Chemical composition and biodegradability of crop residues colonized by white‐rot fungi

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Abstract
Two experiments were conducted to study the degradation of rice (Oriza sativa L) straw and maize (Zea maize L) stover by selected white‐rot fungi, using loss of organic matter, disappearance of cell wall constituents, and in‐vitro dry matter digestibility (IVDMD) as indices of rumen microbial degradation. Four white‐rot fungi—Cyrathus stercoreus, Dichomitus squalens and two strains (cellulase‐less mutant and a wild type) of Phanerochaete chrysosporium—were selected for the first experiment. After 30 days of solid state fermentation. C stercoreus increased IVDMD of both substrates from 373 g kg−1 (rice) and 393 g kg−1 (corn) to approximately 540 g kg−1 by selectively degrading hemicellulose as opposed to cellulose. The highest IVDMD was obtained with a minimal loss of dry matter (33 g kg−1). In contrast, both strains (wild and mutant) of P chrysosporium degraded cellulose and hemicellulose indiscriminately, and thus lowered the IVDMD to 276. 257 as opposed to 373 for rice straw and 301. 307 as opposed to 393 g kg−1 for maize stover. All the fungi showed a preference for degrading maize stover compared with rice straw, although the materials were similar in cellulose and lignin concentration. No direct correlation was found between lignin degradation and the improvement of substrate IVDMD of either substrate.The objective of the second experiment was to determine the rate and extent of digestibility of rice straw colonized by C stercoreus during two incubation periods (15 and 30 days). Dry matter disappearance (DMD) was determined by ruminal incubation of the substrate contained in nylon bags. The 30‐day incubation period was superior (1.3 ×) to that of the 15‐day period in improving in‐situ DMD. Treatment of rice straw for 30 days not only improved the extent of dry matter digestibility (616 as opposed to 389 g kg−1), but also increased (1.5 ×) the rate of disappearance of substrate.