Dunno, I'm finding out! It's been a couple of weeks now and the underneath is smelling rich, but it's still green, it hasn't rotted. It's going to have had virtually all the nutrients washed out, but with two currently overweight horses it's probably going to be just what I need. I've been told that as long as you don't start turning it, it can lie for quite some time.
If you never managed to remove the grass it could stop the new growth coming through. We'll get these fields baled even if we have to just get a big baler in and then dump them in a corner to rot down.