Comparison of digestible and available phosphorus release values for a novel phytase determined with fecal phosphorus digestibility and bone mineralization in weaner pigs

The objective of this study was to compare digestible and available P release from phytase through measuring fecal P digestibility and bone mineralization. A total of 336 weaner pigs (7.4 +/- 0.9 kg) were used in a randomized complete block design. Three diets were established by including 2.5 (control), 6.0 or 9.5 g/kg monocalcium phosphate (MCP) to establish the standard response curves. The other 4 diets included a novel phytase at 500, 1000, 2000 or 3000 phytase units (FYT)/kg feed to the control. Each diet was fed to 6 pens of 8 pigs each for 21 days. All diets included 3 g/kg TiO2, and fecal samples were collected on d 13-15 of trial. At the end, one pig per pen was sacrificed for the collection of tibias. The results showed that increasing supplementation of MCP linearly (P < 0.001) improved growth performance (body weight gain, feed intake and gain:feed and bone mineralization (percent bone ash and weights of dry bone, bone ash, and bone Ca and P). These parameters responded to the supplementation of phytase both linearly and quadratically (P < 0.01). Supplemental phytase or MCP increased apparent total tract digestibility (ATTD) of Ca and P as well as the amount of ATTD P, but reduced the ATTD Ca-to-ATTD P ratio both linearly and quadratically (P < 0.01). Both MCP and phytase linearly (P < 0.05) increased percent bone ash P. The true total tract digestibility (TTTD) coefficient of P in MCP was estimated to be 0.94 by regressing the ATTD P against the dietary P concentration. The available P release increased from 1.58 to 2.21, 1.77-2.62, 1.17-1.82 and 1.25-1.97 g/kg feed when body weight gain, gain:feed, percent bone ash and bone P weight were used as the response variables, respectively, with increasing phytase dose from 500 to 3000 FYT/kg feed, whereas the digestible P release by phytase calculated from ATTD coefficient of P increased from 1.05 to 1.99 g/kg feed. By multiplying the TTTD coefficient of MCP (0.94), the available P release values based on bone P weight were transformed to their equivalent digestible values of 1.18-1.86 g/kg feed. In conclusion, the available P release from phytase depends on the response variables. Using the TTTD coefficient of P in MCP to convert the available P release values to digestible P values for phytase did not generate comparable results as measured from ATTD coefficient of P.