Hi, in my land i let everything to grow and i have a lot of johnson grass . Some years ago somebody in this forum recomended me not to kill johnson grass until i replace it with another plants because it has a purpose.
I think the point of the advice was to make sure you have a strategy to keep the soil covered so the sun doesn't bake it dry and kill all the soil life.
So if you want to kill it, do so, but do it by dropping a lot of mulch atop it, like wood chips. And replace it with something else useful, something that fits your needs.
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein