Physiological and behavioural responses of broilers to controlled atmosphere stunning: implications for welfare

Abstract

Abstract Controlled atmosphere (gas) stunning (CAS) has the potential to improve the welfare of poultry at slaughter but there is a lack of consensus about which gas mixtures are most humane. The aim of this study was to evaluate the welfare consequences of different gas stunning approaches. Individual broilers were exposed to gas mixtures capable of inducing unconsciousness and euthanasia while their behavioural, cardiac, respiratory and neurophysiological responses were measured simultaneously. The approaches investigated included anoxia (N 2 or Ar with < 2% residual O 2 ), hypercapnic anoxia (30% CO 2 in Ar, 40% CO 2 in N 2 ) and a biphasic method (40% CO 2 , 30% O 2 , 30% N 2 for 60 s followed by 80% CO 2 in air). Evaluation of the welfare implications of each approach centred on the likelihood of them inducing negative states or experiences during the conscious phase. Hypercapnic mixtures were associated with strong respiratory responses, while anoxic mixtures induced vigorous wing flapping. Electroencephalogram analysis using the correlation dimension (a non-linear measure of complexity) suggested that anoxic wing flapping occurred during periods in which a form of consciousness could not be excluded. Hypercapnic hyperoxygenation (biphasic approach) exacerbated respiratory responses but eliminated the possibility of vigorous behavioural responses occurring during a conscious phase. The relative importance of respiratory discomfort versus the potential to induce significant distress due to convulsive wing flapping and associated trauma is a matter for debate. We argue that respiratory discomfort is unpleasant but may be preferable to the risk of vigorous wing flapping and associated injury while conscious in poultry during CAS.