POSTHATCHING GROWTH AND PECTORALIS-MUSCLE DEVELOPMENT IN BROILER STRAIN CHICKENS, BANTAM CHICKENS AND THE RECIPROCAL CROSSES BETWEEN THEM

Body weight, pectoralis muscle weight, pectoralis protein and DNA conc entration, and plasma GH and IGF-I concentrations of broiler chicks (B rBr), bantam chicks (BaBa) and reciprocal crosses between them (BaBr a nd BrBa) were measured between 0 and 42 days after hatching. At hatch, body weight and pectoralis weight of the two types of chicks from bro iler eggs (BrBr and BaBr) were equal to each other but greater than th e two types of chicks from bantam eggs (BaBa and BrBa), which were not different from each other. BrBr chicks grew more rapidly than crossbr eds and BaBa chicks grew more slowly. Weights of the reciprocal crosse s (BaBr and BrBa) were markedly different at day of age, but converged by day 14. The increase in pectoralis muscle mass of BrBr chicks exce eded that of the reciprocal crosses which in turn exceeded that of BaB a chicks. The increase in pectoralis DNA content and protein content f ollowed the same pattern. The DNA unit size, as expressed by the prote in:DNA ratio, was markedly lower in pure bantam chicks from 14 to 42 d ays of ages, whereas the unit size did not differ between the intermed iate sized reciprocal crosses and the large bodied broiler chicks. Dif ferences in muscle mass were primarily achieved by differences in the number of DNA units although a difference in unit size was also a fact or. There were no clear relationships between growth and plasma growth hormone or insulin-like growth factor I concentrations. Thus while sa tellite cell proliferation is primarily responsible for genotypic diff erences in muscle mass, the plasma growth hormone-IGF-I axis does not appear to be regulating their proliferation.