DEGRADABILITY OF CELL-WALL POLYSACCHARIDES IN MAIZE INTERNODES DURINGSTALK DEVELOPMENT

Limitations to degradation of forage cell-wall constituents in maturin g plant tissue are not fully understood, which limits progress in deve loping cultivars with improved digestibility. We determined in vitro d egradability of cell-wall components by rumen microorganisms in develo ping internodes of maize (Zea mays L.). Plants were grown in the growt h chamber and harvested at the 15th-leaf stage of development. Individ ual internodes were divided in half and separated into rind and pith f ractions. Cell-wall degradability was measured on the internode sample s and correlated with cell-wall degradability. Degradability of all ce ll-wall polysaccharide components, except galactose and mannose, decli ned with maturation of maize internodes. Degradabilities of glucose an d xylose residues exhibited large differences between 24- and 96-h fer mentation intervals (31 to 58% additional degradation in 72 h), wherea s the arabinose and uronic acids potentially degradable in 96 h were a lready almost completely degraded in 24 h. Maize rind, consisting of e pidermal and sclerenchyma tissues and many vascular bundles, was less degradable than the pith (736 vs. 811 g kg(-1), respectively), which h ad thinner-walled parenchyma tissue and fewer vascular bundles, Lower portions of internodes were younger and more degradable than the upper halves (805 vs, 743 g kg(-1), respectively). Ferulate ether concentra tion was negatively correlated with cell-wall polysaccharide degradabi lity in young, elongating internodes but not for internodes in which e longation had ceased to occur. This pattern is in agreement with the h ypothesis that ferulate cross-linking limits cell-wall degradation, bu t that the effect is only observable in young tissues before dilution of ferulate ether concentrations by secondary wall deposition of ligni n and polysaccharides obscures the relationship.