Stunning pigs with different gas mixtures: Aversion in pigs

The objective of this study was to assess the aversion to exposure of 90% argon, 70% N₂/30% CO₂ and 85% N₂/15% CO ₂ by volume in atmospheric air in 24 halothane-free slaughter-weight pigs using aversion learning techniques and behavioural studies in an experimental slaughterhouse. Pigs were subjected to the treatments individually during 2 separate trials of 12 animals each. The time of exposure to the gases was 46 and 32 s, respectively. When the pit contained any of the 3 gas mixtures, the time taken to cross the raceway and enter the cradle (TCREC) increased compared with the training sessions (atmospheric air). The incidence of pigs showing retreat and escape attempts and gasps and the number of times that this behaviour was performed was lower in 90% argon than in the gas mixtures with N₂ and CO₂. On the other hand, the time to loss of posture was lower with 70% N₂/30% CO₂ than with argon. The second exposure to all gas mixtures was more aversive than the first and the loss of posture also occurred earlier in the second exposure. In conclusion, pigs showed more aversion to gas mixtures with N₂ and either 15% or 30% CO ₂ by volume than 90% argon by volume.