Cell Wall Carbohydrates of Flaccidgrass Plant Parts. I. Neutral Sugar Composition of Fermented Residues1

Abstract

Poor quality subtropical pasture grasses limit animal productivity, especially in mid‐ to late‐summer in the southeastern USA. The fermentability of structural carbohydrates (SC) is an important indicator of quality. Cleavage of the SC into monomers was examined for flaccidgrass (Pennisetum flaccidum Griseb.) plant parts (leaf blade, leaf sheath, and stem) having a range of fiber concentrations. Cleavage was achieved by sequential hydrolysis using 2.0 mol L−1 followed by 0.25 mol L− trifiuoroacetic acid (TFA) of neutral or acid‐detergent extracted cell wall residues (CWR) at 121°C for 1 h at each step. Hydrolysate monomer concentrations were determined by gasliquid chromatography of trimethyisilylated derivatives. Samples at four maturities were fermented in an in vitro rumen fluid bioassay for 0, 20, 40, and 72 h prior to CWR preparation. Plant parts differed widely in their fiber fractions, with neutral detergent fiber ranging from 644 to 817 g kg−1; acid detergent fiber from 336 to 501 g kg−1; hemicellulose from 277 to 353 g kg−1; cellulose from 282 to 383 g kg−1; and lignin from 47 to 112 g kg−1. In vitro dry matter disappearance ranged from 459 to 778 g kg−1. In immature (preheading) plants, the leaf sheaths consistently had a lower disappearance than leaf blades or stems, while mature (postheading) plant stems had the least disappearance. With increasing fermentation time, less SC monomers were hydrolyzed by TFA from the CWR. There was a significant (P ≤ 0.01) linear maturity effect for both the individual (arabinose, xylose, galactose, and glucose) and total neutral sugar concentrations. Significant (P ≤ 0.01) linear and quadratic incubation time responses were found for these variables. Concentrations of SC monomers generally differed among the plant parts. Arabinose, xylose, and galactose in leaf blades were fermented more than in sheaths or stems. Glucose maintained a high level of disappearance, even in mature plants (83 to 93 g kg−1). Xylose fermented the least of the SC monomers (25 to 63 g kg−1 in mature forage). Its resistance to fermentation was a major limitation to the efficient utilization of mature flaccidgrass forage.