Effects of Dietary Metabolizable Energy and Protein on Early Growth Responses of Broilers to Dietary Lysine

Two studies evaluated effects of metabolizable energy (ME), digestible Lys (dLys), and amino acid (AA) balance on broiler performance. In experiment 1 diets contained 3 levels of ME (3,000, 3,100, and 3,200 kcal/kg) in combination with 4 levels of dLys (1.05, 1.13, 1.21, and 1.29%). A fixed proportion of dLys relative to CP and key indispensable AA was maintained in graded increments of CP from 21.9 to 26.9%. There was no interaction of ME and dLys for 21 d BW gain or adjusted feed conversion ratio, which improved linearly with dietary dLys. Increasing the dLys or ME had no effect on feed intake, and the linear improvement in performance was attributed to a step-wise increase in dLys when diets contained a balance of AA and CP. Experiment 2 evaluated broiler response to 20 d of age when diets contained graded increments in dLys while maintaining a fixed proportion of dLys relative to CP and indispensable AA (balanced CP), or when dLys was increased in diets by supplementing synthetic L-Lys to 1 of 2 basal diets with 22.0% CP (low CP) or 27.0% CP (high CP) without adjusting concentrations of other AA or CP. The BW gain of broilers fed the low CP diet series followed a quadratic response, and the dLys requirement was estimated to be 1.19 ± 0.03% (1.30% total Lys). By contrast, BW gain on both the high CP and balanced CP diet series increased linearly. The higher BW gain and continued response to dLys above 1.19% when CP and AA concentrations were increased confirmed that the dLys requirement of broilers was dependent on the dietary CP. When a fixed ratio of dLys to CP was applied and indispensable and dispensable AA were not limiting, broiler BW gain and adjusted feed conversion ratio responded positively to incremental dLys up to at least 1.32% (27.2% CP) and was independent of the dietary ME over a range from 3,000 to 3,200 kcal/kg.