Effect of age on the occurrence of muscle fibre degeneration associated with myopathies in broiler chickens submitted to feed restriction

To evaluate how the age affected the occurrence of muscle fibre degeneration (MFD) associated with white striping and wooden breast, Pectoralis major muscles of 192 broiler chickens differing for genotype (standard vs. high breast yield), gender, and feeding regime (ad libitum vs. restricted rate, 80% from 13 to 21 d of age) were sampled at different ages (14, 21, 28, 35 and 46 d) for histological analyses and using H&E staining to evaluate tissue general morphology, Masson's Trichrome to identify collagen presence, Oil red and Nile blue for lipid presence. The effect of age, genotype, gender and feeding regime on the frequency of chickens showing MFD was evaluated by PROC CATMOD of SAS. Thereafter, significant differences according to the feeding system within age were assessed by χ2 test. On average (data of all slaughters), nor genotype (69.8% vs. 67.7% in standard vs. high breast yield) or gender (68.8% in males and females) affected the frequency of chickens with MFD. Differently, this latter frequency was higher with ad libitum feeding than early restriction (75.0% vs. 62.5% ; P=0.01) and increased with age (18.8% , 28.1% , 75.1% , 96.9%, and 96.9% at 14, 21, 28, 35, and 46 d). In details, at 14 d a similar frequency was found (18.8%) in all broilers; at 21 d, at the end of the restriction, MFD occurred more in broilers fed ad libitum than in those under restriction (50.0% vs. 6.3%; P<0.01); at 28 d a similar trend was observed but differences between the two groups were reduced (87.5% vs. 62.5%; P=0.10) to disappear by the 35th (100% and 93.8%) and 46th d (96.9% and 96.9%). In conclusion, MFD soon occurred in broilers after two weeks of growth and increased dramatically within 28 d of age. Feed restriction was effective in controlling and reducing MFD occurrence only as long as animals were under restriction, but no residual positive effect was recorded after their re-alimentation.