Abstract
A line of Japanese quail selected for increased body weight for 15 generations (C) and an unselected control line (K) were used to examine the impact of selection for body weight on the growth curve of Japanese quail. In addition, the effect of sex on the growth curve in each line was also studied, namely females of C (CF), males of C (CM), females of K (KF) and males of K (KM). The monophasic and diphasic growth models were studied for adequacy in describing growth curves of quail in both sexes of the C and K lines. The monophasic function provided almost the same growth rate for both sexes in both lines. However, the growth rates calculated by means of the diphasic function differed between sexes for both lines, except for those calculated for C during the second growth phase. While there were 2-3 days difference between sexes in age at maximum gain in both lines with a monophasic model, the difference between sexes in the age at maximum gain in both lines became greater according to the diphasic model. There were 5 and 7 days difference between sexes in the age at maximum gain in line C for the first and second growth phases, respectively. A difference between sexes of 18 and 11 days in the age at maximum gain for the first and second phases, respectively, was estimated for line K when the diphasic function was fitted. The use of diphasic functions provides more detailed information on growth patterns. The results showed that the use of the diphasic function was better because it provided greater insights into understanding the biology of growth.