THE EFFICACY OF PHYTASE IN CORN-SOYBEAN MEAL-BASED DIETS FOR LAYING HENS

Microbial phytase hydrolyzes poorly degradable vegetable phytate P in the gastrointestinal tract of poultry; thereby increasing the availabi lity of organic P to an extent that remains to be established. For thi s purpose, the P equivalency value of phytase in corn-soybean meal lay er diets was assessed in three experiments (two short-term absorption studies and one performance trial lasting a complete production period ). In the first absorption study, two basal diets containing 30 or 40 g Ca/kg diet were supplemented with either phytase [0, 250, or 500 phy tase units (FTU)/kg diet] or with monocalcium phosphate (MCP; 0, 0.5, or 1.0 g P/kg diet) and fed to layers from 20 to 24 wk of age. The ile al absorption of Ca and P was measured during the last week. It was sh own that 250 FTU/kg diet hydrolyzed an amount of phytate P that was eq uivalent to 1.3 g; P from MCP. At the highest phytase inclusion level (500 FTU/kg diet), a lower value of equivalency was observed, as P abs orption was almost maximal at the lower level of phytase inclusion (25 0 FTU/kg diet). Phytase hydrolyzed phytate-bound P effectively at both Ca levels, although this degradation was significantly reduced by 12 percentage units at the higher dietary Ca level. The second absorption study, used 0, 250, and 500 FTU phytase/kg diet and 0 and 1.0 g P/kg diet of MCP. All diets were standardized at 35 g Ca/kg diet. The ileal absorption of Ca and P was determined at 24 and 36 wk of age. These v alues were significantly reduced in 36-wk-old hens compared to 24-wk-o ld hens. At 24 wk of age, phytic acid P degradation was significantly improved with increasing levels of phytase up to the maximum inclusion level of 500 FTU/kg diet (maximum phytic acid-P degradation at the en d of the small intestine was 66%). In this experiment, the dose of 250 FTU/kg diet was equivalent to 0.8 g MCP-P.In Experiment 3, either phy tase or MCP-P was added to a corn-soybean meal layer diet, containing 40 g Ca/kg diet and 3.6 g P/kg diet, at levels of 0, 100, 200, and 300 FTU/kg or levels of 0, 0.3, 0.6, and 0.9 g MCP-P/kg, respectively. Pr oduction performance was measured from 18 to 68 wk of age. Diets were consumed acl libitum. Growth, production performances (except kilogram s of feed per kilogram of egg), and tibia parameters were significantl y improved by dietary supplementation of the negative control diet wit h either phytase or MCP-P. Growth, egg production, and feed conversion ratio of the hens from the supplemented groups remained good througho ut the experiment. No phytase dose effects on the production character istics or tibia parameters were observed, indicating that the P requir ements of the laying hens were met throughout the production period ev en at the lowest level of supplementation.