Differences in rumen bacterial degradation of morphological fractions in eight cereal straws and the effect of digestion on different types of tissues and mechanical properties of straw stalks

Abstract

Eight different straw species were hand-harvested and dissected into internode, node within each internode height, leaf sheath and leaf blade. Each morphological fraction was analyzed separately for cell wall constituents and in vitro digestibility. Additionally, sections of internodes were analyzed for shear stress before and after in vitro digestion, and internode cell types were evaluated for histochemical reactions for lignin and degradation by rumen microorganisms using microscopy. In vitro digestibility of cell walls (IVDCW) varied widely between species and internode heights, ranging in values as follows: internodes, 10.3–52.3%; leaf sheaths, 27.8–75.8%; leaf blades, 46.6–81.7%. Acid phloroglucinol (AP) did not stain unlignified parenchyma in straw internodes, and these tissues were totally degraded. In this respect, AP was superior to diazotized sulfanilic acid in indicating recalcitrant cell types. No relationship of IVDCW to loss of entire tissue by microbial degradation (determined by scanning electron microscopy) was found. Lignified, thick cell walls of structural tissue in internodes of some straws were markedly colonized and eroded by microorganisms and, as a result, contributed substantially to total IVDCW of this plant fraction. In internodes, the reduction of shear forces after incubation with rumen microorganisms appeared to be related to microbial erosion of structural tissue. The shear stress of internodes and compression forces to crush nodes increased from upper to lower internodes.