Phytate is the main storage form of phosphorus (P) in many plants, but phytate-bound P is not available to fish. Phytase, an enzyme specific to hydrolyze indigestible phytate, has been increasingly used in fish feed during the past two decades, mainly in response to performance and heightened concerns over P pollution to the aquatic environment. A total of 750 male tilapia (Oreochromis niloticus) of approximately 20g were randomly allocated to 19 tanks (4 replicates per 4 treatment, except treatment NC2+2.500 which had 3 replicates) of 100 l each, using a recirculating aquaculture system. Duration of trial was 81 days. The five dietary treatments comprised a nutrient adequate positive control diet (PC), a negative control diet (NC1) formulated with a 0,20% avP, 0,16% Ca, and 0,56% protein matrix, the NC1 supplemented with 1.500 FTU/kg of phytase (NC1+1.500), a negative control diet (NC2) formulated with a 0,24% avP, 0,18% Ca, and 0,65% protein matrix, and the NC2 diet supplemented with phytase at 2.500 FTU/kg (NC2+2.500). The phytase used in this study was an advanced E. coli phytase (Quantum Blue, AB Vista UK). Diets were extruded to produce 3mm pellets, with phytase applied by post-pellet liquid application. At days 0 and 81 all fish per tank were weighed and feed intake recorded daily in order to calculate total feed intake (FI), body weight gain (BWG), specific growth rate (SGR) and feed conversion ratio (FCR). The efficiency of protein use was determined by the protein efficiency ratio (PER) which is calculated as weight gained/protein intake over the entire feeding period. Over the 81-day feeding trial, performance of tilapia worsened with each nutrient downspec, from PC>NC1>NC2 (Table 1). Addition of phytase to each respective NC diet increased weight gain and improved FCR to a statistically similar level as the PC. Feed intake was not significantly affected by treatment. PER showed a poorer conversion of ingested protein to weight gained in NC2 fed fish, however, PER was restored with the supplementation of 2.500 FTU/kg phytase.
In conclusion, degradation of dietary phytate with phytase supplementation in diets that are reduced in avP and protein resulted in better growth performance of tilapia.