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.

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Production levels in livestock (cattle, pigs and poultry) have been increased considerably, with a correlated increase in gross feed efficiency. However, mature body weight has also increased, leading to higher maintenance costs. Thus, net feed efficiency has been little improved. Breeding for lower body weight has not been successful, but there are possibilities for reduction of feed consumption independent of production and body weight (residual feed consumption). It is as yet uncertain to what extent animal stress susceptibility will be affected by changes in residual feed consumption.

The study described here deals with the quantification of the genetic variation of "residual feed consumption" (RFC) of hens of a White Leghorn population during a 44-wk laying period (20 to 64 wk of age) in 11 time segments of 4 wk each, fed either a commercial or a low-energy diet (11.7 and 10.0 MJ ME/kg, respectively, where 1 MJ = .239 Mcal). The RFC is defined operationally as the difference between the observed feed consumption of a laying hen and its consumption as predicted from a model with metabolic body weight, egg mass production, and body weight gain as independent variables. The RFC was found to be highly heritable in all periods. The heritability of RFC accumulated over the whole laying period (RFC-T) was estimated as .42 to .62. For each time segment between 32 and 56 wk of age, genetic correlations between RFC and RFC-T were estimated to be larger than .91. The genetic sources causing variation in RFC during the first part of lay seem to differ from those causing variation later on, and to be of less importance during the rest of the laying period. It was concluded that RFC shows a considerable systematic and permanent additive genetic variance, and that RFC measurements for selection can be limited to one to three time segments between 32 and 56 wk. Furthermore, less environmental variance and therefore higher heritabilities and genetic correlations seemed to exist for birds fed the low energy diet in comparison with those fed the commercial diet. No clear differences could be found between genetic and phenotypic correlation estimates of RFC with feed consumption, metabolic body weight, egg mass production, and body weight gain.

A large fraction (on average, 25%) of the variance of daily feed consumption among individual laying hens appears unaccounted for by a model with metabolic body weight, daily egg mass production, and body weight gain as independent variables. This unexplained term is referred to as "residual feed consumption" (RFC), defined operationally as the difference between the observed feed consumption of a hen and its consumption as predicted from the model. The study described here deals with the quantification of the phenotypic variation of RFC of hens of a White Leghorn population during a 44-wk laying period (20 to 64 wk of age) in 11 time segments of 4 wk each, fed either a commercial or a low-energy diet (11.7 and 10.0 MJ ME/kg, respectively, where 1 MJ = .239 Mcal). The RFC showed a standard deviation of 4 to 8 g/day. The repeatability of RFC was estimated as .52 to .58. For each time segment between 32 and 56 wk of age, phenotypic correlations between RFC and RFC accumulated over the whole laying period were estimated to be around .8. It was concluded that RFC shows a considerable amount of systematic and permanent variation, and that experimental RFC measurements can be limited to the period between 32 and 56 wk. It was also shown that variation in RFC was caused mainly by differences among hens in maintenance requirements per unit (kilogram.75) of metabolic body weight.(ABSTRACT TRUNCATED AT 250 WORDS)