Ž .
Ž .
induction of breathlessness Gregory et al., 1990 . Raj and Gregory 1995 found that a majority of pigs showed a strong aversion to 90 of CO . After 24 h of fasting,
2
pigs refused to enter an atmosphere of 90 CO for a food reward. Exposing pigs to
2
15–20 s of a 70 mixture of CO in air produced an excitation phase with movements
2
which resemble escape behaviour and this response has been considered unacceptable Ž
. Ž
. Grandin, 1988; Gregory et al., 1990 . However, research by Forslid 1987 and Troeger
Ž .
and Woltersdorf 1991 based on electroencephalogram recordings indicates that the animal is unconscious before reaching this phase of behaviour. Induction of uncon-
sciousness prior to the excitatory response should reduce the concern for the welfare of CO -stunned pigs, although there could still be a window of time between unconscious-
2
ness and insensibility to pain. Exposure to CO is only one aspect of the slaughtering process. In fact, a number of
2
components of the slaughter process may contribute to concern for animal welfare. Aspects of this process that pigs may find unpleasant or aversive and that potentially
impact on welfare include lairage systems and handling systems. Delivery to the CO
2
stunner is often through a raceway at ground level, where pigs enter a well-lit crate, sometimes in small groups. This is different to most electrical stunning systems, where
pigs enter a V-restrainer belt. Aversive handling systems can contribute to acute stress Ž
. prior to slaughter as well as affecting meat quality D’Souza et al., 1998 . Therefore, it
is important when studying effects of stunning methods on welfare and meat quality, to study the differences in aversiveness of the components of the slaughter process such as
handling systems associated with the stunning unit.
Ž . The objectives of this study were 1 to examine the relative aversiveness of CO to
2
Ž . pigs and 2 to compare the aversiveness of a CO stunner crate to the aversiveness of a
2
V-belt restrainer used for electrical stunning.
2. Materials and methods
2.1. Animals and location The experiments took place in an experimental abattoir. There were no other
activities in the abattoir at the time of training and testing. Each experiment consisted of Ž
. Ž
two replicates. Thirty 4-month old boars Landrace = Large White were used 15 in .
each replicate in experiment 1 and another 30 boars were used in experiment 2. Ž
. Twenty-eight 4-month old boars of the same breeding were used 14 in each replicate
in experiment 3. All animals were unfamiliar with all aspects of the abattoir. For the duration of each replicate, the pigs were housed in one large group in a large pen about
20 m away from the CO crate; the pen was adjacent to the abattoir, but not part of the
2
abattoir system. The pigs were housed in this pen from the day before the experiment until the experiment was completed for that particular replicate. The walkway where
ease of movement was observed and the crate itself were visually isolated from the Ž
. group pen by a solid wall. The walkway length of 15 m was bordered by steel panels
and at the end, a left and then right turn led to the CO stunner while a right and then
2
Ž .
left turn led to the V-restrainer experiment 3 . Pigs were individually taken at random
from the group for their training sessions and were returned to an adjacent pen, which was separated by a wire mesh division. The pigs were fed at 0730 h, an hour before the
start of the training sessions, and at 1600 h, after the training sessions were finished.
2.2. Training procedure experiments 1 and 2 The aim of experiments 1 and 2 was to compare the aversiveness of CO with a
2
known aversive stimulus, an electric prodder. In order to be able to do this pigs were first trained to walk into the empty CO stunner. After the training period, they were
2
Ž .
Ž .
exposed to the aversive stimulus CO or prodder or a control treatment no stimulus
2
at the bottom of the CO pit. Their reluctance to enter the stunning crate after treatment
2
would indicate the aversiveness of CO relative to an electric prodder.
2
The pigs were trained to walk into the CO stunner crate, during the first 2 days of
2
the experiment, where they were rewarded with two to three pieces of apple. However, especially during the first few sessions, most pigs showed very little interest in the
reward due to apparent wariness of the novel environment. If the pig was reluctant to move through the walkway, it was encouraged with increasing force by the handler. The
crate was lowered into the empty CO
pit and remained stationary for 10 s at the
2
bottom, after which it would ascend. To ensure that all pigs were trained to a similar level of learning to enter the stunner crate, the pigs were considered trained when they
walked into the CO crate while achieving the following criteria three times during the
2
Ž last four sessions: a time limit of 26 s with a maximum of one or two handling bouts a
. Ž
. push or a slap by the handler andror one or two evasions by the pig baulks and turns
with a maximum of three handling bouts andror evasions. Although these criteria seem somewhat arbitrary, a pilot study indicated that pigs moved readily into the stunner crate
when they where moved within these criteria. There was a maximum of 15 training sessions per day, and once a pig was considered trained no further training occurred
until the next day. The same training procedure was repeated the next day to enforce the learned behaviour. Some pigs did not achieve the training criteria in 2 days, and these
pigs were excluded from the study.
2.2.1. Experiment 1 After the training period, pigs were assigned to three treatments, balanced for ease of
training. The aim of this experiment was to compare the aversiveness of CO and an
2
electric shock. The CO stunner was considered a convenient way of presenting the
2
stimuli to the pigs. Treatment was applied once on the third day, after two additional training sessions on that day to enforce the learned behaviour. Thirty minutes after
treatment was applied, the pigs were induced to enter the stunner crate during testing Ž
. sessions in the same order as when treatment was applied until they again achieved the
Ž previous training criteria
26 s with a maximum of three handling bouts andror .
evasions three times during the last four training sessions. Ease of movement during the testing sessions after treatment was used to measure the aversiveness of the treatment
applied. Not all pigs would achieve the criteria after treatment within the maximum seven sessions.
On the day of treatment, pigs assigned to the CO treatment were treated first,
2
Ž .
because of the time required to fill the CO pit 30 min . Pigs within each treatment
2
were taken from the group pen at random and the control and prodder treatments were applied in a random order. Pigs were tested in the same order during the testing sessions
after treatment as during treatment imposition. The treatments were as follows.
Ž . 1 Control: Pigs were walked individually to the entrance of the abattoir holding
yard and the time and ease of movement were measured from this location to the CO
2
stunner crate. After the pig entered the crate, it descended to the bottom of the CO pit,
2
remained stationary for 10 s and then ascended. This procedure was identical to the training period, except pigs were not rewarded with pieces of apple.
Ž . 2 Prodder: As for treatment 1, but during the stationary period at the bottom of the
Ž .
pit, the pigs were remotely given two brief electric shocks - 1 s with a commercial cattle prodder. The electric prodder was used as a known aversive stimulus.
Ž . 3 CO : As for the control but without a stationary period, with the pit filled with
2
CO providing a concentration of 90 CO at the bottom of the pit. The concentration
2 2
at the top where the pig entered the crate was close to zero. The concentration of CO
2
that the pig was exposed to increased during descent into the pit, with the concentration reaching 60 halfway down the pit, and 90 at the bottom. The descent took about 15
s, and immediately after reaching the bottom, the crate ascended.
2.2.2. Experiment 2 During exposure to 90 CO in experiment 1, pigs became unconscious. There was
2
some concern that their memory may have been affected, which would make the subsequent measurement of the aversiveness of the experience unreliable. Thus, experi-
ment 1 was repeated with a shorter exposure to a lower concentration of CO . Again,
2
pigs assigned to the CO treatment were treated first, because of the time required to fill
2
the CO pit. The treatments were the same as in experiment 1, except for the following
2
differences. In the control treatment, the crate descended halfway down the CO pit,
2
remained stationary for 10 s and then ascended. In the prodder treatment, the two brief electric shocks were given during the stationary period halfway down the pit. In the CO
2
treatment, pigs were exposed to a concentration of 60 CO halfway down the pit. The
2
decent halfway down the pit took about 7 s, and immediately on reaching the halfway point, the crate ascended.
2.3. Training procedure experiment 3 The aim of experiment 3 was to compare the aversiveness of the CO crate and the
2
V-restrainer belt as a restraining system during stunning. In order to be able to do this, pigs were first trained to enter the stunning systems while they were stationary. During
treatment, both the CO crate and the V-belt restrainer were set in motion. Pigs were
2
assigned randomly to treatment before the start of the 2-day training period. Half the pigs were trained to enter the CO stunner crate, where they were held for 30 s while the
2
crate remained stationary. The other half were trained to enter and walk through the
V-restrainer while the belt was stationary and a temporary floor was in place. The training procedure and training criteria were similar to experiments 1 and 2. Pigs were
rewarded in both systems with pieces of apple. The third day started with two training sessions, followed by a session when treatments were applied, in a random order, to both
groups. Following removal of the temporary floor in the V-restrainer, treatment in this system consisted of individually moving each pig to the V-belt, which was then
switched on until the pig reached the other end, where it left the V-restrainer. Treatment in the CO crate consisted of individually moving each pig to the crate, lowering the
2
crate to the bottom of the empty CO pit, where it remained stationary for 10 s and than
2
raising the crate. After treatment was applied once to each pig, the pigs were individu- ally moved to enter the allocated stunning unit during testing sessions until they again
achieved the previous training criteria three times during the last four training sessions. Ease of movement during the testing sessions after treatment was used to measure the
aversiveness of the treatment applied.
2.3.1. Experiment 3 After a 2-day training period, treatment was applied on day 3. On this day, pigs were
subjected to two training sessions before treatment was applied during the third session. The treatments were as follows.
Ž . 1 CO crate: Pigs were walked individually to the entrance of the abattoir holding
2
yard and the time and ease of movement were measured from this location to the CO
2
stunner crate. After the pig entered the crate, it descended to the bottom of the CO pit
2
Ž .
with no CO and then immediately ascended.
2
Ž . 2 V-restrainer: Pigs were walked individually to the entrance of the abattoir holding
yard and the time and ease of movement were measured from this location to the V-restrainer. The V-belt was set in motion at the moment the pig entered the V-re-
strainer. The raceway leading up to the two treatments was identical, with the pigs having to
turn left into the CO crate and right into the V-restrainer. It was not possible to balance
2
Ž .
the location left vs. right of treatments relative to the race since the CO stunner and
2
V-restrainer were permanent fixtures of the experimental abattoir. The entrance to the Ž
. CO
crate consisted of a step down of 10 cm between narrow doors, whereas the
2
Ž entrance to the V-restrainer consisted of a slightly sloping ramp 1.5 m long, 30 cm
. high . The CO crate was in an enclosed area, but was well lit with fluorescent lights.
2
The V-restrainer was in an open space with views through the restrainer into the abattoir. The entry characteristics to the two systems are considered as inherent
components of each treatment.
2.4. Statistical analysis Ž
. An analysis of covariance Genstat 5 was performed to analyse the effects of
Ž .
Ž .
treatment experiments 1 and 2 or system experiment 3 on time to enter the stunning unit. The first two sessions prior to treatment were used as covariates. Each session was
analysed separately, with the individual pig as the experimental unit and the data
blocked on replicate. Although it is recognised that the treatments were not applied at random due to practical constraints, this was not included in the statistical analysis.
3. Results
