Dynamics of relative growth rate in Japanese quail lines divergently selected for growth and their control.

This study was undertaken to test the hypothesis that relative growth is the same in Japanese quail divergently selected for 4-week body weight for 30 generations, and their control. Relative growth rate is the increase in body weight per unit of body weight per unit of time, and it represents the efficiency of an animal as a producer of new body mass material. The maximum relative growth rate of the divergent and control lines were observed during the first week. Selection for high 4-week body weight has resulted in higher relative growth rate for the first two weeks compared to quail from the low line, but by week 3, quail from the low line had higher relative growth rate compared to quail from the high line. Asymptotic body weight increased by about 100 and 70%, respectively in the male and female high lines and declined by about 25% in the low lines. Absolute growth rate is thought to be related to the cumulative growth already achieved whereas relative growth rate is related to the amount of growth remaining. However, selection on absolute weight basis can elicit a response in relative growth rate in the early part of the developmental period which possibly pre-determines the asymptotic body weight. Since relative growth rate depends on the amount of growth remaining, the rate of decline in relative growth is then set accordingly. Furthermore, it is thought that different sets of genes may operate between the developmental period and maturation period.