Evaluation of type of grinding of lucerne hay and wheat straw in diets for growing rabbits with two levels of neutral detergent fibre

To evaluate the effects of insoluble fibre level and its potential interaction with type of grinding on growing rabbits, four diets in a 2 × 2 factorial arrangement were used with normal (NF) and low (LF) neutral detergent fibre (using α-amylase and ash free, aNDFom) level (340 vs. 270 g aNDFom/kg DM, dry matter) and coarse (CG) and normal (NG) type of grinding of lucerne hay and wheat straw (coarse: 9-mm vs. normal: 1-mm). This resulted in a proportion of fibrous large particles (> 315 μm) of 234, 183, 171 and 140 g aNDFom/kg DM for NF-CG, NF-NG, LF-CG and LF-NG diets, respectively. Diets contained a similar digestible [protein/energy] ratio of 10.7 g/MJ. A total of 464 crossbred mixed-sex rabbits (116 per diet) weaned at 25 days of age were used to record growth performance and mortality rate between 25 and 60 days of age, ileal (at 39 d) and faecal (at 39 and 60 d) digestibilities, digestive traits (organs weight and caecal pH at 39 and 60 d, caecal total volatile fatty acid concentration, VFA, at 39 d) and morphology of jejunal mucosa (at 39 d). The reduction of dietary aNDFom from 340 to 270 g/kg impaired growth rate (45.2 vs. 41.8 g/d, P = 0.002) and mortality (12.2 vs. 20.9 %, P < 0.001), with no effect on feed efficiency (0.457 g/g on average). It resulted in lower [villus height/crypt depth] ratio in the jejunal mucosa (5.48 vs. 4.26, P < 0.001) and caecal VFA (65.2 vs. 56.7 mmol/l, P = 0.002), and higher ileal and faecal digestibilities of DM (0.529 vs. 0.599, and 0.671 vs. 0.741, respectively, P < 0.001), and relative weight of caecal contents at 60 days of age (58.6 vs. 69.4 g/kg body weight, P = 0.006). In rabbits fed NF diets, coarse grinding reduced mortality rate (15.1 vs. 9.3 %, P < 0.05). Coarse grinding in LF diets reduced relative weight of caecal contents at 39 days (77.6 vs. 61.6 g/kg body weight, P < 0.05), but not at 60 d. Nevertheless, the mortality still remained higher with the LF-CG diet (19.8 %) than with the NF diets. In conclusion, aNDFom level should not be reduced in diets for fattening rabbits below the minimal requirement (330 g/kg DM), and a coarse grinding of the dietary fibrous ingredients cannot make up for a deficitary dietary aNDFom concentration.