Behavioural responses of calves to long-distance transport: Effects of age and pre-transport milk allowance

The welfare of unweaned calves during prolonged commercial transport is a growing concern in the European Union. This study investigated how calf age and pre-transport milk allowance affect behavioural responses during and after long-distance, multi-modal transport between Ireland and the Netherlands. A total of 138 dairy bull calves were enrolled in a 2 × 2 factorial design with factors age: 2–3 wks old (mean 21.8 d) vs 4–5 wks old (mean 31.4 d) and pre-transport milk replacer allowance: 2 L vs 4 L. Thirty-nine calves were fitted with accelerometers to record lying time, posture changes, steps, and Motion Index (MI) during the 51-h journey (ferry + 13-h rest stop + road) and in the 24 h after arrival. Behavioural observations were conducted for three weeks post-transport to quantify posture, comfort, discomfort/abnormal, and feeding-related behaviours. We hypothesised that older calves and calves receiving a higher milk allowance would exhibit behaviour indicating an improved capacity to cope with transport and faster recovery. Age influenced activity patterns during transport, with older calves standing longer and showing higher step counts and MI at the start of the journey but lying more during later stages, indicating delayed onset of fatigue compared with younger calves. Pre-transport feed allowance affected both transport and early recovery: calves fed 2 L changed posture more frequently during transport and immediately after, when they also had higher MI than those fed 4 L, possibly due to greater restlessness or discomfort. Younger calves displayed more comfort behaviours (self- and allogrooming, stretching, play) in the weeks following transport but also tended to perform more discomfort behaviours (orally manipulating objects, cross-sucking). Across all calves, comfort and non-nutritive oral behaviours increased steadily during the three weeks after transport indicating gradual physiological recovery and adaptation. Calves that received 4 L before transport spent more time lying and showed fewer abnormal oral behaviours in the three weeks post-transport, suggesting that improved pre-transport nutrition mitigated some adverse behavioural outcomes. Overall, both age and pre-transport feeding influenced behavioural responses to transport, although prolonged fasting and the cumulative stress of a 51-h journey led to behavioural indicators of fatigue and discomfort in all calves. Increasing pre-transport milk allowance and transporting older calves may improve coping capacity during extended transport.