The response of fast-, medium- and slow-growing chickens to a low protein diet

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, slow-growing 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 medium- and 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