Dairy cows are highly social animals whose welfare depends substantially on stable social groups, appropriate herd sizes, and management of social relationships.
Disrupting cow social groups through frequent regrouping increases aggression, reduces feeding access for subordinate individuals, and elevates cortisol. Large dairy herds where cows cannot recognise all individuals have fewer stable social relationships and more continuous low-level aggression. Keeping social groups stable from calving through lactation, and minimising unnecessary regroupings, significantly improves welfare. Providing adequate feeding and resting space reduces competition. Social housing of sick cows with compatible companions reduces the welfare cost of necessary medical isolation.