Selecting chicken for improved meat production has altered the relative growth of
organs in modern broiler lines compared with heritage lines. In this study, we compared
the growth and feed efficiency of a heritage line, UIUC, with a modern production
line, Ross 708, for 5 wk posthatch. During this period, the BW and feed efficiency
of the modern strain was higher than that of the heritage line, indicating that the
Ross 708 birds were more efficient than the UIUC birds at converting feed to body
mass. The relative growth of the breast, heart, liver, and intestine were also compared
during these 5 wk. The breast muscle of the heritage line constituted 9% of the total
body mass at 5 wk, whereas in the modern line, the breast muscle was 18% of the total
mass of the bird. In contrast, the relative size of the heart decreased after d 14
in the modern line, suggesting that selection for increased breast muscle has translated
into relatively less weight of the heart muscle. The liver matured earlier in modern
lines, possibly improving nutrient utilization as the birds shift from lipid- to carbohydrate-rich
feed. Finally, jejunal and ileal sections of the intestine were 20% longer in the
modern line, perhaps allowing for increased nutrient absorption.