Effects of extruded and pelleted diets with differing protein levels on growth and nutrient retention of tilapia, Oreochromis niloticus × O. aureus
The present study was conducted to evaluate the effects of extruded diets and pelleted diets with varying protein levels on growth performance and nutrient utilization of tilapia. Six diets, containing three levels of crude protein (CP) of 250, 280 and 310 g kg−1, were prepared by pelleting (sinking) or extruding (floating) and were fed to tilapia juveniles (body weight of 8.0 ± 0.1 g). After 8-week feeding, the weight gain (WG) and feed conversion ratio (FCR) of the six groups (pelleted diets with protein of 250, 280, 310 g kg−1 and extruded diets with protein of 250, 280, 310 g kg−1) were 561.8, 609.4, 686.5, 593.6, 663.7, 683.0 and 1.49, 1.38, 1.22, 1.42, 1.25, 1.23 %, respectively. The WG, protein retention (PRE), lipid retention and energy retention (ERE) of fish fed pelleted diets increased, while FCR decreased as dietary protein increased. In extruded diets, the fish fed 280 g kg−1 CP had a higher WG, PRE, ERE and a lower FCR than fish fed 250 g kg−1 CP and had the similar growth as the fish fed 310 g kg−1 CP. The fish fed 280 g kg−1 CP had the highest apparent digestibility coefficient of dry matter and CP in both pelleted diets and extruded diets. The fish fed extruded diets had better growth and better nutrient utilization than fish fed pelleted diets when dietary protein level was 250 or 280 g kg−1, but when dietary protein level was 310 g kg−1, the fish fed extruded diet showed similar growth as the fish fed pelleted diet. In both extruded and pelleted diets, the tilapia fed 280 or 310 g kg−1 CP had lower activity of aspartate aminotransferase than fish fed 250 g kg−1 CP, and the high-density lipoprotein cholesterol levels of groups of 250 and 280 g kg−1 CP were lower than that of 310 g kg−1 CP group. The above results demonstrate that tilapia fed an extruded diet have a better growth than the fish fed a pelleted diet at a low dietary CP level but not at a high dietary CP level. The dietary protein requirement of tilapia could be 310 g kg−1 for a pelleted diet or 280 g kg−1 for an extruded diet.