Simple SummaryThe impact of pair compared to individual housing of pre-weaning calves has previously demonstrated benefits in terms of socialization and calf development. This study aimed to investigate the impacts of individual and pair housing on a UK commercial dairy farm and establish further behavioural impacts that housing groups can have on calves. Overall, we found an increase in the activity of pair-housed calves, with individually housed calves spending more time with their head out of the front of the pen, as well as spending longer engaged in self-grooming. There was no impact of housing type on average daily liveweight gain, but there was an increase in disease prevalence in individually housed calves, possibly due to the impact of stress induced by social isolation. Overall, pair housing had a positive impact on the health and behaviour of calves on a commercial UK dairy farm.AbstractSocial pair housing of calves has previously demonstrated positive impacts for calves, so this study aimed to compare the health and behaviour of calves kept in individual compared to pair housing on a single commercial UK dairy farm. A total of 457 Holstein and Jersey heifer calves were recruited and systematically allocated to individual and pair housing. Weekly visits were conducted up to 8 weeks of age, with weight and presence of clinical disease measured using both a standardized scoring system and thoracic ultrasonography. A subset of calves (n= 90) had accelerometers attached to monitor activity, with CCTV placed above a further 16 pens to allow behavioural assessments to be made via continuous focal sampling at 1 and 5 weeks of age. During the study, there was a mortality rate of 2.8%, and an average daily liveweight gain (ADLG) of 0.72 kg/day, with no significant effect of housing group (p= 0.76). However, individually housed calves had increased odds of developing disease (OR = 1.88,p= 0.014). Accelerometer data showed that housing group had no effect on lying times, with a mean of 18 h 11 min per day (SD 39 min) spent lying down. The motion index was significantly higher in pair-housed calves (F1,83= 440.3,p< 0.01), potentially due to more social play behaviour. The total time engaged in non-nutritive oral behaviours (NNOBs) was not impacted by housing group (p= 0.72). Pair-housed calves split their time conducting NNOBs equally between inanimate objects and on their pen mates’ body. Individually housed calves spent significantly more time with their head out of the front of the pen (p= 0.006), and also engaged in more self-grooming than pair-housed calves (p= 0.017), possibly due to a lack of socialization. The overall findings of this study indicate that within a UK commercial dairy management system, pair-housed calves were healthier and more active than individually housed calves, while housing group did not influence ADLG or the occurrence of NNOBs.