I guess I'm confused by the question. If it is "well composted," it isn't really horse manure anymore ... or is it? I add horse manure to my compost pile that also includes other thing including hay and chopped brush and some top soil that was scraped up to make the composting site.
So...
Is the question really about compost that contains what was horse manure at one time?
or
Is the question really about using horse manure to directly grow in?
I'd like to know so I don't make any mistakes. For example, R Scott mentioning about the wormers ... I now need to ask my horse-manure-providing neighbor if he gives his horses anything but hay and
feed.