Growth curve analysis using nonlinear mixed model in divergently selected Japanese quails

This study was undertaken to apply non-linear mixed effects model to examine the effect of short-term selection for 8-week body weight on growth parameters in divergently selected lines of Japanese quail and their controls. The parameters of Richards models were utilized to describe growth pattern of Japanese quails from the generation 5 including growth curves; and absolute and relative growth rates. Growth parameter estimates were significantly different in mixed model from in fixed effects model for all lines by sexes, except asymptotic weight and age of inflection point. There was a significant increase and decrease in maturing weights of divergently selected lines over the control for both females and males. However, growth rates and ages of maximum growth rate for lines decreased significantly compared to control. Although Richards model is equivalent to the Logistic model for both females and males of control line, the selection for increased and decreased 8-week body weight resulted in the decline in the values of the growth curve for both sexes. Therefore, the trajectory of the growth curve of females in high line resembles more of the Gompertz growth curve; however, the selection did not alter the shape of growth curves in males of high line and in females and males of low line. Absolute and relative growth rates were also affected by the divergence selection. The high line quails showed significant absolute growth rate through growth period and relative growth rates were found significantly higher for lines compared to control in the first two weeks.