Ideal protein concept and its application in practical diets for Nile tilapia Oreochromis niloticus
Type of DegreePhD Dissertation
Fisheries and Allied Aquacultures
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Protein is considered the most important dietary component as it comprises a significant proportion of whole body dry matter of fish. As it has high cost per unit, provision of feed meeting an exact requirement for protein is a prerequisite for efficient and economical fish production. To do this, the diet has to be formulated properly with regard to amino acid (AA) balance. Like other fish species, research on the balance of indispensable AA (IAA) and dispensable AA (DAA) for promoting maximum growth and protein deposition on Nile tilapia Oreochromis niloticus has been limited and inconsistent. Considering the importance of meeting AA requirement, this research sought to optimize AA balance of the diets offered to Nile tilapia by applying and validating the use of the ideal protein concept. The first study was conducted to evaluate the production performance of Nile tilapia fed graded levels of protein with or without crystalline IAA supplemented in practical diets. Reducing the levels of intact protein from 32 to 24% without IAA supplementation resulted in a significant reduction in growth of Nile tilapia. The supplementation of crystalline IAA to meet AA requirements at two intact protein levels of 29.7 and 27.2% helped the fish to reach comparable growth rates and protein utilization efficiencies with fish fed the reference diet (32% intact protein). Further reduction of intact protein levels to 24.7 and 22.2% of diet, however, induced growth depression of fish which could result from a deficiency of nonspecific nitrogen as a source of energy or limitation of daily IAA intake. ii The following study was conducted to further optimize AA balance of diets for Nile tilapia. The IAA profiles of diets in this study were enhanced to IAA profile of the diet which supported the best performance of fish in the first study. This diet was after that supplemented with DAA at 4% to evaluate the role of DAA in the low protein diet. The results obtained after ten weeks indicated that the supplementation of DAA in the low protein diet is significant to overcome limitations of nonspecific nitrogen. Enhancing IAAs to IAA profile of the diet, which supported the best performance of fish in the first study without DAA supplements, could not help the fish to reach comparable weight gain to fish fed the diet with DAA supplements. The potentially limiting IAA (arginine, threonine, valine, tryptophan and isoleucine) in addition to lysine, methionine and threonine in the ingredient matrix was also confirmed. These IAAs were individually deleted from the IAA profile of the diet with enhanced IAA and DAA supplements. Results illustrated that with the exception of valine, the deletion of the other crystalline IAA supplements (tryptophan, arginine, threonine and isoleucine) did not cause any deleterious effects on growth performance and protein utilization efficiency of fish. Therefore, in addition to lysine, methionine and threonine, valine is limiting in our ingredient matrix and the supplementation of this IAA is necessary to meet the requirements of fish. Even though tryptophan is not limiting in our ingredient matrix, the reported requirement of this IAA is inconsistent so the requirement was confirmed. The tryptophan requirement of juvenile Nile tilapia was confirmed at 0.31% (0.25 - 0.37%), 0.33% (0.26 - 0.39%), 0.25% (0.24 - 0.25%) and 0.27% (0.25 - 0.31%) of the diet for optimal growth, tryptophan deposition, feed efficiency and apparent net protein deposition (95% of maximum value), respectively. iii The final study was conducted to confirm the role of DAA in low and moderate protein diets and evaluate the limitation of daily IAA intake in diets with high inclusion of crystalline AA (CAA) supplemented in the diets. Intact protein was kept constant at 22.2% for all of the diets and CAA was then added to adjust IAA profiles of feed to reach 100, 110 and 120% of the respective requirement of Nile tilapia (NRC, 2011) with or without DAA supplementation. Results confirmed that DAA plays an important role in meeting the nitrogen requirement of fish. Growth performance of fish fed diets without DAA supplementation was not comparable to fish fed diets supplemented with DAA in spite of IAA supplements up to 120% NRC requirement. The results of this study also indicated the inferior growth of fish fed diets with IAA supplements at 100% NRC requirement, which could result from limitation of daily IAA intake. The effect of feeding regimes on the efficacy of CAA utilization was also evaluated in which the feed offered to fish fed 100% IAA without DAA supplements and 110% IAA with 4% DAA supplements was paired for two and four feedings per day. Results indicated that fish can be fed successfully two times per day regardless of high inclusion levels of CAA. 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 profiles of the diets. Well- balanced IAA profiles can be used to reduce the intact protein levels of feed without causing impaired growth of fish. However, if low intact protein levels are to be formulated, the supplementation of DAA might be required to satisfy the nitrogen requirement of fish. In our ingredient matrix, in addition to lysine, methionine and threonine, valine is limiting and the supplementation of this IAA is necessary to satisfy the requirements of fish.