Grass carp fed a fishmeal-free extruded diet showed higher weight gain and nutrient utilization than those fed a pelleted diet at various feeding rates

The present study was conducted to investigate the effects of extruded and pelleted feeds at various feeding rates on growth and nutrient utilization of grass carp (Ctenopharyngodon idellus). Two fish meal-free feeds with the same formula were prepared as pelleted feed and extruded feed, and fed to eight treatments of fish (37.0 ± 0.2 g) with feeding rates of 100%, 90%, 80% and 70% satiation. During the feeding period, the fish of 100% satiation group were fed to apparent satiation, and then the other fish were fed with diets equal to 90%, 80% and 70% of 100% satiation group, respectively. After 10 weeks of feeding, the fish fed pelleted and extruded feeds showed no differences in feed intake (P > 0.05). At the same feeding rate, the fish fed extruded feed showed higher weight gain (WG), protein retention efficiency (PRE), lipid retention efficiency (LRE) and lower feed conversion ratio (FCR) than those fed pelleted feed (P < 0.05). In both pelleted and extruded feeds, WG and LRE reduced with the decreasing feeding rate (P < 0.05), but the fish fed at 80% satiation showed the lowest FCR and the highest PRE. Crude lipid contents, rather than crude protein contents of whole-body and liver decreased with the decreasing feeding rate (P < 0.05). In conclusion, grass carp fed extruded feed showed higher nutrient utilization than pelleted feed, and similar feed intake to pelleted feed. The feeding rate was suggested to be 80% satiation in both extruded and pelleted feeds based on nutrient utilization.