We planned on putting in a garden about 6 years ago so my husband had some friends of our who had 10 horses at the time bring 2 spreader loads full over. My husband and the neighbor boy spent all afternoon tilling it in. Then the engine blew on his truck so we had to put energy into that.
Next spring, the area we had planned for the garden was full of lush, thick white clover while the surrounding area was sand with sparse grass.
I got pregnant, distracted and forgot about the garden until 2 years ago. Out of curiosity we spaded the soil and found rich, thick black dirt where the manure had been placed. I dump about 100 lbs of horse manure on it every year, but the original loads put an excellent base for our soil.
The best stuff is 3 years old if the stables have any in a big pile just rotting. If the horses are given dewormer (which they probably are) best to age it for 6 months before using to the vermicides have a chance to decompose.