Effect of the increase of dietary starch and soluble fibre on digestive efficiency and growth performance of meat rabbits

Effect of increasing dietary starch and soluble fibre on performance and digestive efficiency was studied in 246 cross-bred rabbits from weaning (34 d of age) to slaughtering (75 d). A 3×2 factorial arrangement (i.e. three levels of starch: 60, 103, and 147 g/kg diet; two levels of soluble fibre: 100 and 138 g/kg diet) was used. The rabbits were assigned to six groups and fed the six diets ad libitum. When dietary starch was increased, the coefficient of total tract apparent digestibility (CTTAD) of dry matter (on average, 0.547–0.629) and all nutrients linearly increased (P<0.001), but the CTTAD of soluble fibre did not vary. Weight gain, digestible energy intake, and feed conversion ratio linearly (P<0.01) improved (6.4%, 5.7% and 11.1%, respectively), and feed intake decreased (5.6%). In the caecum, ammonia-N production linearly decreased (7.21–4.93 mmol/L; P=0.01), acetate proportion tended to increase (84.2–85.8 mmol/100mmol volatile fatty acids, VFA; P=0.06), and propionate value decreased (4.00–3.33 mmol/100mmol VFA; P<0.01). As the starch level increased, the CTTAD of NDF and hemicelluloses improved slightly in the diets with 100 g/kg soluble fibre, while at a higher rate in the diets with 138 g/kg soluble fibre (significant interaction, P<0.01). The increase of dietary starch linearly increased slaughter weight (SW) and carcass weight; the proportion of the gastrointestinal tract linearly decreased (196–182 g/kg SW; P<0.001) and the dressing percentage increased (59.3–60.8% SW, P<0.001), as well as dissectible fat (P=0.03). The increase of dietary soluble fibre improved (P<0.001) the CTTAD of dry matter (0.540–0.639) and all nutrients, but the CTTAD of crude protein was not affected; it stimulated daily growth (5.3%; P=0.05) and reduced (P<0.001) feed intake (12.4%) and feed conversion ratio (15.0%). In the caecum, pH decreased (5.88–5.70; P=0.02), total VFA concentration tended to increase (73.8–82.8 mmol/L; P=0.10), and valerate concentration decreased (0.43–0.35 mmol/100mmol VFA; P=0.01). Slaughter weight and reference carcass weight increased (0.03<P<0.07); the gastro-intestinal tract proportion tended to increase (185–190 g/kg SW; P=0.06) without affecting dressing percentage or carcass composition. In conclusion, the use of soluble fibre is recommended in association with (rather than in replacement of) starch to increase the energy value of diets and improve overall farm-feed efficiency and carcass quality.