Patterns of growth and feed intake in divergent lines of laying domestic fowl selected for residual feed consumption.

At Generations 15 (males) and 18 (females) of a divergent selection experiment on residual feed consumption (RFC) in laying poultry, patterns of growth, feed consumption, and associated traits were monitored between the ages of 4 and 34 wk. This monitoring was done to determine how the well-established RFC and feed intake (FI) divergences between adults of the low intake R- line and of the high intake R+ line took place, in relation to the evolution of correlated traits. In males and females, BW and BW gain were higher in the R- line in the first weeks of test, but patterns of BW were quite similar in both lines afterwards. However, R- hens remained heavier than R+ females to the end of the experiment. Line difference for BW was achieved by 28 wk of age, at 2,974 and 2,094 g, and the R- line was then 135 and 133 g heavier, respectively, for males and females. After a fast initial increase in both sexes, FI diverged quickly around 16 to 18 wk of age (sexual maturity), to attain 13 g/d in males and 28 g/d in females, well before the end of the experiment. At the same time, a divergence was observed for wattle length, which was 21% higher in R+ females. Residual feed consumption gradually diverged in the two lines, and the difference became significant at 14 wk of age. At the same age, shanks were 8% longer in R+ hens. Finally, levels of triiodothyronine decreased faster in the R- line, as FI divergence was increasing. These results indicate that the RFC difference between R- and R+, lines obtained in adult birds by divergent selection lines, starts somewhat early in life. In addition, the RFC difference appears to be associated with specific growth periods, around 14 wk and 18 to 20 wk. It is at this time that large differences appear in morphological traits involved in body heat loss. No significant correlations were found between early (6 wk) measures of FI, feed efficiency, and RFC, with adult values (32 to 34 wk) for RFC and FI.