1. HG and LG quail lines selected for high and low relative weight gain between 11 and 28 d of age (RG11-28), respectively, and an unselected C line were compared. Mature body weight of both selected lines was held at that of the C line. Progeny of generation 6 were used for analysis. 2. Divergent selection for RG11-28 brought about opposite changes in the growth rates shortly after hatching. 3. Parameters of the Richards function were used to describe the growth curve. The largest differences between HG and LG lines occurred in age (t+) and body weight (y+) at the inflection point of the growth curve (on average for both sexes 28% and 20%, respectively). For HG quail, the parameter t+ was 5 d later than that for LG quail (18.6 vs 14.1 d for males and 20.6 vs 15.6 d for females, respectively), and consequently the parameter y+ was greater (90.3 vs 84.0 g for males and 104.5 vs 96.1 g for females, respectively). The shape of the growth curve expressed by the y+/A ratio was substantialy different for HG and LG quail (44.8% vs 39.6% for males and 43.5% vs 36.8% for females, respectively). 4. The food/gain ratios for the fattening period (3 to 35 d of age) were 3.21, 3.47 and 3.34 for the HG, LG and C lines, respectively. The HG quail started to utilise food more efficiently than the LG quail as early as 10 to 14 d, that is, at the age when their relative growth rate first became greater. 5. The relative deviations of the HG and LG lines from the C line are discussed.
1. Three outbred lines of Japanese quail and their reciprocal crosses were used. The lines differed in mature body weight or in the shape of the growth curve. Growth was described by body weight (BW) at 0, 4, 7, 14, 21, 28, 35, 42, 48, 56, 63 and 70 d of age and expressed by the parameters of Richards' function. Dickerson's model was used to estimate direct genetic, maternal genetic and direct heterotic effects. 2. The magnitude of BW heterosis was not constant during postnatal growth, it showed a different curvilinear age-trend for each hybrid combination. 3. The age-trend of BW heterosis resulted from the change of the shape of the growth curve. 4. The age-trend of BW heterosis and its maximum magnitude were associated with differences in the growth pattern of parental lines. 5. The heterosis at the inflection point was accompanied by heterosis in egg number.
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