I'm dealing with a Johnson Grass problem, too. I was asking around in my area (NE Kansas) and had several options recommended, but the weed wiper seemed the most feasible. Other options included some very expensive chemical designed to just take out Johnson Grass, and another was overgrazing heavily early in the year, but I was too late last year. This year our drought conditions may not make that feasible, as my neighbor who would have brought in the cattle to overgraze got rid of his herd. Not enough pasture, not enough hay to cut, and hardly any water in the ponds.
It's looking like the weed wiper may have to wait another year. Last years lack of moisture ended up being the problem then, as the Johnson Grass never got tall enough to use a weed wiper. And this year may be worse......