Dietary methionine requirement of juvenile blunt snout bream (Megalobrama amblycephala) at a constant dietary cystine level

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Abstract

A 9-week feeding trial was conducted to investigate the dietary methionine requirement of juvenile Megalobrama amblycephala at a constant dietary cystine level. Six semipurified diets were formulated to contain graded dietary methionine levels from 3.9 to 15.4 g kg−1 in about 2.5 g kg−1 increments in the presence of 2.2 g kg−1 cystine. Results showed that specific growth rate (SGR) and protein efficiency ratio (PER) significantly increased with increasing dietary methionine levels from 3.9 to 12.4 g kg−1 and thereafter kept stable. Maximum protein productive value (PPV), nitrogen retention efficiency (NRE) and liver weight were observed in 8.5 g methionine kg−1 diet. Protein contents in whole fish body were positively correlated with dietary methionine level, while lipid contents were negatively correlated with it. Morphological index and hepatic glutamate-pyruvate transaminase (GPT) activities were independent of dietary methionine levels. However, dietary methionine supplementation significantly improved haematological parameters, plasma methionine and total essential amino acid contents and hepatic glutamate-oxaloacetate transaminase (GOT) activities. Analysis of dose response using broken-line regression on the basis of SGR and PPV versus dietary methionine level estimated the optimum dietary methionine requirements of juvenile M. amblycephala to be between 8.5 and 8.4 g kg−1 of diet (25.0 and 24.7 g kg−1 of protein) in the presence of 2.2 g kg−1 cystine, respectively. Hence, the corresponding total sulphur amino acids requirements of this species were calculated to be 10.7 and 10.6 g kg−1 of diet (31.5 and 31.2 g kg−1 of dietary protein).

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