Body growth response to selection and crossbreeding in dwarf and normal broiler-type chickens.

A normal-bodied broiler population and one dwarfed by the dw sex-linked gene were each sub-divided into two lines. AGB (normal) and D2B (dwarf) were selected only for superior body weight at eight weeks of age. AGE (normal) and D2E (dwarf) were sublines in which prospective dams were selected for large egg weight while prospective sires were selected for eight-week body weight. Selection for large eight-week weight of both sexes in both dwarf and normal populations during each of three generations resulted in superior growth rate to that observed when the dams were selected for egg weight. When the selected dams were tested each generation for their ability to produce superior broiler crossbreds using a commercial broiler sire strain, AGE proved superior to AGB in the second and third generations while D2B and D2E dams produced crossbreds with similar broiler weights. Normal-bodied dams which produced crossbreds having superior growth to that of the dwarf dams in the first generation lost most of this superiority by the third generation in sons and the second generation in daughters. Reproductive fitness was superior within the dwarf lines as well as in their ability to produce more crossbred progeny.