Extensively managed and feral domesticated cattle (Bos taurus) live in a stable, complex social environment. Herds comprise members of various ages, genders, and sexual maturity. Under intensive farm management, dairy cows experience frequent and routine regrouping, which can disrupt affiliative bonds and induce social stress. This study explores the impact of changes in herd composition on milk yield and udder health traits in resident dairy cows that experience the removal of familiar herd members and the introduction of new individuals into the group. Data on milk yield (production trait), electrical conductivity, and blood presence in milk (indicators of udder health) were monitored over 9 years using precision dairy sensors in a sample of 798 cows. Milk yield dropped after the loss of familiar herd members while introducing new cows had no effect. The higher number of withdrawn cows (up to 10 at once) resulted in lower daily milk yield, increased milk conductivity, and a higher probability of blood presence in milk. Thus, besides lowering milk production, social stress is linked to potential udder health issues. Holstein Friesian cows and primiparous and early-lactation cows were more affected by herd changes than Czech Simental and multiparous cows in later lactation stages. These findings highlight the importance of social stability for dairy cow welfare and productivity, emphasising the need for strategies to reduce stress during necessary herd adjustments.