In most Mammals, the neonatal period is characterized by intense mother-young interactions in favor of pair bonding and neonates’ adaptation to the new environment. Here, we investigated the possible consequences of interferences at this particular stage on subsequent mother-young attachment and social development. We choose to investigate this issue on the domestic horse, as mother-young bonds are established rapidly during the first hours postbirth. First, we demonstrated that foals brought to their dams’ teats for the very first suckling bout appeared at later ages (1 to 3 months) to remain closer to their dams and to play less than control foals that had been allowed to suckle spontaneously. Second, we found that foals that were submitted to separation from their dams and handled for a single 1-hour bout post-birth, as compared to foals left undisturbed, showed patterns of unsecure attachment to the dam (strong dependency to the dam, poor play) and lowered features of social competence (social withdrawal, aggressiveness) at least up to adolescence (1 year). We concluded that first suckling and first contacts are crucial events in the development of secure attachment. These findings are discussed in terms of mother-young interactions, timing of interactions and relation between bonding and later social competency.