Colonization of rice straw (Oryza sativa L.) by white-rot fungi: Effects on structure, chemical composition, cell wall components and nutrient digestibility in vitro

A series of studies were undertaken to examine the effects of various species of white-rot fungi (WRF) on botanical parts of rice straw. Three species of WRF, Cyathus stercoreus (Cs), Pleurotus pulmonarius (Pp) and Phanerochaete chrysosporium (Pc) were screened for their ability to improve the quality of leaf and stem of rice straw during 30 d of solid state fermentation. Structural changes in leaf and stem tissues were evaluated before and after ruminal digestion using scanning electron microscopy and transmission electron microscopy. Changes in cell wall monosaccharides and phenolic acids of fungal decayed material were investigated. Ruminal disappearance of DM and cell wall components were also determined on leaf, stem and whole straw decayed by Cs. Finally we investigated nutrient utilization of fungal treated (Cs) rice straw during continuous culture where experimental diets consisted of fungal treated or untreated rice straw (75%): concentrate (25%) on a DM basis.; Control leaf and stem were similar in IVDMD (38%) while IVDMD of fungal decayed material varied depending on the species of fungi employed and the type of substrate (leaf or stem). Of all fungi, only Pp improved the IVDMD of both leaf and stem while Cs and Pc either selectively improved leaf IVDMD or inhibited both leaf and stem IVDMD, respectively. Various responses obtained in IVDMD were related to the selective degradation of cell wall polysaccharides and nonpolysaccharide compounds by the WRF. The analysis of cell wall monomers of decayed material revealed that Pc exclusively utilized glucose as its energy substrate while Pp and Cs utilized arabinose and xylose as energy substrate. Highest lignin and phenolic acid degradation were found in leaf decayed by Cs. Improved IVDMD was due to the increased digestibility of cellulose from cell wall. Ruminal fermentation patterns revealed that molar proportion of propionate and butyrate were significantly increased in fungal (Cs) decayed material while ammonia-N utilization or production was substantially inhibited by fungal treatment. Subsequently, crude protein digestibility was significantly lower in fungal treated diet compared to the control. In conclusion, nitrogen utilization in the form of ammonia-N may have limited optimal utilization of fungal treated rice straw by ruminal microorganisms and requires further investigation.