Multiple fostering in dairy cows: Social attachment can form between calves and their foster cow

The practice of using foster cows to nurse multiple calves simultaneously is becoming increasingly common in European dairy farming, particularly within organic systems. This management context provides a unique opportunity to investigate whether calves reared under such conditions can express attachment responses towards their foster cows, an aspect of calf social and emotional development that remains poorly documented. In this observational study, twenty-five mixed-breed calves and nine foster dairy cows, some of which had moderate lameness or a history of mastitis, were monitored. Calves’ preferential proximity towards their foster cow was recorded on pasture when calves were 2.5, 5.5 and 8.5 months old. At the same ages, they were also tested using a preference/separation paradigm between their foster cow and a familiar cow from the same herd. On pasture, calves were repeatedly observed closer to their foster cows than to other cows (p < 0.001), irrespective of age. During preference tests, calves clearly chose their foster cow over the familiar cow (p < 0.001), although this preference decreased with age (p < 0.01). At all ages during the separation, calves displayed behavioural signs of distress, including increased vocalisations (p < 0.01) and locomotion (p < 0.05), compared to the preference phase. Calves maintained good health throughout the study and showed high average daily gains (826 g/day). Calves repeatedly showed preferential proximity-seeking responses towards their foster cows. These findings are compatible with the hypothesis that foster-cow rearing can support the development of attachment in dairy calves, highlighting the robustness of this social relationship and its potential relevance for animal welfare and sustainable rearing systems.