Johnson grass or Sorghum Almum has its place though its not for everyone. It works wonderful in my heavy black soil bottom land in a pasture that is rotational grazed about every 30 to 45 days. If your hay is for horses, then you hate it. If its for cows it works well. It establishes easily, it reseeds its self all summer long, its does well in marginal soils. It doesn't take a lot of rain, its great to plant where erosion is a problem.
The field noted above had a horrible witch grass problem when I bought the place. I had trouble getting any short grass to establish as the witch grass would out grow everything and shade it out. There is not a herbicide to kill witch grass by itself that I know of and there is a ton of seed in the soil so glyfo only works for a short period before the other seed comes up. Anyway since I have been planting Sorghum Almum down there, things have changed significantly. The witch grass is still there but does poorly as the sorghum out grows it and shades it out. Every year the pasture gets a little better and I have noticed now that some of the plants I tried to introduce like Klein grass are suddenly coming up. I am picking up some SA seed tomorrow to no till drill into my oats as soon as I get it baled. This is late to be planting but the grass will grow until November here so not a big deal. My goal is the same as Fences. I want to graze the SA down hard in September, then plant oats into the lightly disced stubble. This gets me my hay started and gets the seed from the plants knocked down and incorporated into the soil. Hopefully after a few years of this I will not have to replant the Sorghum.
Here is some info on Sorghum Almum if any are interested. Sometimes the seed is easier to find than the Johnson grass. My cost is $69.25 per 50 lb bag. My small seed drill open all the way only puts down 10-12 lbs per acre which is the minimum for new established grass but okay for adding to the existing.