We tried Grazon to eliminate a thick tall stand of Canada thistle in a corner of a pasture. ( about 4 acres) The first year was a nice clean kill, the second year the thistle was still gone and any empty patches filled in nicely with grass. Now going onto the 4th year I see thistle beginning to poke its head up here and there.
It is horribly expensive to use.
Can't comment on Plateau.