Allomorphic relationships from hatching to 56 days in parental lines and F1 crosses of chickens selected 27 generations for high or low body weight.

Allomorphic relationships in chickens selected for high or low juvenile body weight and their reciprocal crosses were examined from hatch to 56 days of age (doa). Those organs whose relationships to whole body weight were most divergent in the parental populations were weights of the gizzard, small intestine, breast, legs, feathers, abdominal fat and lengths of the shank, esophagus and small intestine. Relative weights of feathers, abdominal fat and small intestine were influenced mainly by additive genetic variation, whereas relative weights of breast, legs, and gizzard were influenced by additive genetic variation at younger ages (0 to 24 doa) and by both additive and non-additive genetic variation thereafter. In utilization of resources for high growth velocity and growth rate factor, crosses were 20 to 30% more efficient than their parental populations, demonstrating hybrid vigor. Relationships of rate of development of specific organs to total body weight varied with age and were noted as correlated responses to artificial selection for body weight. Selection for high and for low body weight at 56 days of age resulted in compromises in resource allocations. Specifically, selection for high juvenile body weight favored relatively heavier breasts, legs, abdominal fat depots and small intestines while selection for low juvenile body weight favored relatively heavier feathers and gizzard.