In vitro ruminal fiber digestion as influenced by phenolic-carbohydrate complexes released from sorghum cell walls

Abstract

Objectives were to isolate groups of phenolic-carbohydrate complexes from brown-midrib 6 and normal ‘Redlan’ sorghum ( Sorghum bicolor (L.) Moench) and to investigate the influence of isolated complexes on in vitro fiber digestion. Three field replicates of brown-midrib 6 and normal sorghum stem were harvested 1 week after physiological maturity of grain. Extracts of cellulase-digested neutral detergent fiber (NDF) of both sorghums inhibited in vitro dry matter digestion when added to in vitro ruminal incubations at quantities equivalent to 0.5, 1.0, and 2.0 g sorghum stem NDF. Cellulase extracts separated by size exclusion Chromatograph y (Sepharose CL-6B) yielded two major peaks that absorbed at 320 nm. Phenolic-carbohydrate complexes isolated in Peak 2 were lower in p-coumaric (PCA): ferulic acid (FA) ratios and higher in xylose: arabinose ratios than complexes isolated in Peak 1. When concentrated solutions of isolated fractions were added to in vitro ruminal incubations of sorghum leaf blade (0.25 g) at quantities equivalent to 0.5, 1.0, 2.0, and 3.0 g of sorghum stem NDF, in vitro NDF digestibility of sorghum leaf blade was reduced more by addition of Peak 2 than Peak 1 for both sorghums, but the effect on digestibility was greater for brown-midrib 6. The pH of resulting solutions did not influence in vitro dry matter digestibility. Further separation of Peak 2 by size exclusion chromatography (Sephadex LH-20, for separation of compounds with lower molecular weights) yielded several major peaks that absorbed at 320 nm. One of these peaks, with a low molar p-coumaric: ferulic acid ratio, inhibited fiber digestion when added at quantities equivalent to 5.0 g of sorghum stem NDF. Results suggest that phenolic-carbohydrate complexes low in PCA: FA molar ratio acid and high in xylose: arabinose ratio may inhibit ruminal fiber digestion, although the mode of action is not yet clear.