Response of rainbow trout (Oncorhynchus mykiss) growing from 50 to 200 g to supplements of dibasic sodium phosphate in a semipurified diet

Effects of increasing dietary concentrations of phosphorus on growth, feed intake, feed conversion, composition of gain and concentration of inorganic phosphate in plasma were studied in rainbow trout. Twelve groups of 20 trout initially weighing 53 +/- 0.6 g/fish were fed semipurified diets containing 19.6 MJ digestible energy per kilogram of dry matter. Twelve levels of phosphorus ranging from 1.03 to 10.96 g/kg dry matter were achieved by replacing inorganic binder with dibasic sodium phosphate in 11 graded levels. Nonlinear responses of trout to increasing dietary phosphorus concentration determined over 53 d were described using exponential functions. Feed intake, growth rate and feed conversion ratio as well as plasma inorganic phosphate concentration increased with increasing dietary phosphorus concentration. The concentrations of phosphorus, calcium, magnesium and potassium in weight gain increased, whereas concentrations of lipids and energy in weight gain decreased with increasing dietary phosphorus concentration. Concentrations of protein and sodium in weight gain were unaffected. Different concentrations of dietary phosphorus were required for achieving 95% of the plateau value determined for desired traits. In growth rate and phosphorus deposition, the required phosphorus concentrations were 3.7 and 5.6 g/kg dry matter, respectively. However, dietary phosphorus was utilized most efficiently (88%) at a dietary concentration of 2.5 g/kg dry matter. At the dietary phosphorus concentration that resulted in maximum phosphorus deposition (5.6 g/kg dry matter), phosphorus utilization was about 60%. Supplemental phosphorus from dibasic sodium phosphate was completely available to trout which must be considered in formulating recommendations. Based on this work, 0.25 g available phosphorus/MJ digestible energy is recommended for trout diets.