My real world experience is it's a waste of time unless you go at least 3' deep.  Don't forget the bonus of having a brand new tree row started right where you ripped the roots.  If you do plow 3'+ deep the benefit seems to last about 3 years before it starts coming back with a vengeance, remember these are well established trees not saplings, there's a pretty big factory to push new root growth.   

If you want the tree affect gone, then get rid of the source, the tree.  The correct tool can be a dozer, an excavator, or chemicals but I'll never waste another minute of my life ripping around trees again.