Efficacy of crystalline and protein-bound amino acids for amino acid enrichment of diets for barramundi/Asian seabass (Lates calcarifer Bloch)

Abstract

The efficacy of either crystalline (C-) amino acids (AAs) or casein as sources of protein-bound (P-) AAs for AA enrichment of either high- (540 g kg−1, dry matter) or low- (390 g kg−1, dry matter) protein diets was examined in two 6-week experiments with barramundi Lates calcarifer (Bloch). The AA profile of a lysine (Lys) deficient gluten-rich basal diet was enriched incrementally in five steps either by using a mixture of C-AAs (predominantly Lys) at the expense of starch or by serial substitution of the gluten with casein (a rich source of Lys). These substitutions had a minimal effect on the protein and energy composition of the diet but enriched the Lys content (and that of other critically low essential AA) of the basal diet from 18 to 31 g kg−1 in Experiment 1, and from 12 to 18 g kg−1 in Experiment 2. A high fish meal control diet was included in both experiments. In each experiment, 12 diets were compared using 48 tanks of fish held in a freshwater recirculation system maintained at 28°C and with a 12:12 h light-dark photoperiod and fed once daily to satiety.

Fish growth rate and feed conversion ratio improved quadratically with AA enrichment; the response was most marked for the low-protein diets. Efficacy of AA enrichment was dose dependent. At low dietary supplementation rates (<3.3 g Lys kg−1 for the high-protein diets and up to 6 g Lys kg−1 for the low-protein diets), C-AAs were utilized as effectively as P-AAs. No further enhancement of fish productivity was induced by higher rates of C-AA supplementation with the high-protein diets.