Impact of accessibility and chemical composition on cell wall polysaccharide degradability of maize and lucerne stems

Although lignification of forages is generally accepted as limiting cell wall degradability, prediction of degradation from cell wall composition is often difficult when forages are of similar maturity. It has been proposed that rumen microbe accessibility to potentially degradable cell walls is limited by the presence of non-disrupted cells in forage particles with lignified middle lamella/primary walls acting as barriers to microbial access. We tested this accessibility hypothesis by evaluating the impact of reducing particle size of maize and lucerne stems to the level of individual cells by ball-milling, in order to eliminate accessibility as a limiting factor. While cell wall concentration and composition were not influenced by ball-milling compared with grinding to pass a 1 mm screen in a cyclone-type mill, degradability of total cell wall polysaccharides was dramatically increased. However, only those polysaccharides (cellulose and xylan) which are most abundant in cell types with lignified middle lamella/primary and secondary walls increased in degradability owing to particle size reduction. Degradability of pectins, which are abundant in non-lignified tissues in lucerne, did not respond to ball-milling. Contrary to our expectations, ball-milled forages showed fewer correlations for cell wall composition with degradability than observed for the larger-particle-size grinding treatment. Many components of the cell wall were correlated with polysaccharide degradation for the cyclone-ground samples; however, the results were inconsistent as to which cell wall components were correlated with degradation among and within forages. This observation does not clarify the role of cell wall chemical structure as a limiting agent to wall degradation in the absence of accessibility barriers, but this study does provide support for the hypothesis that lignified middle lamella/primary walls act as barriers to microbial access for degradation.