Mow it. Get a riding lawn mower, or borrow one. Leave the deck all the way up, then mow the whole field. Next, lower the deck, but have it on a high-ish setting and mow it again. Then, find someone with a tractor and have it disked, or one of those tiller things.
Or, find one of those "rent a goat" people and rent 10 goats. They'll have the whole place cleared in less than a week, and they're cute as heck
. As long as they have plenty to eat, they won't mess with the horses or their hay. They will try to steal grain if you feed your horses in the pasture.