Evaluating Long-Term Direct and Correlated Selection Response in White Plymouth Rock Chickens Selected for High or Low 8-Week Body Weight

The increasing demand for poultry meat has led animal breeders to engage in artificial selection of chickens as a way to increase the productivity of poultry. Long-term experiments have been designed to measure rates of genetic response to a trait under selection, and correlated traits, as well as gauge possible selection limits. Two studies were conducted to evaluate response to selection for body weight (BW) in chickens. The chickens were selected for high or low BW at 8 weeks of age. Those that met the criterion were selected as parents for the subsequent generation. In the first study the population structure of the two lines was assessed. Inbreeding coefficients, effective population sizes and relatedness were compared between the lines across the 59 generations of selection. The two lines were parallel in structure, having similar inbreeding levels, founder contributions and family sizes. Such parity allows for reliable comparisons of the performance of the two lines across the selection profile. In the second study, direct response to divergent selection for BW at 8 weeks of age, and its correlated impact on BW at 4 weeks of age, over 56 generations of selection was evaluated. In the analyses a complete pedigree was used accounting for all familial relationships within and between lines. In the high weight selection (HWS) line, both 8- and 4-week BW increased linearly across generations. Even though selection had occurred over an extend time period, substantial additive variation in BW was retained in the HWS line. In the low weight selection (LWS) line, both 8- and 4-week BW decreased in a curvilinear fashion reaching a plateau at around generation 25. Much less variation in BW remained in the LWS than HWS line by generation 56. However, the heritability of BW remained moderately high in both lines. The selection plateau observed in the LWS line therefore likely reflected biological constraints on reproductive fitness rather than the removal of additive variation. Advisor: Ronald M. Lewi