Correlated Responses in Lines of Chickens Divergently Selected for Fifty-Six–Day Body Weight.

Correlated responses for growth, feed intake, and feed utilization were measured in female chickens from lines after 32 generations of divergent selection for 56-day body weight and in sublines in which selection had been relaxed for five generations. The diet used in the present experiment was that under which selection had been practiced (20% crude protein and 2,685 kcal of ME/kg). Feed intake between each selected line and its relaxed subline was equalized by paired-feeding. Ad libitum-fed controls were also maintained. At 21 days of age, differences between the selected lines for ad libitum-fed chickens were 404% for body weight, 279% for feed intake, and 138% for feed conversion ratio (FCR). Respective percentages for the high and low relaxed lines under ad libitum feeding were 267, 223, and 121%. When pair-fed, the growth of the line selected for high weight was unimpeded and FCR improved. When chicks of the low-weight relaxed line were pair-fed, FCR also improved, but growth was reduced and appetite development inhibited. Hyperphagic behavior was observed in the line selected for high weight. In the low-weight selected line, the chicks exhibited hypophagia. Although residual feed consumption was unchanged by selection for 56-day body weight, variation was present for this trait, with lower values under restricted than ad libitum feeding.