Put the manure in the compost bin if it is not ready to use now. Horses, unlike cows, don't process their feed as thoroughly. Horse manure can contain a lot of seeds that will grow in your garden.
When I score fresh stall material of manure, hay and wood shavings, it goes directly into the compost pile and nowhere near my garden. When I get to the stuff that has been in the field for 6 months or more and is very decomposed and ready for putting it directly in the garden, I will do just that.
Just yesterday I scored both. I filled a compost bin with stall material, some shredded paper, garden plant waste and some leaves. The older, more composted stuff went into the garden between my rows on top of the layer of cardboard I put in there a few weeks ago. The composted material will help retain moisture causing the cardboard to decompose faster and helps me by keeping the watering and weeds down. This weekend I plan on placing hay between the rows and around the plants to make my fall gardening an almost no-brainer.