Effects of chemical treatments of rice straw on rumen fermentation characteristics, fibrolytic enzyme activities and populations of liquid- and solid-associated ruminal microbes in vitro

This study was conducted to investigate the effects of treatment of rice straw (RS) with sodium hydroxide (SH) and ammonium bicarbonate (AB) on fermentation characteristics, fibrolytic enzyme activities and populations of liquid- and solid-associated ruminal microbes in vitro. In vitro gas test was performed to evaluate the nutritive value of the treated straw. Specific activities of fibre-degrading enzymes, namely carboxymethyl cellulase (CMCase), avicelase that catalyzes the breakdown of crystalline cellulose (Avicel) to cellobiose and xylanase were estimated from the amount of reducing sugars released from the solid-bound microbes. Total deoxyribonucleic acid (DNA) was extracted from the liquid- and solid-associated ruminal microbes, respectively, and populations of rumen microbes were determined by real-time quantitative polymerase chain reaction (PCR). Populations of total rumen fungi, Ruminococcus flavefaciens and Fibrobacter succinogenes were expressed as a proportion of total rumen bacterial 16S ribosomal deoxyribonucleic acid (rDNA). The cumulative gas production (GP) at all incubation times, potential GP and rate constant of GP were highest for SH-RS, followed by AB-RS and lowest for the untreated RS (P<0.05). Microbial crud protein increased with incubation time, and was higher for the treated straws than for the untreated (P<0.05). Both treated and untreated straws maintained a typical roughage type of fermentation with a high proportion of acetate. The SH treatment increased CMCase and avicelase activities at all incubation times. Both SH and AB treatments significantly increased xylanase activity. Rumen fungi were significantly increased with incubation time in both liquid and solid phases for SH-RS, but not affected by AB treatment. Both solid- and liquid-associated R. flavefaciens were higher in treated straws than in the untreated, with higher solid-associated R. flavefaciens in SH-RS than in AB-RS at early incubation periods. Solid-associated F succinogenes was lower, and liquid-associated E succinogenes was higher in proportion of total ruminal bacterial 16S rDNA in SH-RS than in other two straws, with little difference in solid-associated E succinogenes between untreated and AB straws. Liquid-associated E succinogenes was lower at early incubation in AB-RS. It is inferred that chemical treatments enhance the nutritive value of RS through improving rumen fermentation and fibrolytic enzyme activities that are mainly resultant from more available substrate, and has great influences on rumen microbial distribution and populations, but their fluctuating pattern with incubation time is slightly different between two treated straws. (C) 2007 Elsevier B.V. All rights reserved.