Horse manure is a great amendment. It is also a wonderful input to composting.

Fall is the best time to add it. It is especially nice for rhubarb beds. Aged or composted horse manure is the best.

Fall or winter rye are good cover crops. I seeded a common white clover in my paths this year for fall incorporation before sowing the fall rye. The clover contributes nitrogen and good growth (6-8") controls weeds. The rye is beneficial for weed control and addition of organic matter.

Manure amounts vary with the frequency of application. If you put in 3-4" in the fall you should not have a problem. Combined with a cover crop of green manure will preserve nutrients and improve your soil while also proetecting it from erosion.

Good luck.