Colonization of rice straw by white-rot fungi (Cyathus stercoreus): Effect on ruminal fermentation pattern, nitrogen metabolism, and fiber utilization during continuous culture

Experimental diets consisted of fungal treated or untreated rice straw: concentrate 75:25 on a DM basis and ground through a 1 mm sieve. Fungal treated rice straw (FRS) and untreated rice straw (C) diets were formulated to be isonitrogenous (11% crude protein; CP), but varied in total nonstructural carbohydrates, neutral detergent fiber and acid detergent fiber (18.3 vs. 11.4%; 49.7 vs. 61.0%; 45.0 vs. 47.5%), respectively. Four fermenters were inoculated with ruminal fluid from a ruminally cannulated lactating Holstein cow fed a total mixed ration consisting of 50% silage: 50% grain (DM basis). A randomized complete block design was used with diets replicated within each experimental period and four times across periods. Each period was 10 days in length, 7 days for adaptation followed by 3 days for sample collection. Digesta solid and liquid dilution rates were 0.055 h−1. Apparent and true digestion of DM and OM of the FRS-diet was higher (P < 0.05) compared with the C-diet. Fungal treatment increased (P < 0.05) the digestibility of the major fiber component, cellulose, compared to the C-diet (61.1 vs. 48.8%). Actual quantities of nutrients digested per unit time indicated that fungal treatment increased cellulose digestion by 27% but decreased hemicellulose digestion by 37%, when compared with the C-diet. Arabinose and xylose from FRS-diet were less digestible, however glucose digestion was increased by 38% compared with the C-diet. Increased digestion of carbohydrates from FRS-diet resulted in an increase (P < 0.05) on production of total volatile fatty acids (VFA) compared with the C-diet. Increased VFA production of FRS-diet was due to increased molar proportion (moles 100 moles−1) of propionate and butyrate (28.8 vs. 24.2; 13.1 vs. 7.9), respectively. Branched chain VFA production was not detectable with the FRS-diet when compared with an average of 2.6 moles 100 moles−1 for the C-diet. A similar pattern of inhibition was found with ammonia-N production with FRS-diet compared with the C-diet (0.77 vs. 22.3 mg dl−1). Nitrogen metabolism of FRS-diet resulted in lower true digestion of CP and microbial protein synthesis compared with the C-diet, resulting in increased dietary-N flow. This study demonstrated that fungal treatment increased fiber digestibility by increasing the availability of cellulose but decreasing CP availability for rumen microbial digestion.