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Ten nutritionally-equivalent diets containing 620 g/kg grain sorghum, based on seven blends and three varieties (Block I, HP, Liberty), were offered to a total of 360 male Ross 308 chicks from 7 to 28 days post-hatch. The effects of dietary treatments on growth performance, nutrient utilisation, starch and protein (Nitrogen) digestibility coefficients, disappearance rates and starch:protein disappearance rate ratios in the distal jejunum and distal ileum were determined. On the basis of response surface contour plots, the optimal weight gain of 1439 g/bird would be generated by a 79.2-20.8 blend of HP and Liberty and the optimal feed conversion ratio (FCR) of 1.486 by a 55.6-44.4 HP-Liberty blend. Similarly, the optimal nitrogen-corrected apparent metabolisable energy (AMEn) of 12.38 MJ would be generated by a 32.0-68.0 blend of HP and Liberty and the optimal apparent metabolisable energy to gross energy (AME:GE) ratio of 0.845 by a 31.1-68.9 HP-Liberty blend. Thus Block I sorghum was effectively rejected. Instructively, Block I contained higher concentrations of kafirin, phenolic compounds and phytate than the two other sorghum varieties. Across the ten diets, sorghum concentrations of these three factors were negatively correlated with AME:GE ratios or efficiency of energy utilisation. These negative linear regressions indicate that all three factors may be contributing to sub-standard starch/energy utilisation in broiler chicks offered sorghum-based diets. |
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