Growth-associated traits in parental and F1 populations of chickens under different feeding programs. 4. Growth and thyroid hormones.

Plasma levels of growth hormone (GH), triiodotyronine (T3), and thyroxine (T4) were compared at 25 and 61 days of age in parental lines and F1 crosses between parental lines under ad libitum and alternate day feeding. Parental lines included White Rocks selected for high and low body weight and a White Leghorn population. Differences among populations for hormone levels were more pronounced at 61 than at 25 d of age. Growth hormone decreased and T4 increased with age in all populations except the line selected for low body weight, whereas the pattern for T3 varied with genetic stock. On days when chicks were not fed, plasma GH and T4 increased and T3 decreased. On days of feeding, GH and T4 returned to the level of chicks fed ad libitum while T3 was either identical to or higher than that for the ad libitum chicks. Population by feeding regimen interactions were not significant at either age for any of the hormones measured showing that responses to the feeding regimens were similar for all populations.