Exogenous enzymes added to untreated or ammoniated rice straw:: Effects on in vitro fermentation characteristics and degradability

A study was conducted to evaluate the use of exogenous enzymes as a potential means of improving the cell wall degradation of rice straw. Two developmental cellulases (END), two developmental xylanases (XY), and two commercial enzyme products (EX, supplying a combination of endoglucanases and xylanases; PROT, supplying mostly proteases) were evaluated for their potential to improve in vitro degradation of untreated (URS) or ammoniated rice straw (ARS). The END and XY were added to milled samples (0.45 g dry matter [DM]) of URS and ARS at a rate of 200-300 International Units (IU) of endoglucanase or xylanase. The EX supplied 200-300 IU each of endoglucanase and xylanase, whereas PROT supplied mostly protease. Anaerobic buffer medium and strained ruminal fluid were added to the in vitro incubations and gas production (GP) was measured during 24 h of incubation. Degradabilities of DM, neutral detergent fibre and acid detergent fibre, and volatile fatty acid profiles were determined at the end of the 24 h incubation. Overall, GP and clegradability of rice straw were greatly increased by ammoniation. Adding EX or PROT enzymes increased (P < 0.05) GP and degradability of URS, whereas END or XY had little impact. Adding XY enzymes to ARS increased GP (P < 0.001) starting at 18h of fermentation, as well as DM and fibre degradability at 24 h. Adding EX or PROT also increased GP and fibre degradability of ARS, with greater effects for PROT than for EX. There was a synergistic effect between ammonia pretreatment and exogenous enzymes in the case of XY, EX, and PROT for the in vitro degradation of rice straw. This synergy may indicate that ammoniation removes phenolic compounds and disrupts the lignin-carbohydrate complexes thereby increasing accessibility of the substrate to enzymatic action. When effective, use of exogenous enzymes shifted fermentation towards decreased acetate to propionate ratio. Combining ammonia treatment and exogenous enzymes increased the ruminal degradation of rice straw and is expected to improve the potential of using rice straw as a ruminant feed. (c) 2006 Elsevier B.V. All rights reserved.