Effect of dietary soluble fibre and n-6/n-3 fatty acid ratio on growth performance and nitrogen and energy retention efficiency in growing rabbits

The aim of this work was to study whether the dietary supplementation with soluble fibre (SF) and the reduction of the n-6/n-3 fatty acid ratio or the combination of both influence the growth performance and digestible nitrogen and energy (DN and DE) retention efficiency in rabbits from 27 to 62 d of age. Four diets in a 2 × 2 factorial arrangement were used with two levels of SF (7.8 vs. 13.0%, on DM basis, LSF vs. HSF) and two different n-6/n-3 ratio (13.4/1 vs. 3.5/1, Hn-6/n-3 vs. Ln-6/n-3). Nitrogen and energy retention in the body and in the carcass were determined by bioelectrical impedance. The increase of SF reduced the mortality (38.3 vs. 24.1%; P = 0.018), with no effect of the n-6/n-3 ratio. The feed efficiency in the whole experimental period (27-62 d of age) improved with the increase of the level of SF (+3.5%; P = 0.003) and the reduction of the n-6/n-3 (+2%; P ≤ 0.048). The increase of SF decreased the ratio DN/DE (-2%; P = 0.035). The increase of SF and the reduction of the n-6/n-3 ratio reduced the DN intake (-4%. P ≤ 0.030). However, the amount of nitrogen retained in the body or in the carcass was not affected by treatments (P ≥ 0.26). Consequently, the increase of dietary SF and the reduction of the n-6/n-3 ratio improved the retention of DN in the body and in the carcass (between 3.4 and 5.4%. P ≤ 0.017). It was associated with a reduction of the urinary nitrogen excretion in HSF respect to LSF groups (−13%; P = 0.004) and in Ln-6/n-3 respect to Hn-6/n-3 groups (-12%; P = 0.006). In contrast, the increase of SF increased the faecal nitrogen excretion (+5%; P = 0.004). Treatments had no effect on DE intake and on body and carcass energy retention (P ≥ 0.13). The increase of SF improved the body and carcass DE retention efficiency (+3%. P ≤ 0.042). It was mainly due to the impairment observed in rabbis fed LSF_Hn-6/n-3 respect to those fed LSF_Ln-6/n-3 diet (P = 0.071). In conclusion, the increase of SF improved the survival of fattening rabbits, with no influence of the n-6/n-3 ratio, and both factors especially improved the efficiency of DN but also the DE retained in the body and in the carcass.