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Background  The growth rate often varies among individual broilers of the same breed under a common management condition. To investigate whether a variation in the growth rate is associated with a difference in hormone levels and myogenic gene expression profile in broilers, a feeding trial was conducted with 10,000 newly hatched Ross 308 chicks in a commercial production facility under standard management. At 38 d of age, 30 fast-, 30 medium-, and 30 slow-growing broilers were selected among 600 healthy male individuals. The levels of insulin-like growth factor-1 (IGF-1), triiodothyronine (T3), thyroxine (T4), and growth hormone in the serum or breast muscle were assayed by ELISA or RIA kits, and the expression levels of several representative pro- and anti-myogenic genes in the breast muscle were also measured by real-time PCR.    Results  Results showed that both absolute and relative weights of the breast muscle were in linear positive correlations with the body weight of broilers ( P  < 0.001). Fast-growing broilers had higher concentrations of IGF-1 than slow-growing broilers ( P  < 0.05) in both the serum and breast muscle. The serum concentration of T3 was significantly higher in fast-growing birds than in slow-growing birds ( P  < 0.05). However, no difference was observed in growth hormone or T4 concentration among three groups of birds. Additionally, a decreased expression of an anti-myogenic gene ( myostatin ) and increased expressions of pro-myogenic genes such as  myogenic differentiation factor 1, myogenin, muscle regulatory factor 4, myogenic factor 5, IGF-1 , and  myocyte enhancer factor 2B, C,  and  D  were observed in fast-growing broilers ( P  < 0.05), relative to slow-growing broilers.    Conclusions  Collectively, these findings suggested that the growth rate is linked to the hormone and myogenic gene expression levels in broiler chickens. Some of these parameters such as serum concentrations of IGF-1 and T3 could be employed to breed for enhanced growth.

Association of growth rate with hormone levels and myogenic gene expression profile in broilers