Divergent selection for growth in Japanese quail under split and complete nutritional environments. 8. Progress from generations 18 through 30 following change of selection criterion.

Following 17 generations of divergent selection for 4-wk BW under split- (SD) and complete- (CD) diet environments, two sublines were established from high (H-SD and H-CD) BW lines. Subline H-SDG derived from the H-SD line was selected for BW gain from 2 to 4 wk under the SD environment, whereas Subline H-CDG derived from the H-CD line was similarly selected under the CD environment. These two sublines were maintained and reproduced simultaneously with H-SD, L-SD, H-CD, L-CD lines from Generation 18 through 30. The purpose of changing the selection criterion was to investigate genetic variation in BW gain independent of changes that occur immediately following hatch. Selection progress and heritabilities (approximately .3) in BW lines (H-SD, H-CD, and L-SD) were similar from Generation 18 through 30. However, progress and heritability (approximately .1) was not as great in the L-CD line. These responses indicate that considerable additive genetic variation for 4-wk BW remains in these lines. Selection for increased BW gain from 2 to 4 wk in both sublines (H-SDG and H-CDG) resulted in smaller BW at 2 wk of age than did selection for 4-wk BW. Under the SD environment, H-SDG quail were smaller at 4 wk than H-SD quail following 12 generations of selection, whereas under the CD environment, H-CDG quail were larger than H-CD quail. Results indicate that selection for BW gain was more effective under the CD than SD environment and that the lack of improvement in 2-wk BW accompanying the increase in BW gain may adversely influence the potential benefits obtainable from selection for gain.