RESPONSE OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) GROWING FROM 50 TO 150 G TO SUPPLEMENTS OF DL-METHIONINE IN A SEMIPURIFIED DIET CONTAINING LOW OR HIGH-LEVELS OF CYSTINE

We studied the effect of increasing dietary concentrations of DL-methi onine on growth, feed intake, feed conversion ratio and the compositio n of gain in rainbow trout. Twenty-four groups of 20 trout initially w eighing 51 +/- 0.5 g/trout were fed semipurified diets containing 20.1 MJ digestible energy and either 3.0 or 5.8 g cystine/kg dry matter. A t each level of cystine, 12 levels of methionine (2 to 11 g/kg dry mat ter) were achieved by supplementation with graded quantities of DL-met hionine. During an experiment of 49 feeding days, no significant effec t of the level of dietary cystine was found for any performance trait. Nonlinear responses to increasing dietary methionine concentrations w ere found for feed intake, growth rate, protein concentration of gain and protein deposition, whereas fat concentration of gain decreased co ncurrently. Dietary methionine was utilized most efficiently at a conc entration of 3.5 g methionine/kg dry matter or 0.17 g/MJ digestible en ergy. Ninety-five percent of the plateau deposition of body protein wa s achieved at a dietary methionine concentration of 8 g/kg dry matter or 0.40 g/MJ digestible energy. For achieving 98%, the required concen tration was 9.0 g/kg or 0.49 g/MJ. Recommended dietary methionine conc entration will depend on the trait chosen.