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Adverse early life experiences can permanently "programme" increases in stress reactivity in adulthood. We investigated whether weaning, which involves maternal separation, at different ages, altered subsequent behaviour, physiology and neuroendocrine function in young pigs. The litters of twelve primiparous sows were randomly allocated to weaning at 12 days (W12), 21 days (W21) or at 42 days (W42). Four test daughters from each litter ( n =48, 16 from each treatment) were studied from 11 days of age until 90 days of age. At 90 days of age, prior to culling, half of each treatment were challenged with restraint and isolation stress. There was a long-term reduction in salivary cortisol levels following weaning in all groups. W12 and W21 piglets gained weight more slowly after weaning, but by 90 days weights were similar in all groups. Behaviourally, W12 piglets showed more substrate directed and aggressive behaviours, postural changes, and nosing and belly nosing littermates. Surprisingly, W42 piglets exhibited the onset of belly nosing at 28 days while still in the presence of the sow. At 90 days of age there was no evidence that weaning age had persisting effects on the hypothalamic–pituitary–adrenal (HPA) axis. Restraint stress increased plasma ACTH and cortisol levels and CRH mRNA in the hypothalamus demonstrating that the absence of weaning age effects on HPA function was not due to a ceiling effect. This study is the first to investigate long-term effects of weaning age in pigs using a combination of behavioural, physiological and neuroendocrine measures. The results suggest that variation in the age of weaning does not apparently programme the HPA axis but has potentially deleterious effects on behaviour certainly up to 56 days of age. However, our study also raises complications when considering the welfare implications of weaning age. Specifically more information is needed on the causes and long term implications of the sustained elevations of salivary cortisol we observed over the suckling period. The occurrence of belly nosing in later weaned pigs also suggests that care is required when designing environments where sows and piglets are housed together beyond 28 days. |
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