Free amino acids can replace protein-bound amino acids in test diets for studies in rainbow trout (Oncorhynchus mykiss).

Effects of reducing dietary concentration of protein-bound amino acids on growth, feed intake and composition of gain in rainbow trout were studied in four experiments. Average initial body weights ranged between 29 +/- 0.7 and 55 +/- 0.5 g per trout. Diets contained approximately 20 MJ digestible energy/kg dry matter. Each diet was fed to satiation to four replicate groups of 20 trout. Feed intake and growth rates were recorded for each group. Body composition was analyzed in representative groups at the start of each experiment and in all experimental groups at the end of each experiment. Reduction of dietary protein concentration to < 380 g/kg dry matter caused significantly lower growth rates and reduced protein concentrations of gain, but these reductions in growth could be offset by the addition of 10 crystalline essential amino acids. Fish meal was completely replaced by a mixture of wheat gluten and crystalline amino acids without negative influences on growth. In the absence of fish meal, almost half the wheat gluten could be replaced by crystalline amino acids in diets containing about 32 g N x 6.25/kg dry matter without significant influences on growth. In such diets, concentrations of individual amino acids may be varied widely with no variation in other amino acids or nutrients.