Efficacy of reduced protein diets and the effects of indispensable amino acid supplements for Nile tilapia Oreochromis niloticus

The study was conducted to evaluate the production performance of Nile tilapia Oreochromis niloticus fed graded levels of protein with or without indispensable AA (IAA) supplemented in practical diets. A control diet containing 32 % intact protein was modified to meet the respective requirements of IAA based upon NRC recommendations for Nile tilapia. From this diet, two series of diets were formulated to confirm the efficacy of crystalline AA (CAA) used as supplements in the practical diets of fish by applying "ideal protein concept". In the first series of diets, intact protein was reduced gradually from 32 to 24 % of the diet without IAA supplementation. In the second series of diets, the same dietary protein levels were formulated; however, the IAA profiles of these diets were modified to meet the respective requirement of Nile tilapia (NRC, 2011). Five commercially-available AAs, including methionine, lysine, tryptophan, histidine and valine, were supplemented if necessary to attain the targeted AA profile. The supplementation of all deficient IAAs was obtained in the last diet to evaluate if there is further benefit of using IAAs other than commercial AAs to balance the AA profiles of diets. Four groups of Nile tilapia (12.7 +/- 0.2 g) were fed each diet four times daily to near satiation. The results obtained after eight weeks indicated that fish fed diets with IAA supplementation had significantly higher weight gain compared to fish fed diets without IAA supplementation. Fish fed intact protein levels above 27.2 % with the supplementation of IAA up to the reported requirement (NRC, 2011) showed comparable performance to fish fed the control diet. The reduction of intact protein further than that, however, resulted in reduced growth rate (thermal-unit growth coefficient) of fish even though the IAA profiles of these diets met the reported requirement of tilapia for optimal growth (NRC, 2011). Based on the data obtained from this study, it can be concluded that ideal protein concept can be applied in formulating the diets for Nile tilapia to optimize AA profile of the diets. Wellbalanced IAA profile can be used to reduce the intact protein levels of feed (32 to 27.2 %) without causing impaired growth of fish.