Effect of increasing soluble fibre and starch and reducing acid detergent fibre in diets at different protein level for growing rabbits

From weaning (at 36 d of age) until slaughter (at 79 d), 282 commercial crossbred rabbits (951±31 g) were assigned to six experimental groups and fed ad libitum six diets formulated according to a bi-factorial arrangement with three (soluble fibre+starch)-to-ADF ratios (0.6, 1.2 and 2.0) and two protein levels (15.0% and 17.0%). The dietary soluble fibre (5.0%, 7.0% to 9.0%) and starch (10.0%, 14.0% to 18.0%) levels jointly increased at the expense of acid detergent fibre (ADF) (22.0%, 18.0% to 14.0%). The growth performance, and the in vivo nutritive value of diets were recorded. The caecal fermentation traits were measured on 36 rabbits slaughtered at 45 d of age. When the (soluble fibre+starch) to ADF ratio raised, daily growth linearly increased (P=0.05) and feed intake decreased (P<0.001) and, thus, feed conversion improved (P<0.001); the digestibility of dry matter (50.8 to 71.5%), gross energy (50.3 to 70.7%) and fibre fractions linearly increased (P<0.001); at caecum, the concentration of total volatile fatty acids (VFA) raised (63.0 to 78.5 mmol/L; P=0.02), whereas the proportion of propionate decreased (4.0 to 3.5 mol/100 mol VFA; P=0.04). When the dietary protein increased, feed conversion improved (3.47 to 3.37; P<0.001) and the digestibility of dry matter increased; (60.3% to 61.6%; P<0.001) at caecum, the N ammonia level tended to decrease (P=0.07), whereas total VFA production and VFA molar proportions were not affected. The increase of the (soluble fibre+starch) to ADF ratio reduced the mortality rate (15.8%, 4.5% and 4.9%; P=0.02), whereas the dietary protein level did not affect rabbit health. In conclusion, the contemporary increase of dietary soluble fibre and starch at the expense of ADF increased digestive feed utilization and growth performance, and enhanced fermentation activity at caecum and health status. The increase of dietary protein moderately improved productive results and did not affect health status.