Residual feed consumption in laying hens. 2. Genetic variation and correlations.

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.