I used to have several 15 - 25 acre fields of Canadian thistles in days gone by. When I put them into alfalfa or alfalfa/grass and made 4-5 crops of hay for several years; the thistles are gone. In your case cut and spray repeatedly each season and the y will diminish exponentially. 1000s of farmers ought to be concerned about thistle control because if you use government programs YOU ARE REQUIRED to control noxious weeds; Canadian thistles! I travel between NW Illinois and NE Ohio to visit my in-laws. Several times when going in the summer the fence lines along Route 30 especially in Indiana and Ohio are loaded with Canadian thistles thicker than hair on a dog all fuzzing out and blowing where-ever the wind carries them. I'll bet more than a dollar most are growing along government program entered crops. If someone decides to report this some guys are NOT going to be happy.