Effect of increasing lignin in isoenergetic diets at two soluble fibre levels on digestion, performance and carcass quality of growing rabbits

Abstract

To assess the effect of increasing dietary lignin in isoenergetic diets at two soluble fibre (SF) levels on digestion, performance and carcass quality of growing rabbits, four diets were formulated according a 2 × 2 factorial design: low SF-low lignin (LSF/LL), low SF-high lignin (LSF/HL), high SF-low lignin (HSF/LL) and high SF-high lignin (HSF/HL). On average, in HSF diets SF was increased by 49 g/kg DM, mainly replacing starch (−53 g/kg DM), and in HL diets lignin was increased by 40 g/kg, mainly reducing starch (-78 g/kg DM), with increasing EE (+31 g/kg DM). Two hundred and sixty crossbred weaned rabbits (35 days old) were assigned to the experimental diets, individually housed and fed ad libitum until 63 days of age. Digestibility (from 49 to 53 days old), growth performance (from 35 to 63 days old), carcass quality (at 63 days old) and caecal environment (at 63 days old) were studied in 12, 65, 45 and 16 rabbits per diet, respectively. High SF diets showed higher CTTAD of fibrous fractions (+0.206±0.011, +0.207±0.015, +0.214±0.011 and +0.167±0.015 for aNDFom, ADFom, hemicelluloses and cellulose, respectively, P < 0.001), OM (+0.042±0.004, P < 0.001) and GE (+0.055±0.005, P < 0.001), resulting in high DE content (10.6 vs. 9.30 MJ/kg DM). In contrast, CTTAD of CP was lower (−0.023±0.009, P = 0.013), as well as the DP content (96.9 vs. 103 g/kg DM). This dietary variation reduced the DM content of caecal digesta (−28±3 g/kg, P < 0.001), besides increasing its VFA concentration (+18.0±4.0 mmol/L, P < 0.001) and reducing its pH (−0.28±0.05, P < 0.001). Feed intake and LW gain decreased, with an improvement of feed to gain ratio (−13.8 %, −4.7 %, −9.4 %, respectively; P < 0.001). The proportion of gastrointestinal tract was increased, with a subsequent reduction in dressing out (+19±2 g/kg LW and −15±2 g chilled carcass weight/kg LW, respectively, P < 0.001). High lignin diets showed lower CTTAD of OM (-0.055±0.004, P < 0.001) and GE (−0.034±0.005, P < 0.001) without affecting DE and DP contents. This dietary variation increased DM content of caecal digesta (+21±3 g/kg, P < 0.001), but did not affect the other caecal digesta traits. Feed intake was higher (+4.9 %, P < 0.001), although differences were dependent on the growth phase and the SF level (maximum difference at 35–49 days with low SF diets, +11.0 %, P < 0.001; minimum difference at 49–63 days with high SF diets, +1.0 %, P = 0.689), but did not affect LW gain and consequently impaired the feed to gain ratio (+5.1 %, P < 0.001). No effect was observed on dressing out, but the dissectible fat proportion increased (+6.7±1.1 g/kg reference carcass weight, P < 0.001).