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In my last article, I described how newly-weaned pigs encounter a number of abrupt, simultaneous stressors (challenges) that can compromise disease status and immune function, and therefore pig health and welfare, and can have long-term consequences for some piglets in terms of their subsequent performance, survival, disease state and responses to stressors experienced at a later occasion (or occasions) in life. I discussed how preparing the correct physical environment, easing the nutritional transition from milk to solid feed, and optimising health management can reduce the extent of the negative effects weaning can have on production and welfare. However, and despite these efforts, the fact remains that the common practice of mixing unfamiliar (non-littermate) piglets at weaning causes aggression and fighting that can additionally cause adverse welfare, health and performance outcomes, concurrent with activation of the piglets’ stress responses.
A number of different strategies have been examined to reduce aggression at weaning, for example, mixing piglets into pens according to size (e.g., into light, medium and large bodyweight), the use of selective drugs and compounds, pheromones, masking odours and tranquilisers, increasing dietary tryptophan levels to modulate brain serotonin levels, and decreased shed illumination. However, these methods have tended to be impractical or unsuccessful, and where some success is observed, responses inclined to be variable and short-lived. An alternative approach to reducing fighting at weaning to improve piglet welfare, and enhance socio-cognitive development, health and production, is to familiarise non-littermate piglets with each other before the point of weaning, in lactation, so that when they are weaned and housed together in the nursery, social adaptation is improved and antagonism of non-littermate piglets towards each other is reduced. This may have further benefits for post-weaning adaptation beyond only behaviours, e.g., greater pre-weaning familiarisation with creep feed may reduce latency to the first meals after weaning. This notion of pre-weaning socialisation mimics, to some degree dependent upon the system implemented in the farrowing shed, sow and piglet behaviour in free-ranging (natural) conditions. Here, piglets are predisposed to form social bonds and engage in various behaviours, including beneficial play, as young as two weeks of age.
The concept of pre-weaning socialisation (or co-mingling) of piglets, for beneficial post-weaning welfare, health and (or) production outcomes, is not necessarily new, dating back over 25 years. However, its widespread implementation has been limited by practical considerations in the farrowing room environment and perhaps an unawareness by industry to its potential roles and benefits in modern-day pork production. Nevertheless, and driven by issues including the future viability of the ‘traditional’ farrowing crate, greater emphasis on welfare, increased litter size, and our understanding of the need for newly-weaned pigs to have a good start in the nursery, there has been growing interest in this concept more recently. This is reflected in the growing number of peer-reviewed publications and articles appearing in the last 5-10 years.
How, then, can pre-weaning socialisation (or co-mingling) of piglets be done? Practically, piglet socialisation methods in indoor production systems have typically comprised: (i) systems that allow only the piglets to mix while the sow remains penned in her crate, e.g., by creating piglet doors between adjacent farrowing pens, by removing the partitions between adjacent farrowing pens, and (or) by providing access to an additional piglet area (Figure 1; e.g., “kindergarten” system); or (ii) group-housing systems (“multi-suckling” systems), where both sows and piglets can freely interact, e.g., by removing a barrier at the entrance of the farrowing pen, by removing the farrowing pens themselves, or even by transferring sows and piglets from single housing to group housing (Figure 2). In either system, possibilities exist for an extended weaning age, an increase in the available creep space area, and mating of sows during lactation. As would be anticipated, there are numerous advantages and disadvantages associated with both types of systems, and the practical implementation of either requires much consideration and thought given the investments (e.g., in labour, in redesign) that would be required. Naturally, in outdoor pork production systems, some of these constraints do not exist.
A common feature of mixing non-littermate piglets with each other in lactation is increased aggression (Figure 3), with disruptions also to sow behaviour including possible teat damage and suckling frequency, the sucking behaviour of piglets, and poorer piglet growth, but these effects tend to be transitory. To the contrary, findings indicate that socialised piglets learnt social skills which benefited them in the longer term, enabling them to more rapidly form stable dominance hierarchies during future aggressive encounters with unfamiliar pigs after weaning. Furthermore, socialising piglets early in life by mixing litters before weaning not only can have a positive effect on behaviour (more play, less fighting) and performance after weaning, but can also impact physiology. A recent study by Saladrigas‑García and colleagues (2021) examined the impacts of early socialisation and environmental enrichment (with toys) of suckling piglets on the caecal microbiota and metabolic responses before (-2 days) and after weaning (+ 3 days). These authors’ reported a lack of effects of pre-weaning socialisation and enrichment, but after weaning, differences in the microbial structure, a reduced jejunal expression of the TLR2 (toll-like receptor 2) gene, a transmembrane receptor that plays a fundamental role in pathogen recognition and activation of innate immunity, and decreases in metabolites including triglycerides and fatty acids suggested an impact on reduced energy metabolism consistent with a reduction in aggressive behaviours in these animals.
All in all, experimental data and anecdotal experiences would appear to suggest that creating a physically and socially enriched environment in early life, such as by socialisation or co-mingling, can positively modify piglets’ responses after by means of diminishing the impacts of social stress.
