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This article in AJ

  1. Vol. 70 No. 4, p. 550-554
    Received: June 8, 1977

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Concentration, Composition, and in vitro Disappearance of Hemicellulose in Tall Fescue and Orchardgrass1

  1. C. S. T. Daughtry,
  2. D. A. Holt and
  3. V. L. Lechtenberg2



Hemicelluloses are complex groups of cell wall polysaccharides which comprise significant proportions of the dry matter of many forages. Factors affecting hemi-cellulose concentration in grasses could influence efficient utilization of fibrous feeds by ruminants. The purpose of this research was to evaluate the effects of N-fertilization, maturity, and plant tissue on the disappearance of cell wall constituents (CWC), hemicellulose, and the components of hemicellulose in two forage gzasses during in vitro rumen fermentation.

Tall rescue (Festuca arundinacea Schreb.) and orchardgrass (Dactylis glomerata L.) plots on a Chalmers silty clay loam (fine silty typic argiaquolls) were fertilized with either 0 or 112 kg/ha of N as urea. The grasses were labeled with 14CO2 on 21 May 1975 (heading stage) and were sampled on the 1st, 7th, and 19th days after labeling. Cell wall constituents were prepared by neutral detergent extraction of lyophilized, ground tissue before and after the tissue had been incubated in buffered rumen fluid for 48 hours. Hemicellulose was hydrolyzed in situ with 1.0 N H2SO4. Sugar residues of hemicellulose were separated and identified by paper chromatography and the radioactivity was measured by liquid scintillation counting.

The concentrations of each cell wall component of tall rescue and orchardgrass generally were not significantly different (P = 0.10). Leaves of N-fertilized grass contained 51.1% CWC and 29.5% hemicellulose compared to 58.1% CWC and 24.0% hemicellulose in leaves of non-fertilized grass. Concentrations of CWC and hemicellulose in stems were 10 and 5 percentage units, respectively, higher than in leaves. Xylose and arabinose were the major components of hemicellulose, accounting for 37.8 and 23.6%, respectively, of the hemicellulose. Glucose, galactose, and uronic acids formed the remainder. The relative proportion of xylose in the hemicellulose of leaves declined an average of 14% during the sampling period.

Disappearance of CWC and hemicellulose was 52.6 and 69.1%, respectively, during in vitro tureen fermentation. All of the hemicellulose components were not fermented equally. The ratio of xylose:arabinose increased from 1.6 in the original leaf tissue to 3.0 after rumen fermentation. Arabinose in vitro disappearance was 74.7% compared to 53.7% for xylose. 14C-Xylose accounted for 50% of the radioactivity in hemicellulose before rumen fermentation and more than 70% of the radioactivity in hemicellulose after fermentation. Greater than 96% of 14C-arabinose and nearly all of the radioactivity in glucose and galactose were lost during fermentation, further indicating that the constituents of hemicellulose are not equally available to rumen microorganisms. An inverse relationship between hemicellulose in vitro disappearance and the xylose: arabinose ratio suggests possible selection criteria for improving forage utilization by ruminants.

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