When weve had weaners we tended to send them away at the end of the year so we never planted straight after. Our land is quite hard but the pigs did a reasonable job of clearing most growth but not all. They also left deepish holes where we made wallows and they made dust baths. They also have a toilet....
So, what we do (did) was to rotovate the next spring and had a full growing season. Subsequent years have seen more weeds growing and Im not sure if the pigs rooting made weed seeds come to the surface.
im sure someone more knowledge able will come along soon and give their opinion.
Whatever, enjoy your pigs!!!