If you can borrow a metal detector, it would be a lot faster than taking a metal garden rake across 20 acres, no matter how flat and treeless you are.
That's one big Owwee I hope she heals soon but that looks like you're done for this season
There are some horses that just seem like they should come with bubble-wrap
One of my four is like that but his latest injury was human-induced by a Farrier that thought he knew more than me. Right now the poor horse is in the side yard with a leg/hoof wrap big enough to be another leg. I don't know what kind of drugs the vet injected into him this morning but he's "gimpin' around" pretty darn good - I should've asked for some of whatever it was