Selection for body weight at eight weeks of age. 18. Comparisons between mature and immature pullets at the same live weight and age.

Body component and compositional traits were measured at the same age and live body weight in sexually mature and immature pullets from several genetic stocks. Populations included parental lines of White Plymouth Rocks selected for 25 generations for high and low juvenile body weight, their reciprocal F1 crosses, and an F2, as well as dwarfs from the parental lines. Although matched for age and body weight, percent liver weight and percent breast weight were greater for mature than immature pullets. Population by physiological stage interactions for percentages of abdominal fat and carcass lipid resulted from differences between mature and immature pullets, which were greater in the parental than in the cross and dwarf populations. Moreover, the pattern was reversed in the parental populations. The data supported the hypothesis that compositional requirements, independent of age and body weight, are necessary for the onset of egg production, and that these requirements vary among populations. They also demonstrated physiological buffering properties of heterozygosity and of the sex-linked dwarfing allele.