LAUSR

The aim of the present study was to evaluate the effect of two dietary protein levels on performance, carcass characteristics, and meat quality parameters in fast(Ross 308), medium(JA757), and slow-growing (ISA Dual) chickens to assess the interaction of the two factors. Each genotype was divided into a control group fed a commercial type of feed mixture and an experimental group fed a low-protein diet (LP). The trial was terminated after a common period of fattening of each genotype, and 20 chickens per group (sex ratio 1 : 1) were selected for the carcass and meat analysis. The results indicated that the LP diet decreased growth (P < 0.001) and increased feed consumption (P < 0.001) more in the fast-growing than in the slow-growing genotypes; however, reduced mortality was detected in fast-growing chickens. The LP diet had a negative effect on the European performance efficiency factor (P < 0.001) in fast(–10%) and medium-growing (–6%) but not in slow-growing chickens. The main effect of the genotype on the carcass characteristics included the highest (P < 0.001) dressing out and breast percentage in fast-growing chickens and the highest (P < 0.001) percentage of thigh and abdominal fat in the slow-growing genotype. The LP diet had only a minor effect on the carcass traits. Regarding meat quality characteristics, slowgrowing chickens were characterized by higher contents of dry matter (P < 0.001) and crude protein (P < 0.001) and lower contents of ether extract (P < 0.001) and cholesterol (P < 0.001) compared to mediumand fast-growing chickens. The individual effects of the genotypes were manifested by the largest cross-sectional area of the muscle fibres of pectoralis major in fast-growing chickens (P < 0.001). The results of the present study indicate a significant interaction of the dietary protein levels and genotypes in growth performance and a negligible effect on the carcass composition and physical and chemical quality of meat.