Need to get rid of tough grass

[Homesteader]

When I purchased this property several years ago it was covered in what I was told was tough grass. It is about 4 feet high and in this condition cannot be cut with a mower; riding or otherwise. We've slung and hoed this stuff til we got a really good sized front lawn. But in the not so used areas this stuff is coming back. Help. We tried burning it; but it won't.