Back in the early 80's (83-84) my Dad and I took over a ½ section that was covered in johnson grass, I mean almost solid. We used a lot of roundup those years trying to kill that stuff. It always took at least 2 shots and sometimes 3 to get it to kill that stuff. The PIK program helped clean that farm due to only having part of it in crops. Where we had crops growing we used Fusilade.

Kind of surprises me it took this long to get resistant.