Performance of Layer-Type Chickens as Related to Body Conformation and Composition. 1. A Static Analysis of Shank Length and Body Weight at 20 Weeks of Age

Abstract The growth of an animal part (y) relative to the whole (x), is expressed in terms of an exponential equation, y=αxβ, where α and β are growth constants. For the present study x is body weight and y is shank length determined for 10 lines of chickens at 20 weeks of age. The derived values of α and β, assumed to be limiting relative growth constants for bone, were estimated by least squares of the log-transformed (linear) exponential equation. From the means of α and β, calculated for each of seven Leghorn and three Fayoumi lines, individual values of αi and βi were calculated for more than 24,000 pedigreed birds included in 9 years of records. The individual values (i) were calculated as: α i = antilog ( log y i − β log x i ) and: β i = log α − log y i log x i If the initial growth constant, α, is the primary hereditary growth parameter governing conformation, then the parameter, β, should not be as highly heritable as α. To test this hypothesis, the heritability (h2) of both α and β were calculated for each of the 10 lines. For α sire component h2 values ranged from .12 ± .05 to .45 + .10 and for β from .05 to .41. The pooled h2 estimate of α over lines (.31 ± .02) was not significantly different from that for β (.31 ± .02). Compared with the original data on body weight and shank length, the transformation significantly reduced the heritability estimated. We conclude that the parameters, α and β, are useful only in describing phenotypic and not genetic differences in relative growth.