Coping behaviour in pigs : consequences for welfare and performance

Abstract

In this study we have investigated individual differences in coping with management-related stressors such as cross-fostering, weaning and mixing. Animals differ in the way they cope with stressors. An active coping strategy is characterized by an autonomous response, with higher heart rate, blood pressure and blood glucose levels, necessary for a fight or flight response, while a reactive (or ‘passive’) coping strategy is associated with conservation and withdrawal and characterized by a HPA response with elevated cortisol levels. The coping style is the preference of each individual for a certain coping strategy. Coping styles are determined by the personality, and the coping style and the situation determine the coping behaviour. In pigs, the backtest can be used as an instrument to measure (a dimension of) the coping style of the pigs at a young age. In this test, a piglet is put on its back and during 1 minute the number of escape attempts is recorded (mean 2-3, range 0-10). Active copers respond with many escape attempts (HR, high resisting), while reactive copers resist less (LR, low resisting). We performed backtests at 3, 10 and 17 days of age. Our study results confirmed that weaning and mixing of pigs induced acute, physiologically measurable stress responses, while moving of pigs did not. Cross-fostering and mixing of pigs resulted in lower cell-mediated immune responses at 9 weeks of age, and more active behaviour at 10-12 weeks of age. Mixing enhanced humoral and cell-mediated immune responses shortly after the stressor, but mixed animals were more susceptible for infection with Salmonella later on. Correlations between successive backtests were about 0.4. The backtest is relatively consistent in a stable environment, but our cross-fostering study showed that backtest response in young piglets could change in an extreme social environment (uniform HR or LR pens). We found no associations between backtest response and salivary cortisol rise after weaning or mixing. Backtest responses were predictive for a human approach test at 5-7 weeks of age, but not for a novel object test or an open door test, or for these group tests at 10-12 weeks of age. Possibly, these tests measure different behavioural characteristics. HR animals showed a better cell-mediated immune response after weaning and a lower risk of infection with Salmonella. HR animals also showed better performance results, with a higher daily weight gain in the fattening period, and a higher lean meat percentage and better carcass classification at slaughter. However, it cannot be concluded that one coping style is "better" than the other. HR animals showed better performance results in mixed HR/LR groups. HR animals showed better cell-mediated immunity and lower susceptibility for infection with Salmonella, but once infected they might show a lower humoral immune response. Because of the association with lean meat percentage and daily weight gain, selection for production parameters will probably favour HR animals in the current husbandry systems. The organic farming systems which offer more stimuli (group housing, straw, outdoor area), probably favour the LR sows, because they may be better equipped to cope with a less predictable environment, as other studies showed. Furthermore, in group housing systems, the most active and aggressive HR animals will be excluded in the selection process.