An experiment determined the effect of 6 different lysine concentrations (40, 52, 58, 65, 90 and 130 g/kg CP) in two protein concentrations (210 and 260 g/kg) on the efficiency of energy utilisation of broiler chickens and turkeys. Ninety-six male Ross 308 broiler chicks and 96 male BUT Big 6 turkey poults were fed at 90% of ad libitum during a 12 d bioassay period. At the end of the bioassay, birds were slaughtered and carcase crude protein, fat and gross energy were measured and nutrient metabolisability determined. 2. Broiler chickens had lower AME and AME(N) than turkeys. AME was higher in the 260 g/kg than in the 210 g/kg protein diet; however, no difference was evident in AME(N) between any of the diets in both species. A significant species x lysine interaction indicated that the decrease in AME and AME(N) with increasing lysine was greater in turkeys than in broiler chickens. 3. Turkeys had lower weight gains and efficiencies of energy retention than broiler chickens. The effects of different lysine concentrations on growth and the efficiency of energy retention were significant and growth responses were quadratic but there were negative linear responses in the efficiency of energy retention in both species at both crude protein concentrations. The efficiency of energy retention was increased with decreasing lysine concentrations throughout the wide range of concentrations used in the experiment.
