The optimum threonine requirement in diets of juvenile hybrid grouper (epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂)

A six‐week growth trial was conducted to evaluate the optimum threonine (Thr) requirement in diets of juvenile hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂). Six isoenergetic, isoproteic and isolipidic diets were formulated to contain 5.4, 7.5, 10.3, 11.5, 13.7 and 15.2 g/kg dry matter dietary Thr levels. Results showed that and fish fed 5.4 g/kg dietary Thr had the lowest weight gain percentage (WG%) or protein productive value (PPV) among all experimental treatments. Analysis by the quadratic regression indicated that the optimum Thr requirement for maximum WG% or PPV in diets for hybrid grouper was estimated to be 11.48 or 13.55 g/kg of dry matter. The mRNA expression of hepatic growth hormone receptor 1 (GHR1) in fish fed 11.5 and 13.7 g/kg dietary Thr was higher than that of fish fed other dietary Thr levels. Fish fed 5.4 g/kg dietary Thr had the lowest whole‐body protein contents and the lowest expression of hepatic target of rapamycin (TOR), ribosomal protein S6 kinase 1 (S6 K1) among all experimental groups. In conclusion, the optimum dietary Thr requirement for best growth of hybrid grouper was estimated to be 11.48 g/kg dry matter (corresponding to 23.27 g/kg dietary protein).