Aversion to carbon dioxide stunning in pigs: effect of carbon dioxide concentration and halothane genotype
Aversion to the dip-lift stunning system and to the inhalation of 70 and 90% carbon dioxide was assessed in 18 halothane-free (NN) and 14 heterozygous halothane (Nn) slaughter weight pigs using aversion learning techniques and behavioural studies in an experimental slaughterhouse. Pigs were subjected to the treatments individually. When the dip lift system contained atmospheric air, the proportion of pigs that entered the crate voluntarily increased on subsequent days, indicating that pigs habituate to the stunning system. Based on the number of attempted retreats, for the first descent into the well with atmospheric air, Nn pigs were more reactive than NN pigs. On repeating the descent, Nn pigs showed greater habituation to the procedure. When the pit contained (either 70 or 90%) carbon dioxide, the time taken to enter the crate and the incidence of pigs that attempted to retreat increased on subsequent days, indicating aversion to the carbon dioxide concentrations. The aversion was higher when the stunning system contained 90 as opposed to 70% carbon dioxide due possibly to increased irritation of the nasal mucosal membranes and more severe hyperventilation. Conversely, a decrease in the concentration of carbon dioxide increased the time to loss of posture and, therefore, lengthened the perception of the aversive stimulus till the animal lost consciousness. These results suggest that stunning with carbon dioxide is not free from pain or distress. The degree of aversion depends on the carbon dioxide concentration. Therefore, if higher concentrations of carbon dioxide are recommended for rapid induction of anaesthesia, it needs to be assumed that this may be more aversive to pigs.
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