Effects of two protein levels on the performance of chicken males with different growth intensities

At present, genetic selection programs produce chicken genotypes with different growth intensities, which might have variable requirements for dietary protein. The objective of this study was to compare the response of three different genotypes to two levels of crude protein in feed mixtures. Cockerels of fast-growing Ross 308, medium-growing Hubbard JA 757 and slow-growing ISA Dual chickens were used in the study. Each genotype was fed diets that differed in protein level: the control group (C) received commercial feed, and the experimental group was fed a diet with a 6% lower protein content (LP). The daily weight gain (DWG) and feed conversion ratio (FCR) were significantly affected by the interaction of genotype and feed protein level. A greater percentage of DWG depression was observed in fast-growing cockerels than in medium-growing cockerels (10% and 6%, respectively), whereas the percentage of slow-growing cockerels negligibly increased (2%). A low-protein diet impaired the FCR only in fast-growing birds (-5%), whereas in the medium- (-2%) and slow-growing groups (+2%), the differences were not significant. Carcass composition significantly influenced only genotype and thigh meat pH. In terms of meat colour, significant interactions revealed that in fast-growing Ross 308 chickens, redness and yellowness did not differ according to diet group; however, in both genotypes with slower growth, significantly greater redness and yellowness were detected in the low-protein diet group than in the control group. The results indicate that genotypes with slower growth have lower protein requirements for growth performance, but lower diet protein has an effect on physical meat quality parameters in these genotypes.