Effect of dietary DP/DE ratio on apparent digestibility, growth and nitronean and phosphorus retention in rainbow trout Oncorhynchus mykiss (Walbaum)

The effect of DP/DE ratio in diets for rainbow trout, Oncorhynchus mykiss (Walbaum), was investigated. To evaluate growth and body composition, groups of trout were fed three experimental diets with a constant level of gross energy (25.4 +- 0.12 MJ kg-1 dry matter (DM) and different digestible protein/digestible energy (DP/DE) ratios (diet A 16.35; diet B 17.21; diet C, 18.23 g MJ-1. Fat, protein and energy digestibility coefficients were not affected by the DP/DE ratio of the diets. Growth and feed utilization improved markedly ad dietary DP/DE ratio increased (P< 0.01). The efficiency of fat, protein and energy utilizationtended to increase with increasing DP/DE ratio of the diets. Nitrogen discharge in effluent water per kg of weight gain was not affected by dietary treatments (mean values for: diet A, 29.9; diet B, 29.8; diet C, 29.1 gN kg-1 weight gain), while phosphorus discharge in efflunt water fell using diets with a higher DP/DE ratio (mena values for: diet A, 7.3; diet B, 6.7; diet C, 5.9 g P kg-1 weight gain).