Pre-weaning socialization and environmental enrichment affect life-long response to regrouping in commercially-reared pigs

Weaning and other regrouping events as routine work in commercial farms cause stress to pigs and compromise their welfare. Several studies found positive outcomes to mitigate weaning stress when piglets were socialized (i.e. co-mingled) or raised with enrichment materials in research settings. However, research in commercial settings is lacking. We aimed to investigate the effects of early-life socialization and environmental enrichment on pigs’ life-long response to regrouping under commercial conditions via behavioral observations, aggression-associated skin lesions, ear biting lesions, and salivary stress biomarkers. A total of 661 Danbred pigs were studied from birth to slaughter. Two treatments were differentiated pre-weaning: the control group (CON; 24 litters), where sows and their litters were individually housed in barren farrowing pens; and the enriched group (ENR; 24 litters), where six enrichment objects per litter were provided from birth and two neighboring litters were socialized from Day (D) 14. Pigs were regrouped on D25 (weaning), on D71 (finishing), and at pre-slaughter, while keeping animals from the same treatment together, except at pre-slaughter. Behavioral observation took place on D15, D22, D29 and D36. Lesions were scored on D13, D15, D24, D26, D27 and on carcasses. Saliva was sampled on D24, D26 and D27. Ear biting lesions were scored on D69 and on carcasses. ENR showed 1.6-times more pen and object exploration (P = 0.03) pre-weaning, and CON showed 2-times more agonistic behavior (P = 0.01) post-weaning. Lesions increased after socialization and weaning in both treatments (P < 0.0001), but the increase after weaning was 3.3-times greater in CON (P < 0.01). Salivary cortisol (CORT) and chromogranin A (CgA) increased after weaning in both treatments but the rise was significant only in CON (CORT: 1.5-times; CgA: 6.2-times, both P < 0.0001). Salivary α-amylase increased after weaning in CON (P = 0.05) but decreased in ENR (P < 0.0001). On carcasses, CON had more lesions on the head (P = 0.05) and front parts (P = 0.02) whereas ENR had more lesions on the rear (P = 0.05). Ear biting was unaffected and ear biting lesions did not differ between treatments on D69 and on carcasses. The present study showed a lasting positive effect of enriching the neonatal environment both physically and socially, on piglet object exploration pre-weaning, mitigation of weaning stress, and reduced aggression post-weaning until slaughter. These results, obtained under commercial conditions, provide a promising avenue for improving life-long welfare of pigs.