REQUIREMENTS FOR SULFUR AMINO-ACIDS AND UTILIZATION OF D-METHIONINE BY RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

The methionine (Met) requirement and replacement values of cystine (Cys) and D-Met for L-Met were estimated at 15-degrees-C for fingerling rainbow trout (Oncorhynchus mykiss) by feeding diets containing various levels of L-Met, D-Met and L-Cys. Four tanks, each containing 28 (Experiment 1) or 30 (Experiment 2) trout (11-15 g initial weight), were assigned to each diet. Weight gain and nitrogen retention during a 6-week period increased with increasing levels of L-Met up to 0.5% of diet, when the diets contained excess L-Cys (0.5%). Feed efficiency increased as L-Met increased, but no significant (P > 0.05) differences were detected at levels above 0.4%. With diets containing 0.5% L-Met, the Cys requirement was found to be 0.3% of the diet. Increasing D-Met level above 0.27% in a diet containing 0.23% L-Met did not significantly (P < 0.05) influence weight gain, feed efficiency or nitrogen retention. Analysis of the results showed that 1. the Met requirement of fingerling rainbow trout is 0.52 (1.49)% of diet (dietary protein), when a diet contains excess Cys; 2. the Cys replacement value (on an equal molar sulfur basis) of L-Cys for L-Met is approximately 42%; 3. the requirement of trout for total sulfur amino acids is about 0.8 (2.3)% of diet (dietary protein); and 4. rainbow trout utilize D-Met as efficiently as L-Met for weight gain but less efficiently for protein accretion.