Dietary sulfur amino acid requirement of juvenile yellow perch fed the maximum cystine replacement value for methionine

We conducted three separate experiments designed to determine the dietary m ethionine requirement, ability of cyst(e)ine to spare methionine, and the t otal sulfur amino acid requirement (TSAA) of juvenile yellow perch when fed the maximal amount of cyst(e)ine. The purified basal diet used in each exp eriment contained 33.6 g of crude protein/100 g diet and 12.0 g of lipid/10 0 g diet. In the first experiment, L-methionine was added to eight diets pr oviding methionine concentrations ranging from 0.37 to 1.77 g/100 g diet in gradations of 0.2 g/100 g diet. Diets were fed for 12 wk to juvenile yello w perch initially weighing 4.7 g/fish. Broken-line analyses of weight gain and feed efficiency data indicated that the dietary methionine requirement was 1.0 g/100 g diet (3.1 g TSAA/100 g dietary protein) and 1.1 g/100 g die t (3.4 g TSAA/100 g dietary protein), respectively. In the second experimen t, various ratios of L-cyst(e)ine and L-methionine were added to the basal diet and fed for 12 wk to determine the cyst(e)ine replacement value of yel low perch initially weighing 19.3 g/fish. Weight gain and feed efficiency ( FE) data indicated that cyst(e)ine spared up to 51% of the methionine requi rement. In the final experiment, graded levels of cyst(e)ine plus methionin e in a ratio of 51:49 were added to the basal diet in gradations of 0.1 g/1 00 g diet (0.5 to 1.2 g TSAA/100 g diet) to determine the dietary total sul fur amino acid requirement. Diets were fed to satiation for 10 wk to fish i nitially weighing 8.1 g. Broken-line analyses of weight gain, feed intake a nd FE data indicated that the dietary TSAA requirement was 0.85, 0.87 and 1 .0 g of TSAA/100 g diet (2.5 to 3.0 g of TSAA/100 g of dietary protein), re spectively. The majority of dietary TSAA requirements of fish are in the ra nge of 2 to 4 g/100 g of dietary protein and are generally similar to those of both birds and swine, but lower than estimates for rodents.