If they want in they are going to get in. You can only slow them down. I have seen some of the best pens made keep them out for a time but they will just dig under them after some time.
If you want to stop them from busting up your feeders you are going to need to hang them from trees, then you can put up a pen under them. Will help with cows but the pigs will get in.
Where are you from? I live in Lubbock.