Controlled atmosphere stunning uses gas mixtures to render poultry unconscious before slaughter. Understanding the welfare advantages and disadvantages compared to water bath electrical stunning informs advocacy for best-practice slaughter standards.
Poultry slaughter stunning welfare involves a comparison between imperfect systems rather than a clear choice between welfare-positive and welfare-negative options. Water bath electrical stunning, while the most common system, has documented welfare failures: pre-stun shocks as birds touch the electrified water, inadequate stunning of birds whose heads do not make proper contact, and the fear and distress of being shackled by legs and transported inverted to the bath. Controlled atmosphere stunning eliminates these specific failure modes by rendering birds unconscious in their transport containers before shackling, avoiding conscious handling in the stunning area. However, CAS using high-concentration CO2 replaces one welfare problem with another: CO2 at concentrations needed for rapid unconsciousness causes an aversive period of breathlessness and distress before insensibility is achieved. Low-concentration inert gas protocols using argon or nitrogen mixtures are less aversive but require higher gas concentrations and longer exposure times. The welfare hierarchy from current evidence places multi-phase low-O2 CAS systems above water bath electrical systems on most welfare indicators, despite the higher capital cost. Consumer and regulatory pressure for improved slaughter welfare is the primary driver of industry transition.