A local USDA guy (as I recall) mentioned a tool for ripping through tree roots around the edge of the field.  He suggested it really should be done every couple years, but few do it.

Most seem to use a bulldozer to remove the "brush" instead of messing with the roots, I suppose.  Removing the trees is better when possible, and shading may also be a factor, but does anyone do this, or know about this?

Apparently some county agency nearby has one that can be used or rented.