Patterns and drivers of uncovered outdoor run use in group-housed sows after weaning

The integration of an outdoor run into group housing systems may contribute to compliance with increasingly stringent welfare regulations for sows. However, there is limited empirical data on post-weaning sows. This study investigated the use of an outdoor run by group-housed sows during the first four days after weaning under commercial conditions. Behavioural data from 85 sows over 14 observation days were collected using ten-minute scan sampling, focusing on outdoor presence, and lying behaviour. Environmental conditions as well as sow- and group-level characteristics were considered to identify the factors influencing outdoor run use. On average, 33% of sows were observed outdoors per scan, with clear diurnal patterns characterised by peak activity in the morning and afternoon. The proportion of sows outdoors declined from 40.3% on day 0 to 21.9% on day 3 post-weaning, while the proportion of sows lying outdoors increased, reflecting changes in functional use of the outdoor run over time. Environmental conditions significantly influenced outdoor use in a time-dependent manner. Precipitation strongly reduced outdoor presence, whereas solar radiation promoted it. However, thermal conditions affected both presence and lying behaviour, with higher thermal load generally reducing outdoor use, particularly during midday and afternoon. Group composition also contributed, as higher parity heterogeneity reduced outdoor presence. Overall, outdoor use of group-housed sows after weaning was driven by temporal patterns and environmental conditions, with distinct effects on activity and resting behaviour. These findings highlight the importance of flexible, weather-adapted outdoor-access systems that support behavioural expression and thermal comfort under commercial conditions.