If you have 20 acres and 1 horse, you won't have much of a problem.
If the 20 acres is subdivided/cross-fenced, rotating the horse from one paddock to another will mostly eliminate the need to harrow or break up the manure, though it is still good practice.
If you are keeping the horse in a stall part time, or have run in sheds to clean out, the manure spreader, either pulled by a tractor or ATV, is an excellent solution.
You can also dig a simple manure pit, or keep it in a pile. Not as efficient as true composting, but it will still break down and make excellent fertilizer, and keeping it in a pile or pit will keep it off the grazing.
Anything you do (simple pit, pile, etc.) to speed composting will pay off as people will absolutely haul composted or aged, rotted manure away for gardens.