Behavioral response of weaned pigs during euthanasia with CO2, CO2 with butorphanol, or nitrous oxide

Authors: CAVUSOGLU, ENVER - Uludag University; RAULT, JEAN-LOUP - University Of Veterinary Medicine; GATES, RICHARD - University Of Illinois; Lay Jr, Donald

Submitted to: Animals
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/27/2020
Publication Date: 5/1/2020
Citation: Cavusoglu, E., Rault, J., Gates, R.S., Lay Jr, D.C. 2020. Behavioral response of weaned pigs during euthanasia with CO2, CO2 with butorphanol, or nitrous oxide. Animals. https://doi.org/10.3390/ani10050787.
DOI: https://doi.org/10.3390/ani10050787

Interpretive Summary: The swine industry is forced to euthanize a significant number of pigs due to injuries, hernias, and unthriftiness. The majority are euthanized using carbon dioxide asphyxiation. However, concerns as to the humaneness of carbon dioxide are prevalent. One alternative is the use of nitrous oxide. We conducted this study to compare euthanasia of neonatal pigs using nitrous oxide or carbon dioxide. In addition, we administered an analgesic to a third treatment that used carbon dioxide to determine if we could elucidate behaviors indicative of pain. Pigs experienced ataxia and loss of posture at the same time, regardless of treatment. However, pigs euthanized with carbon dioxide made more escape attempts, but fewer squeals. Conversely, pigs euthanized with nitrous oxide made fewer escape attempts, but more squeals. Pigs in both treatments experienced heavy breathing and open mouth breathing prior to loss of posture. Administration of an analgesic during asphyxiation with carbon dioxide was unable to elucidate behaviors indicative of pain. It is inconclusive as to whether using nitrous oxide is significantly better than using carbon dioxide; but the results do show that its use is just as good, and possibly more humane.

Technical Abstract: The swine industry is forced to euthanize a significant number of pigs in the first few weeks of life due to injuries, hernias, and unthriftiness. The majority are euthanized using carbon dioxide (CO2) asphyxiation because this is an approved method of euthanasia that can be employed on the farm. However, concerns as to the humaneness of CO2 are prevalent. One alternative is the use of nitrous oxide (N2O), commonly used in dental practice to relieve pain and anxiety. Therefore, we conducted this study to compare euthanasia using N2O (n = 9) or CO2 (n = 9). In addition, we administered an analgesic, butorphanol at 0.2mg/kg BW, to a third treatment (n = 9) that used CO2 (CO2B) to determine if we could elucidate behaviors indicative of pain. Farm staff determined which pigs, approximately 21 d of age, had to be euthanized. Pigs were euthanized in pairs such that 18 pigs per treatment were used. The CO2B pigs received butorphanol 30 min prior to euthanasia. The chamber (61 x 38 x 46 cm) was modified to have two clear plastic sides to allow video recording and direct observation. The N2O pigs spent more time standing compared to CO2 and CO2B pigs (144.6 ± 5.8, 123.2 ± 10.8, and 84.4 ± 3.4 s respectively). Duration of lying, inactivity and locomotion did not differ among treatments (P > 0.10). Pigs in all treatments progressed from performing heavy breathing to open-mouth breathing to loss of posture. The CO2 and N2O pigs spent the same amount of time (26.1 ± 8.4 and 34.4 ± 8.4 s respectively, P > 0.10) performing heavy breathing with CO2B pigs spending less time (2.5 ± 8.4 s, P < 0.03). The CO2 and CO2B pigs spent more time open-mouth breathing (59.4 ± 10.3 and 49.4 ± 7.3 s respectively, P > 0.10) with N2O pigs spending the least (19.3 ± 8.6 s, P < 0.01). The CO2 and N2O pigs lost posture at similar times (latency of 145.0 ± 17.3 and 162.6 ± 7.0 s respectively, P > 0.10), with the CO2B pigs losing posture the soonest (101.2 ± 4.7 s, P < 0.01). The CO2B pigs had more escape attempts than did the CO2 or N2O pigs (16.4 ± 4.2, 4.7 ± 1.6, 0.3 ± 0.2 respectively; P < 0.0004), while CO2 and N2O pigs did not differ (P > 0.10). However, the N2O pigs performed more squeals than either the CO2 or CO2B pigs (9.0 ± 1.6, 2.8 ± 1.2, 1.3 ± 0.6 respectively, P < 0.001) with no difference between the CO2 and CO2B pigs (P > 0.10). The CO2 and N2O treatments caused loss of posture at similar times indicating that they are similarly effective. The shorter time displaying open mouth breathing indicates that the N2O treatment is less stressful, however; the larger number of squeals for these pigs suggests the opposite. This result could be due to the CO2 treatment pigs not being able to squeal when they are open-mouth breathing. Comparing pigs in the CO2B treatment to the CO2 treatment, it was not apparent that any behavior measured was indicative of pain.