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 two strains (S12 and S72) of turkeys by selective matings; to study their post-hatching growth patterns and to relate growth patterns to plasma growth hormone (GH), prolactin (Prl), tetraiodothyronine (T4), and triiodothyronine (T3) concentrations. Selection for high and low M:F body weight ratio (HR and LR) was essentially ineffective in both strains. 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 T3 levels decreased and T4 levels increased over time. Differences in plasma hormones between lines and sexes within strains were noted, but showed no consistent patterns. Correlations between GH and growth were mostly negative. When significant, regressions between body weight and GH or gain and GH were generally negative. Significant correlations between Prl and body weight in males were negative whereas they were of mixed sign in females. Correlations between T3 levels from 2 to 8 wk of age and body weight over this same period were mostly positive in males whereas those at later ages and in females were mostly negative. Few significant correlations between T4 and weight were found. These findings suggest that the regulation of growth in turkeys is quite different than that of mammals and that hormones known to regulate growth in some species may not do so in others.

Turkey muscle growth and focal myopathy.

Muscle structure and blood enzyme activity were studied to 16 wk of age in lines of turkeys selected for rapid growth. The body and carcass weights were measured, frozen sections of breast and leg muscles examined, and plasma creatine kinase (CK) levels determined. Muscle weights were usually proportional to BW except for the relatively larger superficial pectoralis (SP) muscles in the most rapidly growing line. Damaged muscle fibers were found in all muscles examined, especially in the SP of the breast, the gastrocnemius (GA), and other muscles of the leg; these damages became more common from 10 to 16 wk of age. There were more degenerating muscle fibers and higher levels of plasma CK in the rapidly growing lines than in a slower growing unselected line. The findings support the idea that a focal myopathy, unrelated to deep pectoral myopathy or to inherited muscular dystrophy of the chicken, is associated with rapid growth of turkeys.

Genetic effects on embryonic growth in domestic turkeys.

Embryo growth rates for two strain crosses of domestic turkeys were compared. Female parents of both crosses came from the same flock of Nicholas parent stock hens. The strain providing sires for Cross L was approximately twice the adult size of the strain providing sires for Cross S. Egg weight was correlated with late embryo weight (starting by 20 days of incubation), poult weight at hatch, and carcass weight (i.e., yolk sac removed) at hatch. When analyses of covariance were done to remove the effects of egg weight, no significant cross differences were found between embryo weights at 8, 12, 16, 20, and 24 days of incubation, poult weight at hatch, and carcass weight. No significant sex differences were found between poult weights or carcass weights at hatch.