The thing about Johnson grass, it is like bermuda. Hard to kill when you are trying to get rid of it. Just seems to make it mad and grow that much better. To grow horse hay, you need to get rid of it. It will probably take several years of spraying and tillage to get rid of the seedbank. Then you can establish the bermuda and use a wick system or spot spray to get whatever comes back. I think their is something "new" on the market that claims to kill Johnson grass in bermuda. I haven't tried it or seen any results.