Selection for fast-growing and increasing breast meat yield may negatively impact muscle structure and meat quality traits. In the last years, two emerging meat defects have been arisen in breast fillets of fast-growing chicken hybrids: white striping characterized by white parallel striations in the direction of the muscle fibers and poor cohesion of muscle structure with fibers bundles which can be easily pulled away by fingers. A trial was conducted to compare productive performances, cut up yields and incidence of white striping and poor cohesion (absence=normal, presence classified in 2 levels as moderate and severe) in four of the most representative commercial chicken hybrids which were classified as either standard (SBY1 and SBY2) or high (HBY1 and HBY2) breast-yield. A total of 2,160 1-day-old male chicks were equally divided into four experimental groups of 9 replications each, raised in the same experimental conditions and fed the same commercial diets. Birds were slaughtered at about 3.6 kg live weight reached at 48 (SBY1 and HBY2) and 51 d of age (SBY2 and HBY1), respectively. Both HBY1 and HBY2 had a lower daily feed intake if compared with SBY1 and SBY2 (120 and 124 vs 131 and 130 g/day, P<0.01). On the other hand, feed conversion rate was not significantly modified. As expected, HBY1 and HBY2 had higher breast yields than both SBY birds (30.9 and 31.5 vs 28.6 and 28.5%). The total occurrence (moderate and severe) of white striping in all the genotypes was remarkably high (>87%). However, HBY birds had an incidence of about 96% white striped fillets, while its occurrence in SBY groups did not exceed 90%. The overall incidence (moderate and severe) of muscle structure with poor cohesion also resulted extremely high (>90%). Indeed, only 10% of breast fillets from SBY1 birds resulted unaffected by this defect, while 40% of breast fillets from HBY2 showed poor cohesion with severe degree. In conclusion, from this study emerged that all the modern chicken hybrids here tested are affected by a high incidence of breast muscle defects which are particularly pronounced in high breast-yield birds
