ABSTRACT: In 2 experiments, a total of 184 pigs (PIC, initial BW of 10.3 and 9.7 kg for Exp. 1 and 2, respectively) were used to develop an available P (aP) release curve for commercially available Escherichia co- li-derived phytases. In both experiments, pigs were fed a corn-soybean meal basal diet (0.06% aP) and 2 diets with added inorganic P (iP) from monocalcium phos- phate (Exp. 1: 0.075 and 0.15% aP; Exp. 2: 0.07 and 0.14% aP) to develop a standard curve. In Exp. 1, 100, 175, 250, or 500 phytase units (FTU)/kg of OptiPhos 2000 or 200, 350, 500, or 1,000 FTU/kg of Phyzyme XP were added to the basal diet. In Exp. 2, 250, 500, 750, or 1,000 FTU/kg of OptiPhos 2000; 500, 1,000, or 1,500 FTU/kg of Phyzyme XP; or 1,850 or 3,700 FTU/ kg of Ronozyme P were added to the basal diet. One FTU was defined as the amount of enzyme required to release 1 μmol of iP per minute from sodium phytate at 37°C. For all phytase products, the manufacturer-guar- anteed phytase activities were used in diet formulation. All diets were analyzed for phytase activity using both the Phytex and AOAC methods. Pigs were blocked by sex and BW and allotted to individual pens with 8 pens per treatment. Pigs were killed on d 21, and fibu- las were collected and analyzed for bone ash. In both experiments, increasing iP improved (linear, P < 0.01) G:F and percentage bone ash. Pigs fed increasing Op- tiPhos had improved (Exp. 1: linear, P < 0.001; Exp. 2: quadratic, P < 0.001) percentage bone ash, as did pigs fed increasing Phyzyme XP (linear, P < 0.001). In Exp. 2, increasing Ronozyme P improved (quadratic, P < 0.01) percentage bone ash. Using analyzed values from the AOAC method and percentage bone ash as the re- sponse variable, an aP release curve was developed for up to 1,000 FTU/kg of E. coli-derived phytases (Op- tiPhos 2000 and Phyzyme XP) in P-deficient diets. The prediction equation was Y = –0.000000125X 2 + 0.000236X + 0.016, where Y = aP release (%) and X = analyzed phytase (FTU/kg) in the diet. Key words: growth, nursery pig, phosphorus, phytase ©2010 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2010. 88:3631–3644 doi:10.2527/jas.2010-2936 INTRODUCTION The development of exogenous phytase (i.e., phos- phatase enzymes that cleave P moieties from phytate) is one of the most important discoveries in animal nu- trition (Cromwell, 2009). In swine diets, the effect of phytase in improving P digestibility and reducing P excretion is well established (Jongbloed, 1987; Angel et al., 2005). Phytase use is currently an integral part of achieving environmental sustainability and providing a more economical P source in swine and poultry diets (Selle and Ravindran, 2007). Several phytase products currently are commercially available; however, there is confusion regarding how to effectively compare these different sources. First, there is wide variation in recommended dosages for similar P release among different phytase sources. For instance, 1,850 phytase units (FTU)/kg of Peniophora lycii phytase (Ronozyme P, DSM Nutritional Products, Basel, Switzerland) is recommended to replace 0.10% available P (aP). Augspurger et al. (2004) demonstrat- ed that 0.13% aP can be replaced in swine diets with 250 FTU/kg of OptiPhos (Phytex LLC, Sheridan, IN), an Escherichia coli-derived phytase. But 500 FTU/kg Efficacy of different commercial phytase enzymes and development of an available phosphorus release curve for Escherichia coli-derived phytases in nursery pigs 1,2 C. K. Jones,* M. D. Tokach,* S. S. Dritz,† B. W. Ratliff,‡ N. L. Horn,§ R. D. Goodband,* 3 J. M. DeRouchey,* R. C. Sulabo,* and J. L. Nelssen* *Department of Animal Sciences and Industry, College of Agriculture, and †Food Animal Health and Management Center, College of Veterinary Medicine, Kansas State University, Manhattan 66506; ‡Enzyvia LLC, Sheridan, IN 46069; and §JBS United Inc., Sheridan, IN 46069 1 Contribution No. 10-201-J from the Kansas Agric. Exp. Stn., Manhattan. 2 Appreciation is expressed to JBS United Inc. (Sheridan, IN) and Enzyvia LLC (Sheridan, IN) for financial support. 3 Corresponding author: goodband@ksu.edu Received February 23, 2010. Accepted May 25, 2010. 3631 Published December 4, 2014