Effects of levels of insoluble and soluble fibre in diets for growing rabbits on faecal digestibility, nitrogen recycling and in vitro fermentation

Abstract

The effect of neutral detergent fibre (NDF) and neutral detergent soluble fibre (NDSF) on in vivo faecal digestibility and caecal fermentation pattern was studied in growing rabbits, in four diets formulated according to two levels of NDF, 370 (LI) and 450 (HI) g/kg, and two levels of NDSF, 150 (LS) and 180 (HS) g/kg in substitution of starch, in a 2x2 factorial structure.  Twenty four New Zealand White rabbits weaned at 28 d (630 ± 80.2 g weight) were allocated to digestibility cages from 42 to 49 d of age to determine apparent faecal digestibility of each diet (n=6).  Urine was collected for determination of purine derivatives (PD).  Once the digestibility trial finished, rabbits were fitted with PVC neck collars for 24 h total caecotrophe collection.  After one day of recovery, animals were slaughtered and caecal contents were used as inocula for 18 h in vitro gas production and caecal degradation (ivDMcD) study, using an HCl-pepsin and pancreatin pre-digested substrate.  Diet digestibility was also determined by the in vitro three-step enzymatic procedure.  There were no effects of the NDF x NDSF interaction for any digestibility parameter (P>0.10).  Both dry matter and organic matter digestibility (DMD and OMD) decreased from 0.518 to 0.442 and from 0.526 to 0.447, respectively, with the inclusion of NDF (P<0.001), but were unaffected by the NDSF level (P>0.10).  In contrast, NDF digestibility (NDFD) and ivDMcD did not respond to NDF (P>0.10) but increased from 0.156 to 0.200 and 0.141 to 0.210 with a ratio of NDSF (P<0.01).  Weight of caecal contents increased with both NDF (P<0.001) and NDSF (P<0.01).  However, total production of caecotrophes increased from 20.1 to 25.5 g DM/d with NDF (P<0.05), but was not affected by NDSF.  The crude protein (CP) proportion in caecotrophes decreased with NDF (P<0.001) and increased with NDSF (P<0.01), and total CP recycled as caecotrophes tended (P=0.093) to be higher in HS diets, being unaffected by the dietary level of NDF.  Diets rich in NDSF rendered higher gas volumes (P<0.001) than those with LS from 2 to 18 h incubation, whereas inclusion of high proportions of NDF reduced gas volume (P<0.01).  Results indicate that NDF reduces faecal digestibility, whereas NDSF promotes better conditions for caecal fermentation.