Optimizing feed efficiency and nitrogen excretion in growing rabbits by increasing dietary energy with high-starch, high-soluble fibre, low-insoluble fibre supply at low protein levels

A total of 282 crossbred rabbits of both genders were divided into six experimental groups and fed ad libitum six diets formulated in a 2×3 factorial arrangement based on three dietary energy levels (low, approximately 8.45 MJ/kg; medium, 9.9 MJ/kg: and high, 11.5 MJ/kg) and two crude protein levels (15% and 17%). The increase of dietary energy was obtained by a simultaneous increase of dietary starch (9.1%, 13.7%, and 17.9%) and soluble fibre (4.8%, 7.1%, and 9.4%) at the expense of acid detergent fibre (23.0%, 18.0%, and 13.5%). Growth performance and nitrogen excretion from weaning until slaughter, and digestive traits were monitored. The increase of dietary energy enhanced the apparent digestibility of dry matter (50.8%, 60.5%, and 71.5%; P<0.001), gross energy (50.3%, 59.7%, and 70.7%; P<0.001), soluble fibre (73.1%, 77.8%, and 82.9%; P<0.10), and neutral detergent fibre (16.1%, 29.0%, and 48.1%; P<0.001). In the caecum, the total volatile fatty acid (VFA) content increased (63.0, 79.2, and 78.5 mmol/L; P<0.05), and the propionate rate decreased (4.0%, 4.2%, and 3.5% mol VFA; P<0.05); in the jejunum, the villi-to-crypt ratio tended to decrease (4.18, 3.89, and 3.57; P<0.10). The increase of dietary energy supply decreased feed intake (181 g/d, 157 g/d, and 135 g/d; P<0.001), which improved feed conversion ratios (3.98, 3.40, and 2.87; P<0.001), and decreased nitrogen excretion (3.15 g/d, 2.56 g/d, and 1.93 g/d; P<0.001). The increase of dietary energy significantly decreased mortality (15.8%, 4.9%, and 4.9%; P<0.05). When dietary crude protein increased from 15% to 17%, apparent digestibility increased for dry matter, crude protein, total dietary fibre, and gross energy (P<0.01). Growth performance during the trial was not affected, but nitrogen excretion decreased significantly (2.73 g/d vs. 2.37 g/d; P<0.001). In conclusion, the increase of dietary energy by the simultaneous increase of dietary starch and soluble fibre to replace insoluble fibre optimized feed efficiency and controlled nitrogen excretion in growing rabbits with positive effects on digestive traits and health. The reduction in dietary protein level was determined to be an useful strategy to reduce farm nitrogen excretion.