Genetics of social interactions in laying hens : improving survival and productivity

Through social interactions, individuals affect one another’s phenotype. The heritable effect of an animal on its own phenotype is referred to as a direct genetic effect (DGE), while the heritable effect of an animal on the phenotype of a conspecific is referred to as an indirect genetic effect (IGE). Both DGEs and IGEs determine a population’s potential to respond to selection, i.e. the total genetic variance. This thesis focusses on the genetic architecture of survival time in laying hens showing feather pecking and cannibalistic behaviour, a well-known social interaction trait. DGEs and IGEs for survival time were estimated in purebred and crossbred laying hens. Unrelated birds of the same line or cross were kept in groups of four. Beaks were kept intact. Results showed that IGEs contribute around half of the total genetic variance in purebreds and the majority of the total genetic variance in crossbreds (up to 87%). The direct-indirect genetic correlations were close to zero in purebreds and moderately to highly negative in crossbreds. Consequently, unlike purebreds, crossbreds would fail to respond positively to mass selection. To ensure positive response to selection, animals should be selected based on their total breeding value. Moreover, increased response to selection can be obtained when including genotypic information. With genomics, the accuracy of estimated breeding values increased with 20 up to 110%, showing the added value of genotypic information. In addition, the genetic correlation between survival time (individual data) and early egg production (pooled data) was calculated. Results showed that, unlike for individual data, pooled data cannot be used to estimate DGEs and IGEs. However, pooled data can be used to estimate total genetic effects. The default bivariate model did not allow all non-genetic correlations between both traits to be fitted and, therefore, resulted in biased genetic parameter estimates. When this issue was resolved, the genetic correlation between survival time and early egg production was slightly negative (-0.09), but not significantly different from zero. Finally, the interpretation of as a measure of inheritance for social interaction traits was discussed. expresses the total genetic variance relative to the phenotypic variance. Throughout this thesis it became clear that, for social interaction traits, the level of data collection (individual vs pooled data) and the within-group relatedness affects the phenotypic variance and, consequently, affects . Therefore, can differ between experimental set-ups, even though the underlying genetic parameters are the same. This is undesirable for the comparison of studies. For survival time, a 30 up to 40% decrease in was observed when using pooled data instead of individual data. This illustrates that , as a measure of inheritance, should be used with care.