Abstract

1. From each of two populations of White Leghorn laying hens, 6 ‘efficient’ and 6 ‘inefficient’ hens were used in energy metabolism studies involving indirect calorimetry with activity measurement.

2. Hens classified as efficient or inefficient with respect to residual food consumption had comparable body weight and production.

3. Efficient and inefficient hens showed similar energy metabolisabil‐ity, cloacal temperature, shank dimension and egg composition.

4. However, efficient hens produced less heat; activity‐related heat production accounted for 29 to 54% of the difference in total heat production between efficient and inefficient hens.

5. Plumage quality also explained some of the difference in heat production, but to a lesser extent (14 to 19%).

6. Adjustment for heat increment of production changed the heat production difference slightly (—5 to 1%).

7. What was finally left unexplained (100 — 54 — 14 + 5 = 37% to 100 — 29 — 19 — 1=51%) was of the same magnitude as activity‐related HP and is discussed in relation to basal metabolic rate.