Phytate-bound phosphorus (phytate-P) is a major anti-nutrient in plant-based aqua feeds due to its negative effect on fish growth and nutrient utilisation. Phytases (PHY) are enzymes that remove P groups from the phytate molecule, thereby making phytate-P more available to the animal and reducing the anti-nutritive effects of phytate. Applying higher PHY doses can result in the complete dephosphorylation of phytate, with the aid of intestinal alkaline phosphatase to remove the final P group, and the release of myo-inositol (MYO). Classified as a vitamin-like nutrient, MYO is often added as a dietary supplement to support fish growth. Therefore, the objectives of the current study were to determine whether high PHY doses can provide sufficient MYO and P to support growth and nutrient utilisation in tilapia.
A total of 360 male tilapia (Oreochromis niloticus; average weight 28g) were allocated to one of 4 diets, with 3 replicate 100L tanks per diet and 30 fish per tank. Water temperature was set at 28±1°C. Low fishmeal (5%) diets were fed to satiation (3 to 5 meals a day) over a 42-day period. Treatments were arranged in a 2 x 2 factorial, including a control diet (total P, 0.79%; phytate-P, 0.30%) and the same diet with 400 mg/kg of MYO, 5000 FTU/kg of PHY (Quantum Blue) or a combination of 400 mg/kg of MYO + 5000 FTU/kg of PHY. Data was subjected to ANOVA using the fit model platform in JMP Pro 13.0, with treatment means separated using Student’s T-test. Survival was analysed using a non-parametric Wilcoxon signed-rank test. Significance accepted at P≤0.05.
Feeding phytase at 5000 FTU/kg more than doubled (P<0.001) daily weight gain of tilapia and reduced (P<0.001) FCR by 31% (Table 1). Supplementation of MYO had no effect on performance parameters.
Inositol content in the muscle was significantly influenced by a QB x MYO interaction (P<0.006). Higher levels of muscle MYO in PHY diets suggest that complete dephosphorylation of dietary phytate led to the release of bioavailable MYO. Feeding MYO directly had no positive effect on muscle MYO content.
In summary, only PHY improved growth and feed efficiency of tilapia, as well as muscle MYO content. These findings suggest PHY supplementation can improve availability MYO as well as P from phytate.