matter disappearance (IVDMD) is important in defining a physiological basis for quality differences among subtropical forage grasses. This study was conducted to examine the relationships on a plant part basis between trifluoroacetic acid (TFA)- hydrolyzed cell wall monomers of flaccidgrass (Pennisetum flaccidum Griseb.) and the traditional quality estimates of IVDMD, neutral detergent fiber, and hemicellulose. Structural carbohydrate (SC) monomers were determined following TFA hydrolysis by gas chromatography on residues recovered after 0, 20, 40, and 72 h of in vitro fermentation. Neutral detergent extracted cell walls were prepared from flaccidgrass plant parts (leaf blade, sheath, and stem) of four maturities (early season's growth to maturity). A unit increase in IVDMD was associated with a unit decrease in the cell wall concentration (r = −0.98). With increasing fermentation time, less SC monomers (consisting of arabinose, xylose, galactose, and glucose) were hydrolyzed from the CW residues. The CW monomers most strongly related to IVDMD were xylose and glucose (r = −0.94 and −0.87, respectively). In the latter relationship there was a significant quadratic response. The total CW hydrolyzed sugars were also strongly related to IVDMD (r = -0.98). These strong relationships were retained when the analysis was performed on 72-h digested CW residues (r = -0.93, -0.87, and -94 for IVDMD vs. xylose, glucose, and total monomers, respectively). Similarly, the total hydrolyzed sugars of cell walls were good predictors of hemicellulose determined empirically by subtracting acid detergent fiber from neutral detergent fiber (r² = 0.98). Carbohydrate monomers responsible for variation in IVDMD were identified and the carbohydrate concentration of flaccidgrass cell walls was found to be intricately associated with IVDMD.