Bower, it comes down to what the horses ate. I have researched so much about adding manures to any growing medium, and there are so many thoughts out there that it becomes confusing. Two of the most highly acclaimed manures are rabbit and chicken manure. A large amount of sites says that chicken manure needs to be hot composted. I personally don't want a hot compost of chicken manure going on near my home. Rabbit manure can be used without composting and is high in N and P with some K. Horse manure pales in comparison, but more is not always better.
I chose one site at random that has a comparison table
http://www.plantea.com/manure.htm