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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 O-8O0/o. 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 ran e of heat production (HP) between strains is found to be 44-118 kJ kg-' s7' d-' ; variation among individuals within strains is of the same magnitude (standard deviation: 16-125 kJ kg-0.75 d-I). The range of maintenance requirements (ME,) is reported to be 41-113 kJ kg-0,75 d-'; standard deviation among individuals is 23-80 kJ kg-0.75 d-'. Variation of heat increment of production (constituting the difference between HP and ME,) seems to be small. The range of fasting heat roduction (FHP) between strains of laying hens appears to be 18-130 kJ kg-'qR d-l, whereas the standard deviation among individuals has again the same magnitude (22-51 kJ kg-0+75 d-' 1. It is concluded that variation of heat increment of maintenance (ME, -FHP) is small. The main component of HP variation and of RFC variation seems to be the variation of ME, : 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.

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 metabolisability, 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.

A selection experiment with a selection and a control line maintained for eight generations was set up to study efficacy of selection for a short interval from weaning to estrus after weaning the first litter and to estimate genetic variation in the Dutch Landrace population. Intervals were recorded without truncation (i.e., intervals up to 234 d were observed). A mating scheme with 10 mating groups was used to avoid inbreeding. Distribution of intervals from weaning to estrus was described by a mixture of a normal and an exponential distribution. Between lines, both underlying distributions were similar, but the contribution to the total distribution was different. From generation 4 onward, intervals were significantly shorter in the selection line than in the control line. Variances within lines, generations, and mating groups varied to a large extent. Response per unit selection differential was calculated for original data and three sets of transformed data, both with and without correction for unequal subclass numbers. Standardized responses were more precise and linear, when data were log-transformed, and when corrected for unequal subclass numbers. The assumed best estimate of the realized heritability was .17. The heritability in the foundation population was estimated at .36 f .05, using an animal model including all genetic relationships in the data. It was concluded that genetic selection decreases the average interval from weaning to estrus by reducing the number of sows with a record in the exponential part of the distribution.

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