Comparative welfare science of pre-slaughter stunning for different livestock species
Pre-slaughter stunning — rendering animals unconscious before killing — is required by law in the EU (Council Regulation 1099/2009), UK, and most developed nations. The welfare quality of stunning varies enormously between methods and their implementation. Understanding the science of stunning is essential for both regulatory policy and slaughter facility management.
Penetrating captive bolt (cattle, sheep, pigs, goats): A bolt driven into the frontal bone by compressed air or gunpowder. When correctly placed and adequate velocity used, induces immediate unconsciousness. Key welfare parameters: correct placement (frontal position for cattle, modified for sheep), adequate bolt velocity (check regularly), correct charge, and immediate sticking after stunning.
Non-penetrating captive bolt (for rendering only): Used for calves and pigs; does not fully penetrate skull; requires immediate exsanguination; not suitable as sole method for cattle.
Electrical head-only stunning (sheep, pigs, poultry): Current through brain induces epileptic state; animal appears unconscious but may recover if not immediately killed. Cardiac arrest does not occur. Requires immediate sticking post-stun.
Electrical head-to-back stunning (cardiac arrest): Current through head and body induces cardiac arrest; animal cannot recover; higher welfare assurance. Becoming more common in pig and poultry slaughter.
CO2 stunning (pigs, poultry): High CO2 concentration causes unconsciousness. Welfare concern: CO2 is aversive — pigs show distress behaviors during induction. Low-O2/inert gas (argon, nitrogen) is less aversive but more expensive. CAS (Controlled Atmosphere Stunning) with high-argon gas mixtures provides better welfare than CO2 for pigs.