Roundup will kill it and everything else it comes in contact with. That's what I am using now as a spot treatment in the fields, and although it seems to kill it, in 2 weeks up it comes again.
2,4-d seems to just feed Johnson Grass! I have used this product on the 2 acres we mow, and it has done great killing crabgrass, but the Johnson grass is thriving and just crowding out everything else.
Thanks for your input! Any other ideas??