In nature, piglets are exposed to a wide range of bacteria that promote a balanced (healthy) gut microbiota. On the farm, routine cleaning can limit exposure to beneficial bacteria leading to an unbalanced gut microbiota. An unbalanced gut microbiota increases the risk of health problems and alters animal behaviour and stress reactivity. Supplementing animals with beneficial bacteria (probiotics) early in life promotes the development of a balanced gut microbiota but the effects on health, growth, feeding behaviour and stress reactivity in pigs are not known. This project aims to investigate the effects of probiotics supplementation on the gut microbiota, growth, food intake and stress responses in pigs. Our hypothesis is that probiotics will increase stress resilience, which promotes growth and health. We will investigate the optimal route of administration under commercial conditions and perform a cost-benefit analysis to assess the economic feasibility of using probiotics on farms.