Genetic analysis of a selection experiment on the growth curve of chickens

A selection experiment on the shape of the growth curve was performed on meat-type chickens through combined selection on juvenile and adult BW. Line X-+ was selected for low BW at 8 wk (BW8) and high BW at 36 wk (BW36). Line X+- was selected for high BW8 and low BW36. Line X++ was selected for high BW8 and BW36, and X-- was selected for low BW8 and BW36. Line X00 was maintained as an unselected control. Data on the first 14 generations (i.e., 38,693 birds) were used. The growth curve was modeled using a Gompertz function on 7,143 birds that were weighed regularly. Selection for higher BW8 increased BW from 4 to 16 wk, initial specific growth rate, and maturation rate and decreased age at inflection. Selection for higher BW36 resulted in increased BW36, asymptotic BW, and estimated BW at hatching. Body weights were more modified in Lines X++ and X--, but the growth curve parameters changed more in Lines X-+ and X-- than in Lines X++ and X+-.