Feed efficiency data for growth selected and unselected and also stocks divergently selected for body weight were reviewed with special emphasis on changes immediately after hatching. Examination of feed efficiency differences between genetically diverse stocks from hatch to market age normally reveal only small differences in the efficiency of feed utilization. On the other hand, differences in feed intake appear to be the dominant factor in growth-related changes of these genetically diverse stocks. However, when data were segmented into periods which allowed investigation of differences at early ages, especially immediately after hatching, major differences in feed efficiency between genetically diverse groups were observed.
These data indicate that feed efficiency plays a more important role in explaining genetic differences in growth than indicated by previous investigations. The appearance of the greatest differences between genetically diverse stocks immediately after hatching indicates that early feed efficiency changes may allow for greater subsequent feed intake. Therefore, it appears that changes in early feed efficiency along with changes in feed intake both play major roles in explaining genetic differences in growth. The role of feed efficiency is apparently obscured by the maintenance requirements associated with large body sizes of selected stocks when measurements are obtained at older ages.