Behavioral Response of Weaned Pigs during Gas Euthanasia with CO₂, CO₂ with Butorphanol, or Nitrous Oxide

The swine industry is often forced to euthanize pigs in the first few weeks of life due to injuries, hernias, or unthriftiness. The majority of pigs are euthanized using carbon dioxide (CO₂) gas asphyxiation but concerns as to the humaneness of CO₂ are increasing. This study compared the euthanasia of weaned pigs using N₂O (N₂O; <i>n</i> = 9) or CO₂ (<i>n</i> = 9), at 50% and 25% min⁻¹ exchange rate, respectively. In addition, we administered an analgesic prior to euthanasia with CO₂ (CO₂B) exposure as a third treatment (<i>n</i> = 9) to elucidate behaviors indicative of pain. Pigs in the CO₂ and N₂O treatments lost posture at similar times (latency of 145.0 ± 17.3 and 162.6 ± 7.0 s respectively, <i>p</i> > 0.10), while the CO₂B treatment pigs lost posture the soonest (101.2 ± 4.7 s, <i>p</i> < 0.01). The pigs in the CO₂B treatment made more escape attempts than the CO₂ or N₂O pigs (16.4 ± 4.2, 4.7 ± 1.6, 0.3 ± 0.2, respectively; <i>p</i> < 0.0004). However, pigs in N₂O squealed more often than either the CO₂ or CO₂B pigs (9.0 ± 1.6, 2.8 ± 1.2, 1.3 ± 0.6, respectively, <i>p</i> < 0.001). Given the similar time to loss of posture and shorter time displaying open mouth breathing, N₂O may cause less stress to pigs; however, the greater number of squeals performed by these pigs suggests the opposite. It was not apparent that any behavior measured was indicative of pain. In conclusion, N₂O applied at a 50% min⁻¹ flow rate can be an alternative to CO₂ for pig euthanasia.