Influence of dietary benzoic acid addition on nutrient digestibility and selected biochemical parameters in fattening rabbits

The effects of two inclusion levels of benzoic acid (5 and 20 g/kg diet; B5 and B20, respectively) vs. a control (C) and an antibiotic (tiamulin) supplemented (A; 150 mg/kg) diet on nutrient digestibility and selected biochemical parameters were investigated in 48 weaned (n = 12 per treatment) rabbits (35 days old). Blood samples were obtained (at 45 and 85 days of age) to measure erythrocyte glutathione peroxidase (GSH-Px) activity, plasma alkaline phosphatase (ALP) activity and Ca and P levels. At 56 days of age total tract apparent digestibility (TTAD) was determined. At the end of the trial (85 days) rabbits were slaughtered for subsequent measurements. GSH-Px activity was also determined in liver homogenates post-mortem. Benzoic acid inclusion level decreased linearly (P<0.05) the weight of total full gastrointestinal tract, resulting in a trend (P=0.058) to increased dressing yield. The TTAD of organic matter (OM) increased linearly (P<0.05), whereas that of dry matter, crude protein and gross energy tended to increase (P=0.067, P=0.090 and P=0.098, respectively) with benzoic acid increment. The TTAD of aNDFom, hemicelluloses and cellulose was affected quadratically (P<0.05) by benzoic acid supplementation, with the B5-fed rabbits showing the higher values. Plasma ALP activity and P levels were linearly reduced (P<0.001) by benzoic acid inclusion, but no influence was observed on GSH-Px activity in erythrocytes or liver. In conclusion, dietary benzoic acid affects the TTAD of OM and fibre fractions in a dose dependent manner, without having any adverse effect on the systemic markers (biochemical parameters) examined.