Correlated responses to long-term divergent selection for 8-week body weight in female White Plymouth Rock chickens: Sexual maturity.

Reported here are correlated responses for reproductive traits to long-term divergent selection (54 generations) for 8-week body weight (BW8). Comparisons involved both selected and relaxed lines. Traits measured were age at first egg (AFE), body weight at first egg (WFE), and ratio of body weight and age at first egg (WAFE). Although sexual maturity was delayed in the selected lines, the effect was more pronounced in the low than high selected and relaxed lines. Selection for low BW resulted in decreases in WFE and WAFE. Correlated responses to selection for high BW were increased WFE and WAFE. Minimum AFE, WFE, and WAFE in relation to sexual maturity were line specific and influenced by selection for BW8. WAFE provided a "yardstick" for target body weights that were optimum for successful attainment of sexual maturity and higher reproductive rates. Such may be line specific. There was opposition between relaxed and artificial selection, resulting in a higher reproductive performance and fitness for the former.

Selection for juvenile body weight in chickens: Standardizing for scaling.

The influence of scaling on phenotypic responses to long-term selection was evaluated in lines of chickens selected for high (HWS) and low (LWS) 8-week body weight. The focus of the experiment was on phenotypic correlations between means and standard deviations, and unadjusted vs. standardized responses. Traits measured were body weight at 4 (BW4) and 8 wk (BW8). Correlations between means and standard deviations for BW8 (the selected trait) were positive and greater for BW4 in LWS than HWS. In both lines, scaling masked the degree more than the pattern of response; however, after standardization, it was greater in LWS than HWS. Selection changed the standard deviation units of juvenile BW with a 2-fold difference in BW8 for both HWS and LWS in which the latter reached a physiological limit. Standardization is a robust method to adjust for scaling, thus enhancing the interpretability of responses in long-term selection experiments. Because of changes in variances associated with those in phenotypic means, this method to correct for part-whole correlations was useful.