Behavioural responses of broiler chickens during acute exposure to gaseous stimulation

Controlled atmosphere (gas) stunning has potential to improve poultry welfare at slaughter but raises concerns about exposure of birds to aversive gaseous environments during the induction of unconsciousness. The aim of this study was to compare the immediate aversiveness of gas mixtures of potential use in gas stunning systems, utilising an innovative approach based on the interruption of ongoing feeding behaviour. Broiler chickens trained to feed from a dish attached to a gas delivery outlet in a custom-built apparatus were exposed to 10 s pulses of test gases whilst feeding. Test gas delivery matched a constant air flow present pre- and post-stimulation and was not associated with any other cues. Behaviour (feeding, headshaking, respiratory disruption, withdrawal and ataxia) was observed before (10 s), during (10 s) and after (20 s) gas delivery. Three identical experiments examined responses to either CO2 in air (10, 25, 40, 55, 70%), CO2 in nitrogen (25, 40, 55 and 40% CO2 with 30% O2) or argon and nitrogen (100%). Stimulation with CO2 in air or nitrogen produced similar responses with evidence of the gas being detected at 10%. As CO2 concentrations increased, headshaking increased monotonically while time spent feeding decreased. Respiratory disruption (apparent increased inhalation depth and duration) was exhibited at all concentrations of CO2 tested but was not dose-dependant. Withdrawal from the food dish (assumed to indicate aversion) was observed in a small number of birds during CO2 stimulation and was related to concentration. Withdrawal was transient with all birds returning to feed within 20 s of air reinstatement. The addition of 30% O2 to the 40% CO2 in nitrogen mix was associated with increased time spent feeding and reduced headshaking. Other than headshaking, no immediate response to delivery of the inert gases nitrogen and argon was observed, but a delayed ataxia (loss of balance, loss of posture) was exhibited by some individuals. Collectively, the results suggest mild or at most moderate immediate aversion to CO2 as indicated by cessation of feeding and withdrawal at some concentrations. The notion that the respiratory disruption induced by CO2 inhalation is potently aversive is not supported by our observations since this response was not associated with withdrawal or even the cessation of feeding in some cases. These results inform the welfare debate surrounding aversiveness of initial gas exposure and have implications for the levels of CO2 to which conscious birds should be exposed during controlled atmosphere stunning.