That the farmer intends to kill the Bull
The farmer is not suggesting to the Bull that he is going to kill him or his heifers.
It is not clear that the Bull ever decided to listen.
There are a lot of truths but they are not necessarily trying to hide the lie. Although they may be trying to hide the lie.
It would only make the situation worse for the bull and the farmer. The bull would have a shit life because he knows he will get killed for food and the farmer will have a less quality of meat.
The Crow is trying to tell the Bull the whole truth in order for him not to be deceived by the Farmer.