The effect of different levels of management on the assessment of differences between varieties of chickens. 1. Eight-week body weight

Abstract

1. Analyses of records of eight-week body weight in broiler chickens have been carried out to estimate the effect of management level (measured by mean body weight of all varieties on each farm) on the components of variance due to differences between and within varieties, and to find out if there is any evidence of genotype-environment interaction.2. It was found that as management improved, the between-variety genetic variance remained constant or possibly increased whilst the withinvariety (genetic and environmental) variance decreased markedly. The cause of these regressions is not known.3. In four cases out of six there were significant genotype-environment interactions. In two cases, the genetic correlation between the performance of varieties on different locations was significantly different from 1·0.4. It is suggested that on the basis of these findings it would be necessary to select between varieties on records taken from chickens maintained under conditions similar to those in which they will eventually be used. However, fewer chickens would be required to measure, with a predetermined level of accuracy, mean differences on well managed farms than on poorly managed ones.