Growth patterns for three generations of an intercross between red junglefowl and chickens selected for low body...

Growth is a complex and dynamic process that may be measured at a specific point or over a period of time. Compared was the growth of male and female chickens over a three-generation period. Involved were red junglefowl (RJF; Gallus gallus), a line of White Plymouth Rock chickens (LWS; Gallus gallus domesticus) selected for low body weight, and their reciprocal F₁ and F₂ crosses. In both sexes, Gompertz's description of growth showed that RJF had significantly lower asymptotes, earlier inflection points, and faster growth rates than LWS. Heterosis for these measures was positive for asymptote and negative for growth rate and inflection point. The RJF commenced egg production at a significantly younger age and lower body weight than LWS. Although F₁ and F₂ reciprocal crosses were similar for body weight and for age at first egg, the F₁ reciprocal crosses began lay at significantly younger ages than the F₂ crosses and parental lines. When viewed on a physiological basis where age and body weight were simultaneously standardized, both parental lines and reciprocal F₁ and F₂ crosses had differing rapid and lag growth phases. Overall, sexual dimorphism increased in all populations from hatch to sexual maturity. The LWS males had a longer growth period consistent with their female counterparts who became sexually mature at older ages. Comprehensively, these results indicate additive and nonadditive genetic variation for distinct growth patterns and changes in resource allocation strategies over time.