It is the nature of a donkey to be very set in it's ways. ( don't say stubborn because donkey and mule folks don't like that term) Almost any donkey will make a calf breaker because they prefer to lead over being led. They like it only when it is their idea. Most donkeys will improve the more they are used, but some are just better than others. I prefer gelded jacks over all natural jacks, and I prefer jennets over gelded jacks. Most problems occur because the donkey gets mean and bites, not because they can"t get a calf broke to lead. Hope this answers your question. Earl