Phytases have been employed in animal feeds for several years, used primarily for reducing the environmental impact and phosphorus loads of farm effluents. 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). To determine nutrient retention, 30 fish were euthanized at the start of the feeding trial with 10 fish being pooled into one of 3 sample replicates. On termination of the growth period, 10 fish/tank were euthanized and samples pooled. Whole body homogenates and diet samples were analysed for dry matter, nitrogen (N), P, ash, lipid and gross energy. The nutrient retention in Tilapia over the 81-day feeding period is shown in Table 1. Retention of N was decreased with each nutrient downspec applied, even though the analysed protein downspec was similar in both NC1 and NC2 diets. This suggests that the greater P deficiency in NC2 diets was limiting protein utilization. Comparing the higher phytase levels of 2.500FTU/kg on top of the NC2 diet increased N retention to a level which was comparable to both the PC and NC1. Both phytases treatments gave significantly greater P retentions than all other treatments. Consequently, calculated P excretion was also lower in the phytase treatments compared to the controls.
In summary, by supplementing an appropriate level of phytase, the inclusion level of these nutrients can be reduced in diets, thereby creating diet cost savings, while preserving nutrient retention and having a positive impact on water pollution.