Effects of extruded and pelleted diets with differing lipid levels on growth, nutrient retention and serum biochemical indices of tilapia (Oreochromis aureusxTilapia nilotica)

This study was conducted to evaluate the effects of extruded diets and pelleted diets with varying dietary lipid levels on growth performance and nutrient utilization of tilapia. Six diets, containing three levels of lipid at 40, 60 or 80gkg(-1) (with the supplemental lipid of 0, 20 or 40gkg(-1), respectively), were prepared by extruding or pelleting and then fed to tilapia juveniles (8.0 +/- 0.1g) in cages (in indoor pools) for 8weeks. The results indicated that the fish that were fed the diet with 60gkg(-1) of lipid had a higher weight gain (WG), specific growth rate (SGR), protein efficiency ratio (PER), lipid retention (LRE), energy retention (ERE), apparent protein digestibility, apparent dry matter digestibility and a lower feed conversion ratio (FCR) than those fed the diet with 40gkg(-1) lipid in both the extruded diet and pelleted diet (P<0.05). As the dietary lipid level increased from 60 to 80gkg(-1), these parameters were not further improved, even digestibilities of the crude protein and dry matter decreased (P<0.05). With the dietary lipid level increased, whole-body lipid content significantly increased (P<0.05), serum aspartate aminotransferase, alkaline phosphatase, total cholesterol and low-density lipoprotein cholesterol (LDL-C) tended to increase (P>0.05), whereas whole-body protein content, serum triglyceride (TG), high-density lipoprotein cholesterol (HDL-C) and HDL-C/LDL-C tended to decrease (P>0.05). Fish fed with the extruded diets had a higher WG, SGR, hepatosomatic index (HSI), PER, protein retention (PRE), LRE, ERE, TG, apparent digestibility of protein and dry matter, as well as a lower FCR, than those fed with the pelleted diets at the same dietary lipid level (P<0.05). These results suggested that tilapia fed with the extruded diets had a better growth and higher nutrient utilization than fish fed with the pelleted diets, when dietary lipid level ranged from 40 to 80gkg(-1) and at dietary crude protein level was 280gkg(-1). The optimum dietary lipid level was 60gkg(-1) in both the pelleted and extruded diets, and extrusion did not affect dietary lipid requirement of the tilapia.