Colonization of crop residues by white-rot fungi: Cell wall monosaccharides, phenolic acids, ruminal fermentation characteristics and digestibility of cell wall fiber components in vitro

Objectives were to evaluate the effect of fungal treatment on monomers of structural carbohydrates, major cell wall phenolic acids and ruminal fermentability in vitro during small and large scale treatment process. Samples from two previously conducted experiments were used in the present study as follows: Samples from botanical fractions (leaf and stem) of rice straw colonized by Cyathus stercoreus (Cs), Phanerochaete chrysosporium (Pc) and Pleurotus sajorcaju (Ps) were evaluated for in vitro fermentation characteristics, in vitro digestibility (IVD) of cell wall fiber constituents and changes in cell wall phenolic acids (PA) and monosaccharides. The same analyses were repeated for the second set of samples obtained from a large scale trial where rice straw was decayed with Cs in 5 kg batches. The effect of processing of straw, straw pre-soaked in water to achieve 80% moisture, also compared with unprocessed rice straw which served as a positive control. Xylose:arabinose ratio was higher for control rice stem than for leaf which suggests that arabinose substitution with xylose may differ between leaf and stem. Stem contained more p-coumaric acid (p-CA) and less ferulic acid (FA) than leaf, but total PA was higher for stem. Hemicellulose (HC) in rice leaf, calculated as the sum of monomers, was selectively and extensively consumed by Ps and Cs while Pc selectively consumed only glucose derived from cellulose. Arabinose and xylose were solubilized during Pc growth on leaf, but not metabolized. Consequently, less total PA degraded by Pc was found in leaf than with the other two fungi. IVDMD and p-CA were highly correlated (r = -0.73) yet other factors such as the cellulose: p-CA ratio also contributed to the variation in IVDMD. Uronic acids were only partially degraded by fungal treatment. Of all fungi, Cs and Ps improved IVDMD of rice leaf; primarily because of increased digestion of cellulose. Increased IVDMD was further evident by a 1.7 fold increase in the total volatile fatty acid production. Processing of straw prior to fungal treatment decreased IVDMD by more than three-fold, but no apparent mechanism was found. Results of these experiments indicate that improvement of rice straw quality was dependent on fungal species, botanical fractions, and preparation of substrate prior to fungal decay.