Caecal microbial activity, digestion and gut health were analysed in the young rabbit, in response to fibre substitution by starch, in diets with high proportions of rapidly fermentable polysaccharides (pectins þ hemicelluloses:acid-detergent fibre (ADF) ratio of 1·7). A range of five diets corresponding to a 60 % linear reduction of the ADF level (230 to 92 g ADF/kg) without changes in the fibre quality, and to a corresponding linear increase in dietary starch, was given ad libitum to young rabbits from 18 d until 70 d of age. A one half reduction of the ADF level resulted in a sharp increase in energy digestibility (þ 25 units), associated with a lower feed intake ( -35 %) and to an increase of the mean retention time (þ 6 h) in the whole digestive tract. Despite large variations in the fibre intake (20 to 59 g ADF/d), the fibre digestive efficiency remained similar among the five diets. Starch ileal concentrations were low after 4 weeks of age (,5 %), and variations with age were significant when the dietary starch level was over 19 %. A 65 % lower biomass production was measured when the ADF level progressed from 230 to 165 g/kg, and no precise relationship was found with fermentative activity. Reducing the fibre intake led to a linear decrease of caecal volatile fatty acids concentrations, and to higher pH and NH 3 levels. An increased occurrence of mortality by diarrhoea was registered with the lowest fibre intake. It can be concluded that a sufficient supply of fibre, with high proportions in rapidly fermentable polysaccharides, stimulates the maturation of microbial activity and reduces the occurrence of diarrhoea.

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-Four high-starch diets (20%) varying only by the source of starch (wheat = diet W; barley = diet B; maize = diet M; extruded maize = diet EM) were given ad libitum before weaning (at 29 d old) from 18 days of age, to 8 litters (2 litters per diet) of 8 rabbits caged separately from their mother. Starch concentration in the ileal digesta and caecal fermentative characteristics were assessed at weaning and at 50 d of age, while faecal digestibility was measured between 46 and 50 d of age. The energy digestibility coefficient was the highest for the barley based diet while the lowest were observed for wheat and maize based diets (P < 0.05). At weaning, the level of starch in the ileum was high and similar for W, B and M diets (meanly: 8.8% DM), but was 5 points lower for the EM diet. At 7 weeks of age, it remained high (>10% DM) with the maize based diet, while it decreased for the other groups (less than 6% DM). The faecal digestibility of the glucidic fraction (NNCC) was almost complete (> 92%) and significantly higher with the EM diet followed by the B diet. NNCC digestibility was negatively correlated with ileal starch concentration (r = -0.63, P < 0.001). The fibre and protein digestibility remained unaffected by the dietary treatment, and no significant correlation was detected between the ileal starch level and fibre digestibility (NDF or ADF). Caecal VFA concentration increased by 11 mmol (+20%) from weaning to 7 weeks of age, while ammonia level decreased by 15%. Compared to weaning, at 7 weeks of age, the butyrate proportions were 5 units higher. The starch source did not significantly affect the caecal traits or fermentations, except for a slight increase in the empty caecal weight for EM and B compared to the W and M diets (P < 0.05). The ileal starch concentration was not correlated with caecal fermentative activity. rabbit / digestion / starch nature / extrusion / caecal fermentation Résumé -Effet du type de céréale et de l'extrusion dans l'alimentation du lapin en croissance. I -Conséquences sur la digestion et l'activité fermentaire caecale. Pour étudier les effets de la nature de l'amidon alimentaire sur la digestion du lapin en croissance, quatre aliments riches en amidon (20 %) variant seulement de par l'origine de l'amidon (blé = groupe W, orge = groupe B, maïs = groupe M, maïs extrudé = groupe EM) ont été distribués à volonté avant le sevrage (29 j) à partir de 18 jours d'âge, à 8 portées de 8 lapereaux (2 portées par régime) logés séparément de leur mère. La concentration en amidon iléal ainsi que l'activité fermentaire caecale ont été mesurées au * Corresponding author: gidenne@toulouse.inra.fr 56 T. Gidenne et al.sevrage et à 50 jours d'âge, tandis que la digestibilité fécale des aliments a été mesurée entre 46 et 50 j d'âge. La digestibilité de l'énergie du régime à base d'orge (groupe B) est la plus élevée, alors que celle du régime à base de maïs est la plus faible (P < 0,05). Au sevrage, la concentration en amidon iléal est assez élevée et similaire entre les groupes W, B ou M ...