How do you stop a calf from cross sucking?

Separate the calf and his mother from the herd and put them in a separate pen by themselves for a few weeks. That may teach him to stop stealing milk from other cows and just depend on his dam for milk.

However, you will have to pay attention to his dam. See if she is producing enough milk for him to rely on. If not, then this may force him to have to find other cows to suckle on. Also, some cows are not as strict about having other calves suckle on them as some are: these type of cows should be noted to being potential surrogate dams if you happen to have a calf that needs another mother to take care of it.

If it's the case that the calf's dam isn't producing enough milk, then it may be a wise idea to not find a way to stop the calf from cross suckling. However, if you haven't find the other cows are going down in condition from having an additional calf on them, you will have to consider separating him and his mother from the herd.