Invited review: A systematic review of the effects of pair housing on dairy calf welfare and productivity

Individual housing of dairy calves has received criticism due to animal welfare concerns. Therefore, it is important to provide researchers and practitioners with accurate and reliable scientific knowledge on alternative housing systems for dairy calves. Our objective was to critically evaluate the scientific knowledge on pair-housed dairy calves. The outcomes investigated were the performance, behavior, reactivity to stress, health, learning abilities, and emotional states of pair-housed calves. Primary research articles were found through targeted Web of Science, ScienceDirect, PubMed, and Agricola searches. Studies were included if peer-reviewed, published in English, based upon original data, and pair-housed calves were compared with individually housed calves. After applying the inclusion criteria, 48 articles were left. Most articles did not report differences in behavior of individually and pair-housed calves. Few exceptions included social behavior, behavioral reaction to husbandry procedures, and behavior in the human approach test. Social behavior tended to be better developed in pair-housed calves, which also showed more flexible behavioral reactions to several husbandry procedures. However, pair-housed calves interacted less with a human in the human approach test. We found no consistent evidence of impaired health or improved productivity, learning abilities, and emotional states in pair-housed calves. In conclusion, pair housing provides calves with more benefits than individual housing and improves or does not affect variables of a high value to the producer. We encourage research on longer-term effects of pair housing, such as social behavior or productivity of dairy cows, because the number of long-term studies is limited.