THE ROLE OF PLASMA GROWTH-HORMONE, PROLACTIN, TRIIODOTHYRONINE AND TETRAIODOTHYRONINE IN THE REGULATION OF GROWTH AND SEX-DIFFERENCES IN BODY-WEIGHT OF TURKEYS

An attempt was made to alter male:female (M:F) body weight ratios in t wo strains (S12 and S72) of turkeys by selective matings; to study the ir post-hatching growth patterns and to relate growth patterns to plas ma growth hormone (GH), prolactin (Prl), tetraiodothyronine (T-4), and triiodothyronine (T-3) concentrations. Selection for high and low M:F body weight ratio (HR and LR) was essentially ineffective in both str ains. Sex differences in body weight were first significant in S12 at day of age and in S72 at 6 wk. Plasma GH and Prl rose during the first 2 wk of age and fell thereafter while T-3 levels decreased and T-4 le vels increased over time. Differences in plasma hormones between lines and sexes within strains were noted, but showed no consistent pattern s. Correlations between GH and growth were mostly negative. When signi ficant, regressions between body weight and GH or gain and GH were gen erally negative. Significant correlations between Prl and body weight in males were negative whereas they were of mixed sign in females. Cor relations between T-3 levels from 2 to 8 wk of age and body weight ove r this same period were mostly positive in males whereas those at late r ages and in females were mostly negative. Few significant correlatio ns between T-4 and weight were found. These findings suggest that the regulation of growth in turkeys is quite different than that of mammal s and that hormones known to regulate growth in same species may not d o so in others.