Evaluating Phosphorus Release by Phytase in Diets Fed to Growing Pigs That Are Not Deficient in Phosphorus

Microbial phytase is widely used to enhance digestibility of phytate-P. By tradition, diets with P content well below requirement are used to quantify phytate-P release by phytase, but P-adequate diets may be more physiologically relevant. The objective of this study was to investigate the effects of phytase on P digestion and metabolism and develop a P release curve for phytase in P-adequate diets (above requirement according to NRC, 2012), and to compare these effects in a P-deficient diet. Three replicates of 24 barrows each (BW = 23.0 ± 1.8 kg) were randomly assigned to 1 of 8 dietary treatments, housed in individual pens for 21 d, then moved to metabolism crates for 5 d urine and fecal collections. A basal corn-soybean meal diet (P-adequate, A) was formulated at 0.36% standardized total tract digestible (STTD) P and total Ca:STTD P of 1.83. Phytase was added to A at 200 (A200), 400 (A400), 600 (A600), and 800 (A800) phytase units (FTU)/kg. A positive control diet (PC) was formulated using monocalcium phosphate (MCP) to increase STTD P by 0.16% to 0.52%, the expected STTD P release of 800 FTU/kg. A P-deficient diet (D) was formulated by reducing MCP to achieve 0.21% STTD P, and 200 FTU phytase/kg was added to D for D200. Pig was the experimental unit, and replicate and dietary treatment were fixed effects. Orthogonal polynomial contrasts were used to test linear and quadratic effects of phytase within A, A200, A400, A600, and A800. Phytase increased percent apparent total tract digestibility (ATTD) and STTD of P (quadratic P < 0.001), and quantity of absorbed P (linear P < 0.001; quadratic P = 0.069). Urinary P increased linearly with phytase (P < 0.001) and retained P also increased (linear P = 0.001, quadratic P = 0.094). Phytate-P release was estimated to be 0.049, 0.080, 0.093, and 0.09% STTD P for 200, 400, 600, and 800 FTU/kg, respectively. It appears that the effect of phytase may be lower in P-adequate diets as compared to P-deficient diets, given that there was a 12% improvement for A200 versus A, and a 28% improvement in STTD P for D200 versus D. In conclusion, phytase improved P digestibility and retention in P-adequate diets, and P digestibility was used to estimate the quantity of P released by phytase. Further research investigating P release by phytase in P-adequate diets, rather than P-deficient diets, may be preferable.