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This article in CS

  1. Vol. 27 No. 5, p. 1069-1076
    Received: July 28, 1986

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Cell Wall Carbohydrates of Flaccidgrass Plant Parts. III. Near Infrared Reflectance Spectroscopy Determination, Rate, and Extent of Fermentation1

  1. J. M. de Ruiter and
  2. J. C. Burns2



Cell wall carbohydrate fractions of forage plants strongly influence their rate and extent of fermentation. Two different analytical methods were evaluated to establish the rate and extent of structural carbohydrate in vitro fermentation in a subtropical forage grass species, flaccidgrass (Pennisetum flaccidum Griseb.). The procedures involved either chemical determination [cell wall hydrolysis with trifluoroacetic acid (TFA), derivatization and analysis by gas-liquid chromatography], or near infrared reflectance spectroscopy (NIRS) of partially fermented cell wall residues. The chemical hydrolysis products were carbohydrate monomers of noncellulosic origin. Coefficients in a fermentation model derived from chemical data indicated major differences in the in vitro disappearance of cell wall xylose and glucose. Cell wall xylose disappearance was 32% slower than glucose when averaged over the range of flaccidgrass maturities. In addition, between 41 and 73% of the xylose was fermented, compared with 82 to 98% of the cell wall glucose hydrolyzed by TFA. Plant maturity and plant part (leaf, sheath, or stem) had pronounced effects on the digestion model coefficients. When averaged over all plant parts, total cell wall monosaccharides losses from mature plants were 26% less than in immature plants, with an overall 40.5% reduction in the disappearance rate. There was an average of 19% difference in the extent of fermentation of leaf and sheath, while stems fermented 38% slower than leaf. Fermentation losses and the cell wall carbohydrate components remaining at different fermentation periods were predicted accurately and rapidly by NIRS. Predicted and observed values for the level of fermentation were highly correlated (r = 0.95) as were carbohydrate monomers (r > 0.73). The method can be used to estimate the digestion (in vitro and in vivo) of forage species on the basis of their structural composition, provided calibrations are precise. Both the digestion model and NIRS method gave good predictions of the concentration of carbohydrates in partially fermented cell walls, as well as the extent of dry matter loss during fermentation.

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Copyright © 1987. Crop Science Society of America, Inc.Copyright © 1987 by the Crop Science Society of America, Inc.