Possesion is 9/10's of the law! If the cows are on your property, i would say there wouldn't be much room in the freezer for venision with all of that beef. We had the same problem with the amish horses getting out and wandering throughout our property. A friend of ours, a State Police officer said "possesion is 9/10's of the law" meaing i guess we could have had a couple of horse's after that. It didn't come to that.
I would say approch the farmer and let him know his cows slip the fence and maintenance need to be done to it. If i keeps up and becomes a nuciance, notify the local police and let them handle it. I have beef cows and if mine got out routinly i would be fixing my fence, making it better, and having to pay for nieghbors landscaping.