Performance comparison of dwarf laying hens segregating for the naked neck gene in temperate and subtropical environments

In poultry selection programmes, birds are generally raised in a uniform environment to record the selected traits and evaluate genetic values. Thus, most commercial populations are obtained from breeding farms with a controlled environment and are delivered to production farms with variable environments across the world. As a consequence, genotype × environment (GxE) interactions may occur. Under subtropical environments, growth rate as well as egg production are generally depressed by high ambient temperature [1,2]. Introduction of some major genes, such as the naked neck (NA) gene, in chicken lines can be used to alleviate heat stress, as discussed in several studies carried out at high ambient temperatures [1,3-5]. Heat tolerance of laying hens is an important issue for the impact of G × E on egg production because the productive period is long and the impact of heat stress increases with the hens' age [6].At INRA, a selection experiment was undertaken to improve egg production of brown-egg layers by increasing clutch length, in the presence of two major genes known to improve heat tolerance, the naked neck gene [7] and the sex-linked dwarf gene [8]. After 16 generations, significant genetic progress on egg production was observed in two dwarf lines, one carrying the NA gene and one not carrying it [9-11]. The reduction of feather mass due to the NA gene is lower in heterozygous birds than in homozygous birds, i.e. 27% and 22% respectively for heterozygous females and males, and 41% and 33% respectively for homozygous females and males [12]. Heat tolerance is significantly improved by the presence of the NA gene and significant genotype by temperature interactions were observed for egg production, egg mass and feed intake under a 32°C constant temperature environment, which represents a severe temperature stress [3]. The purpose of this paper was to study the laying performance in real subtropical conditions of the genotypes obtained from the selection experimen