Effects of extruded and pelleted diets with different protein levels on growth performance and nutrient retention of largemouth bass (Micropterus salmoides)

Abstract

This study investigated effects of extruded diet (ED) and pelleted diet (PD) with differing dietary protein levels on growth performance and nutrient utilization in juvenile largemouth bass (LMB). Nine hundred LMB (initial body weight of 6.47 ± 0.1 g)were fed for eight weeks in five levels of crude protein (40%, 44%, 48%, 52% and 56%) with pelleting or extruding processing technology. The results showed the weight gain (WG, 380.65%), apparent digestibility of crude protein (ADCP, 55.76%), dry matter (DM, 89.70%) and whole-body crude protein (66.32%) of LMB fed with ED52 group were significantly higher than those of other groups (P < 0.05), and FCR (0.96) was lowest. The WGmax was obtained when dietary protein level was 51.6% in ED. Activities of digestive enzymes in liver of LMB fed ED groups were generally higher than those fed PD groups. In addition, lipase, tryptase and α-amylase activities were at higher values between ED48 to ED56. The levels of total cholesterol and triglycerides in serum of LMB were lowest at ED52 group. The liver glycogen content, GLUT1, P-GSK-3β and GYS2 gene expression level in ED groups were significantly higher than those of in PD groups at same protein level (P < 0.05), and the gene expression levels of LMB were lowest in ED52 group. However, the gene expression levels in ED56 group were significantly higher than those of the other groups (P < 0.05). These results indicated that juvenile LMB achieve maximum growth when fed diet for ED at 51.6% protein level.