Defining characteristics of immersion carbon dioxide gas for successful euthanasia of neonatal and young broilers

This study investigated how the carbon dioxide (CO₂) concentration within a chamber affects the efficacy of CO₂ euthanasia and how the efficacy of CO₂ induction methods changes as birds age. In experiment 1, pairs of broiler chicks (n = 192; 0, 3, and 6 D of age) were immersed into a chamber prefilled with 70, 80, 90, or 100% CO_2. For experiment 2, 3- and 6-day-old broiler chicks (n = 88) were immersed in pairs into 100% CO₂ or exposed to CO₂ gradual fill in a chamber with a displacement rate of 28% chamber volume per minute. Latency to performance of headshaking (HS) and gasping (GS) as potential indicators of distress, loss of posture indicative of insensibility, and the cessation of rhythmic breathing (CRB) and cessation of movement (COM) as the indicators of death were monitored (live focal sampling/video recordings). The duration and frequency of HS and GS were assessed. For both experiments, behavior data were analyzed for CO₂ method and age (4 × 3 factorial). Age and CO₂ concentration interacted for latency to CRB and COM, with longer latencies for 0-day-old chicks immersed into 70% CO₂ than other concentrations and ages. CO₂ concentration did not affect latency to HS, GS, or loss of posture but affected CRB and COM, with latencies longest for 70% and shortest for 90 and 100% CO₂. Newly hatched chicks had a longer latency to CRB and COM and longer duration and frequency of distress behaviors than older chicks. At all ages, initiation of all behaviors occurred later with gradual fill compared to immersion. There was an increased duration and frequency of distress behaviors with gradual induction compared with immersion. Overall, immersion into 90 to 100% CO₂ resulted in the shortest time to insensibility and death, with a decreased duration and frequency of distress behaviors. Chicks immersed into 70% CO₂ had the longest duration of GS and time to death. Age affects the efficacy of CO₂ euthanasia, with increasing age decreasing time to death and the duration and frequency of distress behaviors.