Dietary threonine requirement of juvenile blunt snout bream (Megalobrama amblycephala)

A feeding trial was conducted to determine the dietary threonine (Thr) requirement of juvenile blunt snout bream and to assess the effects of Thr on the growth performance, feed utilization, whole body composition, plasma composition and target of rapamycin (TOR) pathway gene expression. Five isonitrogenous and isoenergetic semi-purified diets (34% crude protein) were formulated to contain graded levels of dietary Thr (0.58, 1.08, 1.58, 2.08 and 2.58% in dry matter (DM)). l-Amino acid mixture was supplemented to simulate the whole body amino acid pattern of fish, except for Thr. Triplicate groups of juvenHabte-Michael Habte-Tsionile fish (initial weight, 3.01 ± 0.01 g) were fed with the respective diet to near satiation four times daily for 9 weeks. The results indicated that final body weight, weight gain rate (WGR), specific growth rate (SGR), feed efficiency ratio (FER), protein efficiency ratio (PER), protein retention value (PRV), Fulton condition factor (K factor), and whole body protein content significantly (P < 0.05) increased with increasing dietary Thr levels up to 1.58% and thereafter decreased. Plasma free Thr concentration and total protein content rose as dietary Thr levels increased (P < 0.05). High dietary Thr level (2.58%) triggered plasma urea content (P < 0.05). Dietary Thr regulated TOR and eukaryotic translation initiation factor 4E-binding protein 2 (4E-BP2) gene expressions in the proximal-intestine (PI), mid-intestine (MI) and distal-intestine (DI) of juvenile blunt snout bream, which may explain further that dietary Thr enhanced growth performance and protein retention. Based on the second-degree polynomial regression analysis of SGR against dietary Thr levels, the optimum dietary Thr level for juvenile blunt snout bream was estimated to be 1.57% of the diet, corresponding to 4.62% of dietary protein.