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Abstract
1. The object of this study was to identify the concentrations of carbon dioxide (CO2) and hypoxia (induced by argon) which hens could detect and by preference avoid.
2. Hens were housed in an apparatus consisting of an upper roosting chamber connected to two descending passages which led to separate identical feeding chambers. The gas concentration in both these feeding chambers could be regulated.
3. The hens were allowed to feed in either a normal air atmosphere or an altered atmosphere containing a sub‐stunning concentration of CO2 or argon. The birds’ preference for feeding in the two chambers was used as an indicator of the concentration which they chose to avoid.
4. The study involved two experiments. In the first it was found that when the concentration of CO2 was raised above 5%, or the concentration of oxygen was reduced to 10% or below, the occupancy of that chamber was significantly reduced.
5. The results of the second experiment confirmed that a concentration of above 7.5% CO2 or oxygen levels of 10% or below can be detectable to hens and, given a free choice, the hens learnt to avoid such atmospheres.
6. The presence of a dominant bird in the roosting chamber or in the feeding chamber containing air caused the subordinate birds to enter the treatment chamber containing gas. This would imply that the unpleasantness of the stunning gases at the concentrations used in this study was less severe than the social pressure.
7. In the context of gaseous stunning of poultry, these results indicate that exposure to increased concentrations of CO2 or decreased concentations of O2 can be perceived by hens. A rapid induction of anaesthesia is recommended when using CO2 or anoxia in order to minimise any discomfort during stunning.
