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The ontogeny and secretory pattern of plasma insulin-like growth factor-I (IGF-I) in relation to GH secretion were studied in meat-type (broiler) poultry during pre-pubertal and post-pubertal growth. Male and female broiler chickens of two commercial strains (strains A and B) were reared from 1 to 198 days of age. From 1 to 49 days of age birds were reared in raised-wire cages and thereafter in deep-litter pens, with food available ad libitum. Blood samples were taken at regular intervals during growth, and at 29 and 43 days of age representative birds were cannulated and serial blood samples taken at 10-min intervals for 5 to 7 h. Plasma concentrations of GH and IGF-I were measured by radioimmunoassay. Birds of strain A were heavier (P less than 0.05) than those of strain B from 12 to 35 days of age. In general, male birds were heavier (P less than 0.01) than females from 12 to 35 days of age. Plasma GH concentrations were significantly higher (P less than 0.05) from 12 to 35 days of age, while plasma IGF-I concentrations were lower (P less than 0.05) from 6 to 21 days of age in male compared with female birds. Plasma IGF-I concentration increased with age, reaching a plateau at 28 days of age, while plasma GH concentration declined over the same period. Plasma IGF-I concentrations declined in a linear manner from 49 to 198 days of age, and there was no evidence of a pubertal increase.(ABSTRACT TRUNCATED AT 250 WORDS)
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