Effect of long-term divergent selection on growth characteristics in Japanese quail

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Abstract
The current study was undertaken to examine the effect of long-term selection for 4-wk BW on growth characteristics in divergent lines of Japanese quail and their control. Growth rate was significantly higher in males than females in all the lines. There was a significant increase in growth rate of the females selected for increased 4-wk BW over the control females, as well as a significant decline in growth rate of males selected for decreased 4-wk BW compared to their control counterparts. It appeared that selection for increased 4-wk BW did not alter the rate of growth in the males compared to the control males; however, in the females, selection for increased 4-wk BW resulted in an increase in growth rate. On the other hand, selection for decreased 4-wk BW resulted in a decline in growth rate in males but not in the females. The dynamics of the growth curve parameters indicate that selection for decreased 4-wk BW shifted the growth curve for females as well as altering the trajectories of growth in both sexes. However, selection for increased 4-wk BW only resulted in altering the trajectory of growth in the males. Long-term selection resulted in asymmetry of response in the low and high lines. In addition, different genes may respond differently to the same selection pressure in opposite directions. The use of the 4-parameter Richards model to analyze growth data from such an experiment provided a better understanding of how selection can alter the rate and trajectory of growth to affect the genetically determined growth potential of quail. Consequently selection for increased or decreased 4-wk BW affected the sexes differently.