Protein and energy metabolism in broiler chickens selected for either body weight gain or feed efficiency.

1. Correlated responses in protein and energy metabolism were studied in 2 lines of meat-type chickens selected during 6 generations for improved efficiency of food utilisation (line A) or greater body weight (line B). 2. Daily gain was 18% higher in line B but efficiency was 15% better in line A. 3. Metabolisability (ME/GE) was 0.752 in line A and 0.725 in line B. 4. Retained nitrogen as a fraction of nitrogen intake was 0.51 for line A and 0.46 for line B. 5. Energy retention as a fraction of metabolisable energy was 0.38 in line A and 0.43 in line B the difference being primarily attributable to greater fat retention in line B. 6. Selecting for improved efficiency of food utilisation in meat-type chickens having a growth capacity of 50 to 60 g was very effective and produced a lean bird with higher metabolisability of the diet.