My riding friend and I always clean up our manure and either
haul it home or scatter it well away from the parking area at our local trail
heads. We also walk around and clean up other people’s horse manure
because we don’t want the multiple users to decide they don’t want
horses in the parking areas. Both our horse trailers have manure forks in
them and it is just part of our routine to clean up the manure….takes a
couple minutes.
The horse camp at the Mt Adams ride in Washington has cool
manure bunkers—worth investigating if that is a good solution for your
area.