I have used generic glyphosate to kill elm stumps and they never re-sprouted. Have also used Tordon on woody shrubs. According to a work sheet I have from the USDA willow can be controlled satisfactorily with Triclopyr (Garlon 3A) Picloram (Tordon or Pathway) Imazapyr (Chopper, Stalker or Aesenal) by painting the stump immediately after cutting. For oil based applications use Gaelon 4, Chopper or Stalker or 2,4,D + 2,4 DP (Patron 170.) You only need to paint the outer inch or so, not the entire stump.