Johnson Grass Problem - The best I can tell, from simple observation throughout the years, Johnson grass thrives in places where herbicides have been sprayed.

For several years in the 1960s and 1970s, in the bottoms along Green River on Highway 70 just west of Liberty, the University of Kentucky had test plots for Johnson grass eradication. I have no idea what they were utilizing for eradication (assumed it was herbicide of some sort) but the Johnson grass was thriving and taller and greener and growing more thickly than the corn it was supposed to have been eradicated from.

The only thing I have seen that will eradicate Johnson grass from a plot of ground is horses as they will pull up and eat the roots. At least mine did.