الفهرس 
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Abstract
This experiment was carried out at Sakha Animal Production Research Station, Animal Production Research Institute, Ministry of Agriculture, Egypt. The chemical analysis was carried out at Laboratories of the Animal Production Research Institute, Ministry of Agriculture, Egypt. The aim of the present study was to investigate the possibility of improving phytase phosphorus utilization by adding microbial phytase and/or citric acid to laying hen diets to decrease dietary level of phosphorus, phosphorus excretion leading to reduce the environmental pollution Twelve experimental diets were formulated during the laying period using three levels of NPP phosphorus (0.40, 0.30 and 0.20%), two levels of microbial phytase (0 and 500 FTU/kg) and two levels of citric acid (0 and 2%). The birds of the control group (T1) were fed a diet containing 0.40% NPP, while T2 contain 0.30% NPP and T3 contain 0.20% NPP. The birds of T4 to T6 were fed diets having the same NPP levels of T1 to T3 plus 500 FTU/kg diet while the birds of T7 to T9 were fed diets containing similar NPP levels to those fed T1 to T3 plus 2% citric acid, and the birds of T10 to T12 were fed diets containing similar NPP levels to those fed T1 to T3 plus 500 FTU/kg phytase and 2% citric acid A total number of 360 28-weeks-old Dokki-4 laying hens plus 72 Dokki-4 cocks of the same age were used in this study. The following measurements were recorded: live body weight, feed intake, egg quality, nutrients digestibility and some blood parameters, certain semen quality traits, minerals retention and reproductive performance.