الفهرس 
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Abstract
This experiment was designed to evaluate the productive performance and immunocompetence parameters of two commercial Layer strains (Hy-Line W-36 and Hy-Line Brown) under Egyptian environmental conditions. Measurements were recorded for each on day-old chicks (produced at 30, 35, 40, 45 and 50 weeks of age for the breeders), commercial laying flocks (at 30, 40 and 50 weeks of age) and Layers breeders (at 68 weeks of age).
The present results indicated that the brown breeder hens produced significantly heavier chick weight compared to the white ones. Day old brown chicks had significantly higher plasma total protein and globulin compared to the white ones. Whereas, no significant difference between strains for plasma albumen was observed. Plasma protein was significantly affected by breeder age. No significant difference between strains for A/G ratio. The highest value of A/G ratio recorded at 30 and 50 wks of breeder’s age. Lower antibody titers to Newcastle Disease Virus (NDV) were recorded for brown chicks compared to the white ones. Conversely, brown chicks had significantly higher antibody titer for Infectious Bursal Disease Virus (IBDV) compared to the white ones. Also, Antibody titers against NDV and IBDV of brown and white chicks were significantly affected by breeder age. White chicks had significantly higher absolute and relative bursa and thymus weight compared to brown ones. Strain and breeder age had significant effect on residual of yolk sac percentage. It could be noticed that lower residual of yolk sac percentage was recorded for brown chicks compared to the white ones. Consequently, the brown chicks had better absorption of yolk sac than white chicks.
Results demonstrated that brown strain had significantly heavier body weight compared to the white ones. The brown layers significantly consumed more feed compared to the white ones. W-36 layers strain had better feed conversion ratio than that of brown layers strain. Concerning age effect, feed conversion ratio increases as advancing of layers age. Consequently, feed efficiency decreased. Data indicated that the strain had insignificant effect on egg number; conversely, layer age had significant effect on same trait. Egg weight and egg mass of brown eggs were heavier than white eggs. The results confirmed that the brown eggs recorded significantly higher in eggshell quality traits compared to the white eggs. With respect to immune response, cell-mediated immunity to PHA-P injection in toe-web was significantly higher in brown strain than that of white counterparts at all investigated times. The brown layers strain had significantly higher total anti-SRBCs antibody titer at 7 days post secondary SRBC-injection compared to white ones.
Brown layer breeder strain had significantly higher body weight and body measurements compared to the white ones. The brown layer breeder strain had significantly higher plasma total protein and albumen compared to the white ones. Concerning plasma globulin, no significant difference between strains for this parameter was observed.
In conclusion, brown Hy-Line strain had better productive performance and immunocompetence than that of W-36 Hy-Line strain under Egyptian environmental conditions.