Identification of the main factors that influence caecal fermentation traits in growing rabbits

The aim of this work was to determine the main factors that affect caecal pH, the caecal concentrations and molar proportions of volatile fatty acids (VFA) and the weight of caecal contents. The database was constituted of data (n = 92) from three laboratories that included the mentioned traits and the chemical composition and nutritive value of the diets. The laboratory affected caecal pH (P < 0.001) probably because of the differences in sampling time among them. Dietary uronic acids (UA) were measured in 21 diets and were negatively correlated with caecal pH and positively with caecal VFA concentration and the proportion of propionic acid (P = 0.004, 0.06 and 0.03, respectively). When UA was removed from the model, no other chemical characteristics significantly affected caecal pH, and digestible NDF was the variable which was the best correlated with it (R-2 = 0.58; P < 0.001). Only one laboratory detected a relationship between caecal pH and caecal concentrations of VFA( P = 0.044) and N-NH3 (P = 0.14), which accounted for 12% of the variability observed in caecal pH. A linear and quadratic effect of dietary NDF and the degree of lignification of NDF on VFA concentration was found (R-2 = 0.56; P < 0.001), indicating a positive effect of high levels of low lignified fibre on caecal VFA. The weight of caecal contents was quadratically influenced by dietary NDF (R-2 = 0.39; P < 0.001). It was also linearly affected by the degree of lignification of NDF (P < 0.001). Dry matter intake was negatively correlated with caecal content weight (P< 0.001; R-2 = 0.44), once the effects of the laboratory and dietary digestible energy concentration were discounted.