Effect of Dietary Energy to Protein Ratios on Growth Performance and Feed Efficiency of Juvenile Grass Carp (Ctenopharyngodon idella)

A feeding trial of three dietary protein (DP) (25, 35 and 45%) and four dietary lipid (DL) levels (3, 6, 9 and 12%) with different dietary energy to protein (E/P) ratios (from 5.93 to 13.84 kcal/g protein) was conducted to investigate the proper dietary E/P ratio for juvenile grass carp. After acclimation of 2 weeks by a lipid-free diet, fish were reared for 10 weeks in a water recirculation system at 22.2 ± 2.8°C under natural light-dark cycle. The highest growth and feed efficiency were obtained with the diet of 45% DP and 3% DL (E/P ratio of 5.93 kcal/g protein), followed closely by the group with 35% DP and 3% DL (E/P ratio of 7.62 kcal/g protein), without any significant difference between them. At the same DP level, growth and feed efficiency decreased with increasing DL level. However, at the same DL level, growth and feed efficiency enhanced with increasing DP level. But at the 9% and 12% DL level (energy of 321 and 348 kcal/100g diet), there were no significant differences between groups in terms of growth indices, irrespective of the DP level. Except intraperitoneal fat ratio, condition factor, viscera ratio and hepatosomatic index, all studied parameters showed increasing trends with enhancing dietary E/P ratio. Lipid contents of whole body and liver aroused with the increase of DL level at the same DP level. Taking into account the cost of feeding, the diet containing 35% DP and 3% DL (E/P ratio of 7.62 kcal/g protein, GE of 267 kcal/100g diet) is recognized as an optimal diet for juvenile grass carp in the present study. However, some future investigations will need to address the effects of temperature on optimal E/P ratio in grass carp.