Behavioural indicators of hunger in dairy calves

Dairy calves are typically fed only about half of their ad libitum milk intake and are highly motivated to drink more milk. The aim of the current study was to describe the behaviours associated with milk hunger in dairy calves, by comparing animals fed restricted quantities of milk (two milk meals weighing a total of 10% of the calf's body weight; n = 12) or provided milk ad libitum (n = 12). Calves were kept in groups of four and observed from 8 to 14 days of age. Treatment was assigned within group using a computerized milk feeder. Calves fed milk ad libitum gained about four times more weight (P < 0.001) and ingested twice as much milk as the restricted-fed calves (P < 0.001). The restricted-fed calves performed on average 24 unrewarded visits/day to the milk feeder (i.e. visited the feeder but received no milk), 12-times more than the calves fed milk ad libitum (P < 0.001). During rewarded visits, calves fed restricted quantities of milk spent twice as much time on the teat (P < 0.001) and consumed the total amount of milk available in a single nutritive sucking bout that was followed by short non-nutritive sucking bouts. Restricted-fed calves spent 1 h longer standing/day than the calves fed milk ad libitum (P = 0.05), and were more likely to displace other calves from the feeder. Thus, calves fed restricted quantities of milk ingested their available milk allotment more rapidly during a rewarded visit, were more active, more competitive and spent more time at the feeder, indicating that these behaviours can be useful in identifying milk-feeding practices that cause hunger in dairy calves.