Abstract
When calves are reared by cows, separation can be stressful, and it is often advised to use gradual weaning to minimize this stress. In foster cow systems, one gradual weaning method is to remove the foster cows, one by one, from the cow-calf group. The response to one-by-one removal of foster cows of 32 calves housed in 5 groups was studied. Calves were fostered to a designated foster cow individually, in pairs, or in trios, at the age of 1 to 2 wk and then housed in groups of 4 to 6 foster cows and 6 to 11 calves until gradual weaning started at 3 mo of age. Calf behavior and aggression received by calves from cows were recorded using video during 18 h before any foster cow was removed, as well as 18 h before and 18 h after the designated foster cow was removed from the group. Before any foster cow was removed, the calves spent a similar amount of time sucking the designated foster cow and other cows (in total 43 ± 5 min/18 h). The total duration of sucking was similar before and after the designated foster was removed, as the duration of sucking other cows increased. The number of aggressive events received from other cows also increased. The results indicate that milk availability for the group of calves decreases as foster cows are gradually removed and that the older and heavier calves increasingly compete with the younger and lighter calves for the available milk. This weaning method may also compromise the welfare of the foster cows that are left to wean off an increasing number of calves, and this warrants further research.