METHANOGENESIS IN RABBIT CAECUM AS AFFECTED BY THE FERMENTATION PATTERN: IN VITRO AND IN VIVO MEASUREMENTS

Methane formation and caecal fermentation patterns were studied in vivo and in vitro in 16 New Zealand White rabbits (70-80 d and 2.27 +/- 0.064 kg) allocated to 4 diets formulated to have similar neutral detergent fibre (33.8 +/- 0.53%) and crude protein (17.7 +/- 0.33%) content, with 2 different sources of fibre (alfalfa hay, AH; or sugar beet pulp, SP) and starch (wheat or maize). Animals received the diet for 16 to 20 d before methane production was measured in vivo in a respiratory chamber. Animals were subsequently slaughtered at approximately 9:00 a.m. and caecal contents were sampled and used as inoculum for in vitro incubations to determine gas and methane production. Volatile fatty acid (VFA) and purine base (PB) concentrations were determined from both caecal content and incubation medium after 6 h. Total VFA concentration in caecal content decreased (P<0.05) in rabbits fed AH-maize diet compared with rabbits fed AH-wheat and SP-maize diets (37.7 vs. 59.6 mM), with those fed SP-wheat showing an intermediate value (53.0 mM). Fermentation pattern was affected when maize was the source of starch compared to wheat, with lower acetate (0.72 vs. 0.79; P<0.01) and higher butyrate (0.19 vs. 0.14; P<0.001) molar proportions. Fermentation in vivo vs. in vitro showed some differences (molar proportions of acetate, 0.76 vs. 0.73, P<0.001, and propionate, 0.069 vs. 0.091, P<0.001, in vivo and in vitro, respectively), probably due to differences in pH (6.0 vs. 6.7 in vivo and in vitro; P<0.001). Only 2 out of 16 rabbits produced a substantial volume of methane in vivo (on average, 12.6 mL or 0.56 mmol per kg of metabolic weight and day), showing a high inter-individual variability that hindered comparison of treatment differences. In contrast, methane was detected in vitro in all cases and volumes were more homogenous, a higher formation (P<0.05) being observed with maize compared to wheat. A similar effect was shown in total gas production. The low methane production and H-2 recovery suggest the importance of H-2 disposal mechanisms other than methanogenesis, such as reductive acetogenesis. PB concentration in caecal content and the incubation medium, as an index of microbial concentration, was highest when SP was added with maize (P<0.05).