Preslaughter treatment of pigs : consequences for welfare and meat quality

Chapter 1 is a general introduction to this thesis. Welfare of slaughter pigs may be impaired by preslaughter treatment. Frequently used welfare indicators are behavioural and physiological responses. The responses to preslaughter treatment may also have an effect on perimortem muscle metabolism and thereby on meat quality. Behavioural and physiological responses to acute stress could lead to properties of PSE (pale, soft, exudative) meat via an associated increase in muscle exercise, muscle temperature, or elevated hormone levels. Depletion of muscle glycogen by metabolic exhaustion will result in DFD (dark, firm, dry) meat.The purpose of this thesis was to gain more insight in the relative contribution of common preslaughter treatment factors in evoking behavioural and physiological responses indicative of disturbed welfare. Furthermore, attention was paid to the effects of conditions during the fattening period on responses to preslaughter treatment, and to the consequences for meat quality.Results from field observations on pigs in five Dutch and four Belgian slaughter-houses are described in Chapter 2. Brooms and/or electric goads were used to move pigs in all slaughterhouses and in one Dutch abattoir handling with sticks seemed to contribute considerably to skin damage. The level of agonistic behaviour was lowest during the first ten minutes in lairage. In the Dutch slaughterhouses, large individual differences in aggression between pigs were observed. Pigs in Belgian slaughterhouses were tranquillized prior to transport resulting in generally low levels of agonistic behaviour. In both the Dutch and the Belgian slaughterhouses, skin damage was higher in the front and in the middle region than in the hind region of the pig. For the Dutch slaughterhouses skin damage was associated with time kept in lairage and stocking density. It was suggested that to decrease aggression and skin damage and thus to increase welfare in the visited Dutch slaughterhouses, stocking density should be lower and pigs should be slaughtered as soon as possible after arrival.The response of pigs to two simulated lairage events, driving and mixing, was investigated in an experiment described in Chapter 3. Five groups of six and five groups of seven 70-kg pigs were transported for 40 min on a lorry and subjected to one of the following treatments: two groups were driven down a passage; four groups were mixed for one hour (A and B together, C and D together); four groups received the driving treatment immediately followed by the mixing treatment ('combined treatment'). Initial transportation led to increased concentrations of cortisol. Behaviour during driving was not correlated with concentrations of cortisol after driving and cortisol did not increase relative to post-transport levels. Frequency and duration of fighting during mixing were positively correlated with aggression in the home pen and increase in concentrations of cortisol during mixing. One hour after the start of mixing, concentrations of cortisol had decreased relative to post-transport levels. After the combined treatment, all correlations described for the mixing treatment were absent and concentrations of cortisol increased relative to post-transport levels. Skin damage was highest after the mixing treatment. The responses observed indicate that the combination of driving and mixing, which is very common in lairage, leads to a greater response than in the case of each individual treatment. In the Appendix of Chapter 3, it is described that pigs that had high cortisol concentrations after mixing and fought more during mixing, had a higher pH 45 min post-mortem, which is indicative of DFD.Chapter 4 describes behavioural and physiological responses of slaughter pigs to the shot biopsy, a method commonly used to study muscle tissue processes and predict meat quality. From 10 23-wk old gilts, one sample from the longissimus muscle was obtained using a cannula connected to a captive bolt. Ten other gilts were used as a control and received a sham shot. One week later, a second shot biopsy was taken from the experimental group. Behavioural and salivary cortisol responses to both biopsies were the same. All pigs flinched in response to the biopsies. Salivary cortisol was increased after 15 min. In both tests heart rate increased in response to the presence of the technician. In response to the first biopsy heart rate increased but not in response to the second biopsy. The experimental pigs showed a decrease in initiating contact with the technician in the second test. It is concluded that shot biopsy had a significant acute effect on behavior and physiology. Therefore, the usefulness of the technique in studies on the relation between pre-slaughter stress and meat quality is limited.In Chapter 5, the behavioural and physiological responses of pigs to transport and subsequent exposure to slaughterhouse sounds were examined. Fourty-one groups of four slaughter pigs were separately loaded onto a lorry and transported for 25 min. Another 43 groups were loaded onto the lorry which then remained stationary for 25 min. Following unloading pigs were moved to a novel holding pen. Either one of the following sounds was played at 85 dB(A) for 10 min: Squeals of pigs in front of the restrainer, Machines in lairage, White noise, or Control (no sound). Heart rate was higher on the lorry for transported groups. In contrast, heart rate was lower during unloading, the sound exposure period and the post-sound period for transported groups compared to stationary groups. Salivary cortisol concentrations were higher after transport than after the stationary period and remained higher for transported pigs after the sound exposure period. Transported pigs spent less time exploring the race and were less active than pigs from the stationary lorry. Pigs exposed to the Machines and White noise treatment spent more time close to their group-mates compared to Control pigs, with pigs subjected to the Pig sound being intermediate. Heart rate and cortisol levels did not differ significantly between sound treatments. It is tentatively suggested that social support from conspecifics may protect pigs from potentially adverse effects of exposure to lairage sounds.Chapter 6 studies the effects of regular moving and handling during the fattening period on behavioural and physiological responses of pigs during preslaughter treatment, and consequences for meat quality. From the age of 10 wk onwards, 144 pigs were housed in groups of four and subjected 10 times to one of the following treatments. The Environment treatment allowed pigs to move freely outside their home pen and in addition accustomed the pigs to transport in a box. In the Handling treatment, pigs were handled by an experimenter in the home pen. A Control treatment was also included whereby the pigs were subjected to no treatment. During preslaughter treatment, the stockmen needed less time to move Environment pigs out of their pen and into the transport box. There were no differences between treatments in salivary cortisol concentrations before or after transport. Environment and Handling pigs had paler meat than Control pigs. Glycogen content at 1 h post-mortem and waterholding capacity were lower in Environment pigs compared to Control, but this did not lead to a higher incidence of PSE-meat. It is concluded that those pigs that had experience with leaving their home pen were much easier to handle at loading. Pigs that are easier to move are less likely to be subject to rough handling, which implies improved welfare, while the workload for stockmen is reduced. Differences in meat quality due to treatment were slight.In Chapter 7, the effects of moderately enriched housing conditions on behavioural and cortisol responses during preslaughter treatment are described, as well as the consequences for meat quality. A total of 48 slaughter pigs were either raised in intensive housing conditions ("Poor" treatment: standard farrowing crates followed by standard rearing and fattening pens) or in more extensive conditions ("Enriched" treatment: larger farrowing pens followed by larger rearing and fattening pens, all provided with straw). During preslaughter treatment, stockmen needed significantly less time to load pigs from Poor housing conditions. Enriched pigs showed higher salivary cortisol levels before transport, but the cortisol increase in response to transport was higher in Poor pigs. Enriched pigs showed more exploratory behaviours in lairage. Glycogen content 1 h post-mortem showed a trend to be lower in pigs from Enriched pens, while levels did not differ at sticking or on the previous day. There were no differences in any of the meat quality variables measured.It is discussed in Chapter 8 that the behavioural and physiological responses observed indicate that particularly transportation and mixing have a negative impact on welfare. Journeys should be as 'smoothly' as possible, which implies careful driving on motorways. Either pigs should be slaughtered immediately upon arrival at the slaughterhouse or legislation should prohibit to mix pigs during transport and lairage. Further research can be carried out to study the importance of social support from conspecifics during lairage to protect pigs from potentially adverse effects of exposure to lairage sounds.The willingness to move during preslaughter treatment can be significantly increased by regularly moving pigs during the fattening period. Future studies should estimate the minimum number of sessions needed to achieve this effect.The relationship between preslaughter handling, high energy reserves at slaughter and incidence of PSE merits further research. When considerate preslaughter treatment leads to affected carcasses, it should be examined to what extent processing factors such as rapid chilling can reduce the expression of PSE.