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Remote monitoring of electroencephalogram, electrocardiogram, and behavior during controlled atmosphere stunning in broilers: Implications for welfare

A. M. L. Coenen^*, J. Lankhaar, J. C. Lowe and D. E. F. McKeegan^,1

^* Nijmegen Institute for Cognition and Information, Biological Psychology, Radboud University Nijmegen, PO Box 9104, 6500 Nijmegen, the Netherlands; Stork Food Systems, PO Box 118, 5830 AC Boxmeer, the Netherlands; Silsoe Research Institute, Wrest Park, Silsoe, Bedford MK45 4HS, UK; and Division of Animal Production and Public Health, Faculty of Veterinary Medicine, University of Glasgow, Glasgow, G61 1QH, UK

¹ Corresponding author: d.mckeegan{at}vet.gla.ac.uk

This study examined the welfare implications of euthanizing broilers with 3 gas mixtures relevant to the commercial application of controlled atmosphere stunning (CAS). Birds were implanted/equipped with electrodes to measure brain activity (electroencephalogram, EEG) and heart rate. These signals were recorded using a purpose-built telemetry-logging system, small enough to be worn by each bird in a spandex backpack. The birds were euthanized in a scaled-down CAS apparatus consisting of a conveyor belt passing through 2 compartments. Three gas environments were applied (8 birds per treatment): 1) anoxia (N₂ with <2% residual O₂, in both compartments), 2) hypercapnic anoxia (N₂ with 30% CO₂ and <2% residual O₂, in both compartments), and 3) a 2-phase approach with a hypercapnic hyperoxygenated anesthetic phase (40% CO₂, 30% O₂, and 30% N₂, in the first compartment, 80 s) followed by a second euthanasia phase (80% CO₂ in air, in the second compartment). All 3 CAS approaches effectively achieved nonrecovery states, and time to loss of consciousness for each bird was determined by visual determination of isoelectric EEG and by calculation of the correlation dimension of the EEG. Hypercapnic anoxia resulted in rapid unconsciousness and death; both anoxic treatments were associated with early onset prolonged wing flapping and sustained tonic convulsions as displayed in the electrophysiological recordings. These responses were seen in the period when consciousness remained a possibility. Hypercapnic hyperoxygenation (the 2-phase approach) was associated with respiratory disruption, but this treatment eliminated initial clonic convulsions in the stunning process, and tonic convulsions were not seen. These results suggest that the presence of O₂ in the first stage of CAS is associated with an absence of potentially distressing behavioral responses. The respiratory discomfort associated with hypercapnic hyperoxygenation is an issue. We propose that this may be compensated by a more gradual induction to unconsciousness, which eliminates the impact of other potentially negative experiences.

Key Words: broiler • euthanasia • controlled atmosphere stunning • welfare • electroencephalogram