Caecal fermentation patterns in vitro of glucose, cellobiose, microcrystalline cellulose and NDF separated from alfalfa hay in the adult rabbit

A comparison of caecal fermentation patterns of neutral detergent fibre prepared from alfalfa hay (aNDF) with glucose (GLU), cellobiose (CEB) and microcrystalline cellulose (MCC) was conducted with an in vitro batch culture. Caecal contents, freshly collected from adult rabbits, were taken as inoculum. After a 72-h incubation at 39 °C, the highest mean activities of xylanase, acetyl and feruloyl esterases occurred in aNDF (P<0.05) while highest carboxymethyl cellulase activity occurred in GLU (P<0.05). Both GLU and CEB were rapidly fermented and characterized by much lower pH (P<0.01) and greater production of total volatile fatty acids (VFA). The lowest concentration of total VFA occurred in aNDF and MCC (P<0.05). The fermentation of aNDF and MCC resulted in higher molar proportions of propionate, valerate, branched chain VFAs than those of GLU and CEB (P<0.05); the molar proportions of acetate ranked as: GLU > MCC > CEB > aNDF (P<0.01); the molar proportion of butyrate ranked as: CEB > aNDF > GLU > MCC (P<0.01). The total gas production of GLU and CEB were greater than those of MCC and aNDF (P<0.05). Molar CH4 proportion ranked as: MCC > aNDF > CEB > GLU (P<0.01), and the molar H2 proportion in MCC was greater than those in other substrates (P<0.01). These results suggest that fermentation of MCC and aNDF produce more glucogenic acids, especially propionate. Low hydrogen recoveries for all substrates implicated that mechanisms of H2 disposal other than methanogenesis, such as reductive acetogenesis, may exist in the caecum and depend upon the nature of the carbohydrate.