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When multiple linear regression of feed consumption on metabolic body weight (MBW), body weight gain (BWG) and egg mass production (EM) is calculated between or within strains of laying hens, a standard deviation of 47–180 kJ metabolizable energy (ME) per day remains unexplained by these effects. This unexplained fraction, called residual feed consumption (RFC), has a heritability of 0–80%. A survey of the literature was performed to quantify this unexplained variation between strains and among individuals within strains of laying hens with respect to some energy metabolism parameters independent of MBW, BWG and EM. Genetic differences in ability to metabolize gross feed energy are found to be of limited magnitude; the coefficient of variation is 1–3%. From calorimetric experiments, the range of heat production (HP) between strains is found to be 44–118 kJ kg−0.75 d−1; variation among individuals within strains is of the same magnitude (standard deviation: 16–125 kJ kg−0.75 d−1). The range of maintenance requirements (MEm) is reported to be 41–113 kJ kg−0.75 d−1; standard deviation among individuals is 23–80 kJ kg−0.75 d−1. Variation of heat increment of production (constituting the difference between HP and MEm) seems to be small. The main component of HP variation and of RFC variation seems to be the variation of MEm: differences in physical activity, feathering density, basal metabolic rate, area of nude skin (comb, wattles, legs), body temperature and body composition. Net energy per g EM does not play an important role in variation of RFC.
P. Luiting (Sun,) studied this question.