Divergent selection for shape of growth curve in Japanese quail. 3. Onset of sexual maturity and basic characteristics of early lay.

1. The onset of lay, quality of eggs during early lay and gonadal development of both sexes were analysed in meat-type lines of Japanese quail, HG and LG divergently selected for high and low relative weight gain between 11 and 28 d of age, respectively, and constant body weight (BW) at 49 d of age. 2. The LG line was sexually mature at an earlier age and lower BW than the HG line. This corresponded with the trend during embryonic and early postnatal development. 3. Analysis of gonads also confirmed earlier sexual development in the LG than in the HG line. In both lines, the growth of testes was detected about one week earlier than the growth of ovary. 4. Despite the different age and BW at onset of lay, HG and LG quail commenced lay at the same degree of maturity (about 90% of adult BW). This implied that the onset of sexual maturity could be identified as a point on the growth curve which terminates the linear phase. 5. When compared with the LG line, the HG line was characterised by a longer acceleration and shorter retardation phase of the growth curve. This difference is seen as an important determinant of line differences in growth and reproductive performance.