| Summary: | The impact of dietary regimens on productive traits, meat quality, physiological adoption, immunological response and digestibility of nutrients was studied using Ross-308 and Cobb-500 strains. Chickens were reared under normal brooding conditions during most of the experimental period, but exposed to heat stress [(HS); 32 °C and 55% RH] during 20–22 and 28–30 d of age. During 1–18 d of age, broilers were fed a standard-protein diet [(SPD); 22% crude protein (CP) with 12.97 MJ/kg], a high-protein diet [(HPD); 24% CP with 12.97 MJ/kg], or a high-protein with high-metabolisable energy (ME) diet [(HPMED); 24% CP with 13.60 MJ/kg]. During 19–35 d of age, the SPD group was fed 20% CP with 13.42 MJ/kg, the HPD group was fed 22% CP with 13.42 MJ/kg, and the HPMED group was fed 22% CP with 14.06 MJ/kg. Each group within each strain was replicated six times with five chicken males each. For the whole experimental period, Ross and Cobb chickens fed the HPMED showed higher BWG than chickens on the HPD. Broilers fed the HPMED had improved FCR compared to those on the SPD, independent of broiler strain. Further, independent of broiler strain, the HPMED regimen increased the dry matter, protein, and lipid contents of meat compared to the other regimens, and decreased cloacal temperature, respiration rate and Heterophile/lymphocyte (H/L ratio) compared to the HPD regimen. Growth and feed utilisation, percentage thymus, protein in meat were improved of Ross compared to Cobb, but abdominal fat, meat lipid and pH were decreased. On the other hand, Cobb on HPD had lower cloacal temperature and respiration rate than Ross strain, suggesting that production, physiology and immunological response of broiler chickens depend on feeding regimen that should be specific for each strain.
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