CELL-WALL COMPOSITION AND DEGRADABILITY OF STEM TISSUE FROM LUCERNE DIVERGENTLY SELECTED FOR LIGNIN AND IN-VITRO DRY-MATTER DISAPPEARANCE

Lucerne populations that had previously been divergently selected for acid detergent lignin (ADL) concentrations of whole herbage were addit ionally divergently selected for in vitro dry-matter disappearance (IV DMD) of basal stems. Parental plants of the four selected populations were intercrossed and the half-sib progeny evaluated in a replicated f ield trial in 1988 over two harvests. Cell wall composition of basal s tems was determined for detergent fibre components and neutral sugars, uronic acids, Klason lignin and esterified and etherified phenolic ac ids of the total fibre fraction. Lignin polymer composition was measur ed by nitrobenzene oxidation. Degradability of cell wall polysaccharid e components was determined by a 48-h in vitro ruminal fermentation. C ell wall composition was changed by both selection criteria. Klason li gnin proportion of the total fibre did not differ as a result of selec tion, whereas the ADL concentration of neutral detergent fibre was dif ferent among selection groups. The cell wall polysaccharides of lucern e basal stems shifted towards more cellulose (glucose residues) and le ss hemicellulose (xylose residues) with selection for either low ADL o r high IVDMD. While degradability of the cell wall polysaccharides was correlated with various measures of lignin composition and concentrat ion, the results were variable and inconclusive. Surprisingly, Klason lignin and ADL were positively correlated with cell wall polysaccharid e degradability, and esterified ferulic acid was negatively correlated with degradation of the cell wall. Selection for herbage ADL and basa l stem IVDMD of lucerne did alter cell wall degradation, but the assoc iated changes in cell wall lignification were not consistently correla ted with cell wall degradability.