Think you'll find that the law of inverse applies - if you don't want the trees the pigs will leave them and they will thive, if you do want the tress they'll do their best to remove them.
So plant them with an attitude that you don't want them to survive !!

We generally find that when the tree trunk is the size of your wrist they will generally leave them alone, but obviously there are execptions (called Hilary apparently, so don't call you pigs that either!), and if you are newly planting, then it is highly likely that your pigs will lean on them and without established roots will simply knock them over, eat the jucy leaves and roots and kill them.
So suspect this will a no-no !