IN VIVO AND IN VITRO STUDY OF CAECAL FERMENTATION PATTERN AND METHANOGENESIS IN RABBITS

Methane formation and caecal fermentation pattern were studied in vivo and in vitro in 16 white New Zealand rabbits allocated to four diets formulated with two different sources of fibre (alfalfa hay or sugar beet pulp) and starch (wheat or maize). Animals received the diet for at least two weeks before methane production was measured in vivo in a respiratory chamber. Then, animals were slaughtered and caecal contents were sampled for volatile fatty acid (VFA) determination and used as inoculum for in vitro incubations performed to determine gas production and methane formation. Total VFA concentration in caecal contents averaged 52.5 mM, with butyrate content higher when alfalfa hay was the main source of fibre compared to sugar beet pulp (9.97 vs. 7.32 mM). Fermentation pattern was also affected by the experimental treatment, acetate being lower (72 vs. 79%) and butyrate higher (20 vs. 14%) when maize was the source of starch in relation to wheat. Fermentation in vivo vs. in vitro showed some differences (molar proportion of acetate, 75 vs. 72 % in vivo and in vitro, respectively; molar proportion of propionate, 6.9 vs. 9.1 % in vivo and in vitro, respectively), probably due to differences in pH (6.0 vs. 6.7 in vivo and in vitro, respectively). Only 2 out of 16 rabbits produced a substantial volume of methane in vivo, whereas all of them showed some formation in the in vitr