Effects of dietary protein levels on growth, feed utilization, body composition and ammonia–nitrogen excretion in juvenile Nibea diacanthus

Abstract

Juvenile Nibea diacanthus were fed various dietary protein levels, and their growth performance, feed utilization, carcass proximate composition and 24-h postprandial ammonia–nitrogen excretion were investigated. Five isoenergetic and isolipidic experimental diets, containing various protein levels ranging from 36 to 52 % by a 4 % increment, were formulated. Each diet was randomly fed to triplicate groups of 25 juvenile N. diacanthus (initial weight 62.85 ± 0.32 g) for 8 weeks. Highest weight gain, specific growth rate, feed efficiency, protein efficiency ratio and protein retention were obtained at 48 % dietary protein levels (P < 0.05). As dietary protein levels increased, feed intake, hepatosomatic index and viscerosomatic index declined significantly. Similarly, the ammonia–nitrogen excretion rate was largely inversely correlated with dietary protein levels, with the lowest value appearing in the 48 % group (P < 0.05). High-protein diets resulted in higher whole-body and muscle protein contents and produced a significant effect on muscle amino acid profile. Also, total essential amino acid pattern of the muscle was significantly correlated to those of dietary proteins. Analysis of specific growth rate and weight gain by broken-line regression indicated that the estimated optimal dietary levels of protein for juvenile N. diacanthus were 48.66–48.94 %.