Relationship of plasma growth hormone to growth within and between turkey lines selected for differential growth rates.

The ontogeny of plasma growth hormone (GH) was determined within and between genetically-related turkey lines selected for differential growth rates. Blood samples were obtained at weekly intervals from hatching through 195 days of age from male and female poults of a slow-growing, randombred line (RBC-2), representing the base population, and a fast-growing line selected over 20 generations from RBC-2 for increased 16-wk body weight (F). Growth rate of the F line exceeded that of RBC-2 during the very early (0 to 14 day) posthatch period (P less than .05), but was not different from that of the RBC-2 line by 77 to 195 days of age. Males of both lines were larger than females at all ages (P less than .05). Plasma GH increased from hatching until 2 wk of age and declined thereafter in both lines. Differences between overall plasma GH concentrations of the lines were not observed during the 0 to 14 day period, but concentrations were higher (P less than .05) in RBC-2 than F birds thereafter (21 to 195 days of age). Plasma GH concentrations initially (0 to 14 days) were higher in females than in males for both lines (P less than .05), but thereafter (21 to 195 days) declined more rapidly and averaged lower overall in females than in males (P less than .05). Correlations between measures of GH and growth rate within lines over age were strongly positive (r = .83 to .93; P less than .001).(ABSTRACT TRUNCATED AT 250 WORDS)