Disappearance of hemicellulosic monosaccharides and alkali-soluble phenolic compounds of normal and brown midrib sorghum x sundangrass silages fed to Holstein steers.

Four ruminally cannulated Holstein steers fed ad libitum were used in a 4 x 4 Latin square design. Feeds were normal and brown midrib genotypes of Redlan x Piper and Redlan x Greenleaf sorghum-sudangrasses. An in situ trial was run in conjunction with the digestibility trial as part of the data collection for each period. Hemicellulosic monosaccharide and alkali-soluble lignin phenolic compound disappearances were quantitated. The xylose to arabinose ratio was higher in normal genotypes than in brown midribs. Normal genotypes had higher concentrations of alkali-soluble p-coumaric acid and lower vanillin, vanillic acid, and sinapic acid than did brown midrib mutants. Disappearance of p-coumaric acid was higher in steers consuming normal genotypes than in those consuming brown midribs. Neutral detergent fiber, acid detergent fiber, hemicellulose, cellulose, and acid detergent lignin showed no genotypic differences in rate of digestion, but extent of 72 h digestion was higher for brown midrib mutants than for normal genotypes. Extent of digestion of the hemicellulosic monosaccharides, xylose, and uronic acids was higher in situ for brown midrib silages compared with normal genotypes. Differences in alkali-soluble phenolic compound and cell wall monosaccharide profiles between normal and brown midrib genotypes may help explain digestibility differences between the two genotypes.