Partial replacement of starch with acid detergent fibre and/or neutral detergent soluble fibre at two protein levels: Effects on ileal apparent digestibility and caecal environment of growing rabbits

The effects of the composition of peri-weaning diets on apparent ileal digestibility of nutrients and caecal environment were studied in growing rabbits. Eight diets were formulated according to a 2×2×2 factorial design to increase acid detergent fibre (ADF) from 230 to 290gkg−1 dry matter (DM) at the expense of mainly starch, to increase neutral detergent soluble fibre (NDSF) from 145 to 185gkg−1 DM at the expense of mainly starch, and to reduce crude protein (CP) content from 175 to 145gkg−1 DM. A total of 32 litters of eight kits were randomly assigned to the diets at 17 days of age, weaned at 28 days of age and slaughtered at 35 days of age to collect samples of ileal and caecal contents. The substitution of ADF for starch reduced the coefficient of apparent ileal digestibility (CAID) of DM (0.292 vs. 0.229; P<0.01). The substitution of NDSF for starch increased the CAID of NDSF (−0.040 vs. 0.099; P<0.01). An interaction between dietary ADF and NDSF levels was found for the CAID of CP, higher in the low ADF-low NDSF diets than in the rest (+0.074; P<0.05). The reduction of dietary CP content decreased the CAID of CP (0.578 vs. 0.525; P<0.05) and NDSF (0.086 vs. −0.038; P<0.01). Interactions between dietary ADF and NDSF levels were found affecting full caecum weight as well as DM content and pH of caecal digesta. The increase of ADF in the high NDSF diets reduced full caecum weight (−9.1gkg−1 live weight; P<0.01), increased DM content of caecal digesta (+31gkg−1; P<0.001) and reduced caecal pH (−0.10; P<0.05). In the low NDSF diets, higher ADF increased DM content of caecal digesta to a lesser extent (+16gkg−1; P<0.001) and increased caecal pH (+0.08; P<0.05). The increase of NDSF in the high ADF diets increased full caecum weight (+8.5gkg−1 live weight; P<0.01) and reduced caecal pH (−0.14; P<0.001). In the low ADF diets, higher NDSF increased full caecum weight more extensively (+15.5gkg−1 live weight; P<0.001) and reduced DM content of caecal digesta (−16gkg−1; P<0.001). Both ADF and NDSF replacing starch increased total volatile fatty acid (VFA) concentration of caecal digesta (+11.5% and +13.9%, respectively; P<0.01). The replacement with NDSF increased molar proportion of butyrate (0.095 vs. 0.112; P<0.001) at the expense of mainly acetate (0.841 vs. 0.820; P<0.001) and reduced ammonia concentration of caecal digesta (−8.1%; P<0.05). The reduction of dietary CP content decreased total VFA (−14.4%; P<0.001) and ammonia (−23.5%; P<0.001) concentrations of caecal digesta and increased caecal pH (5.87 vs. 5.93; P<0.05). Overall, the three dietary changes under study led to changes in the caecal environment which might be effective in reducing mortality rate in a context of rabbit epizootic enteropathy.