There is no perfect solution to prevent cross-suckling among dairy calves. But prevention is needed: cross-suckling can lead to inter-suckling (sucking the udders of heifers and cows), which can have alarming consequences like udder damage, mastitis, and milk loss.
Luckily, with a few feeding measures and toys, it’s possible to substantially reduce the risk.
Calves are very happy to play with toys that are originally meant to prevent boredom in pigs. they also like to pull out individual straws from hay nets (commonly used in horse stables), that are hung in an over-head position - otherwise the ear tags are in danger.
And especially the toy propellors are in very high demand!