Graded inclusions of sodium metabisulphite in sorghum-based diets: I. Reduction of disulphide cross-linkages in vitro and enhancement of energy utilisation and feed conversion efficiency in broiler chickens

Seven sorghum-based diets containing 0, 1.50, 2.25, 3.00, 3.75, 4.50 and 5.25 g/kg of the reducing agent sodium metabisulphite (SMBS) were prepared in which sodium levels were balanced with sodium bicarbonate. A red sorghum (766.4 g/kg starch, 78.6 g/kg protein) was hammer-milled through a 3.2 mm screen prior to being mixed into complete broiler diets containing 195 g/kg protein with an energy density12.97 MJ/kg, which were steam-pelleted at an 84 °C conditioning temperature. Each of the seven dietary treatments was offered to a total of 294 male Ross 308 chicks in seven replicate cages (6 birds per cage) from 10 to 24 days post-hatch to determine the effects of SMBS on growth performance, nutrient utilisation and nitrogen (N) digestibility coefficients in four small intestinal segments. SMBS did not linearly influence (P > 0.50) weight gains and feed intakes. However, SMBS quadratically improved (r = 0.416; P < 0.05) feed conversion ratios and it may be deduced from the quadratic equation that 4.75 g/kg SMBS improved FCR by 3.06% (1.424 versus 1.469). At all positive inclusions, SMBS significantly enhanced apparent metabolisable energy (AME) and N-corrected AME (AMEn); at the lowest inclusion, 1.50 g/kg SMBS significantly improved AME by 0.36 MJ (13.81 versus 13.45 MJ/kg; P = 0.005), AMEn by 0.45 MJ (12.30 versus 11.85 MJ/kg; P = 0.001) and FCR by 2.47% (1.422 versus 1.458; P = 0.025) on the basis of pair-wise comparisons. SMBS linearly decreased concentrations of disulphide bonds (r = −0.775; P < 0.001) and linearly increased free sulphydryl groups (r = 0.890; P < 0.001) and protein solubility of the diets (r = 0.943; P < 0.001). However, SMBS had little influence on N digestibility coefficients as the sole significant response was a 6.41% increase (0.797 versus 0.749; P < 0.01) in the distal ileum at 3.75 g/kg and SMBS did not influence small intestinal sites of protein disappearance. Consideration is given to the possibility that the sulphite reducing agent, SMBS, caused oxidative-reductive depolymerisation of starch polysaccharides which may have been responsible for the improvements in energy utilisation and feed conversion efficiency.