الفهرس 
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Abstract
The present study was carried out at a private farm in Dakhlia governorate for 4 consecutive months (from December 2011 to March 2012). Chemical analyses were performed at Poultry Department, Faculty of Agriculture, Zagazig University, Zagazig, Egypt. The current work aimed to study the effect of dietary calcium level (3.0 and 3.6%), available phosphorus (0.25, 0.35 and 0.45%) and microbial phytase supplementation (0 and 300 FTU/Kg diet) on the performance and economic evaluation of laying hens during 20 to 36 weeks of age. Results obtained could be summarized as follow:1- Final body weight, egg number, egg weight, egg mass and feed conversion were not significantly affected due to the different dietary calcium levels in laying hen diets.2- Increasing dietary calcium level from 3.0 to 3.6% had significantly (P≤0.05) decreased feed consumption only during 20 to 24 week of age.3- The interactions among calcium, phosphorus and phytase levels had insignificant effects on live body weight at 36 weeks of age and body weight change from 20 to 36 weeks of age.4- Internal egg quality traits were not significantly influenced at 24, 30 and 36 weeks of age by dietary calcium, available phosphorus levels or by microbial phytase supplementation.5- Increasing dietary available phosphorus level from 0.25 to 0.35% significantly (P≤0.05) reduced shell thickness at 36 weeks of age while, increasing available phosphorus level from 0.35 to 0.45 % had no significant effect on egg shell thickness.