Crude protein reduction and digestible methionine plus cystine and threonine to digestible lysine ratios in diets for Nile tilapia fingerlings

The objective of the present study was to investigate crude protein reduction and the methionine+cystine and threonine to lysine ratios in diets for reversed Nile tilapia fingerlings (Oreochromis niloticus), Thai strain. Four hundred and twenty Nile tilapia fingerlings with an average initial weight of 1.83 +/- 0.02 g were placed in a completely randomized design, with five treatments, seven replications per treatment and 12 fish per experimental unit, for 37-days. Two crude protein (CP) levels were evaluated (32 and 28%). The diet with 32% CP had methionine+cystine: lysine and threonine: lysine ratios of 63 and 74%, respectively. Three diets with 28% CP had methionine+cystine: lysine and threonine: lysine ratios of 63 and 74%, 60 and 74% and 60 and 69%, respectively. The diet with 28% CP and had methionine+cystine: lysine ratio of 63% and threonine: lysine of 74% and was supplemented with 0.3% glutamic acid. The fish were kept in thirty-five 130 L aquariums supplied with single-pass flow-through water, individual aeration and fed ad libitum six times per day. Fish fed the diet with 32% CP had lower protein efficiency ratios and nitrogen efficiency ratios. The diet CP level was 28% and the methionine+cystine and threonine to lysine ratios were 60 and 69%, respectively.