From Penn State Extension:
...Horse manure (click here) has been considered a valuable resource rather than a “waste.” Fertilizer value of the 8 1/2 tons of manure produced annually by a 1,000-pound horse is about 102 pounds of nitrogen, 43 pounds of P2O5 (phosphorus pentoxide [phosphate] = 43.7% P), and 77 pounds of K2O (potash = 83% K). The nutrient content of horse manure can also be represented as 12 lb/ton of N, 5 lb/ton of P2O5, and 9 lb/ton of K2O (nutrient values for any manure vary widely so these are only guidelines). Traditionally, nitrate-nitrogen is the component that presents the most pollution potential since it moves freely in the soil. Most of horse manure’s nitrogen is contained in the urine.
These values are an average for horse manure (urine and feces). With the large amount of bedding material mixed with manure in typical stall waste, the fertilizer nutrient value would vary (see Direct Disposal section)....
Ever try bugs? Yeah, bugs. Worms to be exact. Red worms. Why not? The folks that specialize in COMPOSTING WITH WORMS have a little problem when it comes to deworming medicine in the manure.
As I’ve written (click here) many times here on the blog, aged manure is hands-down one of the best materials to add to a vermicomposting system. Horse manure, in particular (especially when mixed with bedding materials), happens to be my personal favorite! Some of the most successful populations of Red Worms (highest densities, largest worms) I’ve ever encountered have been living in old outdoor heaps of horse manure, such as the one pictured above.