Horse manure is what they call hot. It takes a lot of an additive, I can not remember if it is lime or what, I'm not a gardner, just an old horseman. If the horses are stalled they usually have been using wood chips as bedding and there is more wood chips than manure in some cases. Horse manure will kill some plants, and it also has seeds from the feed the horses ate. Palletized and cubed feed comes from all over the country and has seeds from the area grown in them.
Most horse people I know cannot find a place to get rid of the stuff, and will give it away to any one that wants it, just to get it off the place. People I know that do use it will get it in the fall and roto-till it into the ground. Let it set all winter then use it in the spring.