Investigation of welfare impacts of gaseous methods for on-farm euthanasia of suckling piglets : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Physiology at Massey University, Manawatu, New Zealand
Blunt trauma (BT) to the head is the most common method used for on-farm
euthanasia of pre-weaned piglets. When performed correctly, loss of consciousness is
immediate, but the potential for delivery of sub-lethal blows, along with aesthetic
unacceptability to many operators, has lead to the need for alternative methods to be
One recommended alternative is exposure to 100% CO2. Although gas
euthanasia is potentially more reliable and less disturbing to perform than BT, there are
concerns that CO2 may induce breathlessness and pain before loss of consciousness,
thus negatively affecting piglet welfare. This research aimed to evaluate the welfare
impact of alternative gases, relative to CO2, for piglet euthanasia.
A small pilot study was conducted to select appropriate gases for further
evaluation. This identified 100% argon (Ar) and a mixture of 40% CO2-60% argon
(CO2-Ar) as possible alternatives to 100% CO2 (CO2) for piglet euthanasia.
The relative welfare impacts of CO2, Ar and CO2-Ar were evaluated in two
studies. These studies aimed to identify the interval following gas exposure in which
the animal may be conscious, and to identify evidence of welfare compromise within
this interval. Identifying the period of possible consciousness is important in evaluating
welfare impact, as this defines the time period in which the animal is capable of
perceiving potential negative experiences associated with euthanasia.
In the first study, conscious animals were exposed to the test-gases in a
purpose-built chamber. Behavioural and physiological data including escape attempts,
vocalisation, loss of coordination, loss of posture, respiratory effort, convulsions,
gasping, and respiratory arrest were recorded until death. Loss of posture has
previously been used to infer the onset of unconsciousness, whilst escape attempts,
vocalisation and laboured breathing are associated with the experience of pain,
aversion and distress in animals. Piglet behaviour was examined for evidence of
negative experience prior to the onset of unconsciousness.
In the second study, EEG and ECG data were recorded from anaesthetised,
immobilised pigs during exposure to the same test gases used in the first study.
Changes in the amplitude of the raw EEG can provide information on the level of
consciousness. Changes in the EEG power spectrum, derived from mathematical
transformation of the raw EEG, can provide evidence of noxious stimulation in
anaesthetised mammals. EEG recorded during exposure to test gases was analysed to
determine the likely latency to loss of consciousness with each gas, and to determine
whether nociceptive processing occurred. Changes in heart rate, derived from the
ECG, are frequently used as indicators of acute stress in mammals. ECG recorded
during gas exposure was examined for indications of physiological stress responses.
Behavioural data suggested that the latency to onset of unconsciousness did
not differ between gases. However, the changes in the amplitude of the EEG
suggested that loss of consciousness may occur sooner with CO2 than with Ar or CO2-
Ar. Behavioural data indicated that piglets found CO2 exposure more aversive or
unpleasant than exposure to either Ar or CO2-Ar. However, CO2-Ar induced greater
respiratory stimulation than Ar alone, suggesting that Ar caused the least negative
welfare impact of the 3 gases. ECG data showed that heart rate increased prior to
likely loss of consciousness in piglets exposed to CO2 and CO2-Ar but not Ar alone,
suggesting that Ar exposure does not induce a physiological stress response.
Contrary to expectations, there was no evidence of nociception in piglets
exposed to either 40 or 100% CO2, although this may have been influenced by the
Together, these data suggest that whilst CO2 induces more rapid loss of
consciousness than Ar, it also results in significantly greater welfare impact prior to loss
of consciousness. The addition of CO2 to Ar may provide some welfare advantage over
CO2 alone, but not over Ar alone. From a welfare perspective, Ar is preferable to either
CO2 or CO2-Ar for piglet euthanasia.