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

Atpresent, genetic selection programs produce chicken genotypes with different growth intensities, which might have variable requirements for dietary protein. The objective ofthis study was tocompare the response ofthree different genotypes totwo levels ofcrude protein infeed mixtures. Cockerels offast-growing Ross 308, medium-growing Hubbard JA 757 and slow-growing ISA Dual chickens were used inthe study. Each genotype was fed diets that differed inprotein level: the control group (C) received commercial feed, and the experimental group was fed adiet with a6% lower protein content (LP). The daily weight gain (DWG) and feed conversion ratio (FCR) were significantly affected bythe interaction ofgenotype and feed protein level. Agreater percentage ofDWG depression was observed infast-growing cockerels than inmedium-growing cockerels (10% and 6%, respectively), whereas the percentage ofslow-growing cockerels negligibly increased (2%). Alow-protein diet impaired the FCR only infast-growing birds (–5%), whereas inthe medium- (–2%) and slow-growing groups (+2%), the differences were not significant. Carcass composition significantly influenced only genotype and thigh meatpH. Interms ofmeat colour, significant interactions revealed that infast-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.