34 inch hog panels. I know some people despise this practice, but we typically put a strand of barb wire above the panels. We had a problem with the cattle guy overgrazing the place. Had cattle issues and especially so in round pens where we could not get the wire tight. The 2nd pen was built at the end of that guy's grazing lease and the pasture was allowed to go free for a year. When a new cattle guy was secured, the owner placed restrictions on how many cattle could be on the place. That took care of the over-grazing issue. That was two seasons ago and we have not had any issues since then. If this issue ever comes back, I will without hesitation run a string of wire across the top making it so tight you can play a tune. Can't do that with the round pen, but sure can do it with the square pen with 2 3/4" inch corner posts which were driven in with one of those mechanized post drivers which look like a modified chain saw. That there thingie was worth its weight in gold and especially since Klappenbach bought it.
I hear ya and you are right, most of the time cattle will not even attempt it unless they get really hungry.
Now, if they get a taste of protein it sometimes changes everything and you will need a rhino proof pen to keep em out.
As far as a strand of barbed wire across the top goes, I'd do it in a heartbeat if I had a cow getting in!