Gas euthanasia methods are commonly chosen when large groups of animals need to be humanely and efficiently put to death, such as during disease outbreaks. Nevertheless, previous research by our laboratory and others indicates that carbon dioxide gas does not provide instantaneous loss of consciousness and causes levels of distress in animals, including pigs. While the capacity to euthanize animals in groups is a major advantage of gas methods, group euthanasia demands consideration of both direct and bystander effects. The objective of this study was to evaluate the effects of chamber stocking rate on facets of animal welfare and efficacy during gas euthanasia of young pigs. Crossbred pigs (390 neonatal and 270 weaned) designated for euthanasia at production farms were randomly assigned to group sizes of one, two, four, or six pigs. Gas euthanasia of each piglet group was performed in a Euthanex® AgPro chamber. The chamber air was gradually displaced with CO2 gas over 5 min to establish an in-chamber concentration of approximately 80% CO2. Pigs remained in that atmosphere for an additional dwell period of at least 5 min. Higher stocking rates were associated with higher CO2 concentrations after gradual fill for both age groups. While there was no evidence of an effect of stocking rate on latencies to loss of posture or last movement in suckling pigs, there was evidence of an effect on all measured efficacy variables in weaned pigs, with grouped pigs faster to succumb than solitary pigs. This finding is consistent with expected consequences of higher CO¬2 concentration at increased stocking densities. Aversive states and behaviours of focal pigs in the chamber were scored from video. Weaned solitary pigs displayed a high incidence of pacing and may have experienced isolation distress. Escape attempts were absent in neonates and were not linearly affected by stocking rate in weaned pigs. Although the risk of hazardous interactions was correlated with group size, this study provided no evidence that isolation during gas euthanasia would benefit animal welfare.
The objective of the second phase of the study was to evaluate the effects of chamber stocking rate on facets of animal welfare and efficacy during euthanasia of weaned pigs with argon gas. Two hundred and thirty-three weaned pigs designated for euthanasia at a commercial production farm were randomly assigned to group sizes of one, two, or six pigs. Gas euthanasia of each piglet group was performed in a Euthanex® AgPro chamber. The chamber was pre-filled with argon gas for 6 min in order to reduce the oxygen concentration to less than 2%. Pigs were then placed into the pre-filled chamber and gas flow was continued at a high rate to displace any introduced air and re-establish a fatally low residual oxygen concentration. Pigs remained in the chamber for 10 min and then were removed to test for signs of sensibility and life. There was no significant evidence of an effect of stocking rate on focal pig latencies to onset of neuromuscular excitation or last movement, as scored from video recordings. Solitary pigs were more likely to pace and make righting attempts in the chamber than paired or grouped focal pigs, although pigs in higher stocking rate treatments tended to retain posture longer. The results of this study do not support seclusion during argon gas euthanasia as a method of improving animal welfare.
In conclusion, gas euthanasia of solitary pigs was associated with greater distress when using carbon dioxide or argon gases. Stocking density of grouped pigs did not affect distress associated with gas euthanasia when groups comprised 6 pigs or less.