The phytase analytical activity of pelleted diets may not adequately describe efficacy in the bird

Abstract

Most commercial broilers are fed pelleted diets containing an exogenous phytase. Various techniques exist to prevent denaturation of phytase during pelleting, but the potential remains for phytase to lose efficacy in liberating phytate-bound phosphorus. Dietary treatments consisted of positive control and negative control diets differing in nonphytate phosphorus by 0.13%, as well as the negative control diets containing 1 of 9 Escherichia coli-derived mixer-added heat-stable phytases. Diets were steam conditioned at 71°C (160°F), 77°C (170°F), and 82°C (180°F) before pelleting. Feed samples were analyzed for phytase pre- and postpelleting by a commercial laboratory to determine retained activity. Pellets conditioned at 77°C were chosen for further evaluation via a feeding experiment because all 9 enzymes demonstrated high retained activity in vitro (≥60%). Diets were fed to Cobb 500 male broilers from 3 to 21 d. Feed intake, FCR, BW gain, tibia breaking strength, and tibia bone ash weight per chick were calculated. Seven out of 9 enzymes resulted in BW gain similar to that of the positive control diet. Five of those enzymes resulted in decreased FCR by an average of 8 points compared with the other 2. Tibia measures demonstrated additional differences among enzyme treatments. Phytase activity did not positively correlate with performance or bone mineralization measures. Therefore, evaluations of phytase may need to include in vivo bird testing beyond in vitro activity assays to fully assess efficacy. It is important to note that our conclusions in this study were derived using specific phytase products that were analyzed for activity by using specific protocols, and diets were pelleted within a selected range of processing conditions (i.e., 77°C and 10 s of conditioning); using alternative methodologies may result in different conclusions. ©2012 Poultry Science Association, Inc.