Genetics of survival in cannibalistic laying hens : the contribution of social effects

Mortality due to cannibalism in laying hens is a worldwide economic and welfare problem occurring in all types of commercial poultry housing systems. Due to prohibition of beak-trimming and the traditional battery system in the European Union in the near future, mortality due to cannibalism in laying hens may increase. To reduce mortality in laying hens, it is possible to use genetic selection. Mortality due to cannibalism, however, depends on social interactions between group members. Traditional selection methods neglect these social interactions, meaning that they ignore the genetic effect an individual has on its group members. These methods are, therefore, not very effective. The main aim of this thesis is to investigate the effect of social interactions on the heritable variance in mortality due to cannibalism in laying hens and to develop a selection method that takes into account social interactions.
To investigate the effect of social interactions on the heritable variance in mortality due to cannibalism, genetic parameters for direct and associative effects on survival time in three layer lines were estimated. For all three layer lines it was found that social interactions contribute approximately two-third of the heritable variation in survival time. The heritable variation in survival time is, therefore, substantially larger than suggested by the traditional methods currently used in poultry breeding.
To improve traits affected by social interactions in laying hens, a solution is to select individually housed candidates based on the performance of their full sibs kept in family groups. Theoretical results suggest that this selection method offers good opportunities to improve traits affected by social interactions. A selection experiment was applied aiming to improve mortality due to cannibalism in laying hens using selection based on relatives. After one generation, mortality was 10% lower in the selection line compared to the control. In the second generation, no significant effect was found, which seemed to be related to environmental factors.
Results in this thesis suggest that prospects for reducing mortality due to cannibalism by means of genetic selection are good. Using selection methods that incorporate social interactions may lead to substantial reduction of one of the major welfare problems in egg production. Further research is needed to investigate the effect of group size and kin recognition on social interactions.