Rethinking the maximum inclusion levels of exogenous phytase for enhanced turkey production performance

This study evaluated the effects of graded dietary phytase (PHY) inclusion on feed intake (FI), weight gain (WG), feed conversion ratio (FCR), nitrogen-corrected apparent metabolisable energy (AMEn), nutrient and mineral utilisation, and phytate degradation in growing turkeys. A positive control (PC) diet was formulated to meet or exceed breeder recommendations for nutrient content. A negative control (NC) diet was reformulated from the PC to contain the same calcium (Ca) to P ratio, but a reduced total P. The NC diet was subdivided and supplemented with PHY at 0, 100, 1000, 10,000, or 100,000 FTU/kg, resulting in six experimental diets. Seventy-two female BUT Premium turkey poults (mean body weight 1,885 g; SD ± 210 g) were randomly allocated to 36 raised-floor pens (two birds per pen; 0.36 m² per pen). Each diet was fed for 9 days, from 68 to 77 days of age to six replicate pens following randomisation. Birds fed the NC diet exhibited significantly reduced FI, WG, dry matter retention (DMR), and AMEn compared with birds fed the PC diet, confirming the adverse effects of marginal phosphorus supply. Supplementation with PHY at inclusion levels ≥1000 FTU/kg largely alleviated these deficiencies. Increasing dietary PHY concentration resulted in significant positive dose-dependent responses in WG, FCR, DMR, nitrogen retention, and AMEn. Marked reductions (p < 0.001) in excreta inositol phosphates (IP) were observed at PHY doses ≥1000 FTU/kg. Concentrations of IP and free inositol (IN) followed expected responses to increasing PHY dose, involving linear and quadratic components. Mineral retention coefficients were generally lower in turkeys fed the NC diet than in those fed the PC diet, except for phosphorus retention, which did not differ between control diets. Increasing PHY dose produced significant positive linear responses (p < 0.001) in the retention of calcium, magnesium, iron, potassium, manganese, zinc, and phosphorus, particularly at ≥1000 FTU/kg, Overall, the results confirm that turkeys respond to high PHY inclusion in a manner broadly consistent with broilers. Very high PHY levels (super- and mega-doses) provided continued additional benefits for growth performance beyond 1000 FTU/kg, along with continued improvements in nutrient retention, and the data suggest the optimum dose was beyond that investigated in this study.