If it is fresh horse manure, may I suggest that you compost it. Very often horse manure comes from a stable as opposed to a yard area. Manure from the stable contains a large amount of urine which is why I suggest composting. If it is already composted or has been piled out in the weather for a while, it should be safe to use.

My mother grew azaleas for years, an azalea and daylily nursery and a show garden and used cottonseed meal and cow manure or horse manure from our animals, probably had some chicken mature in it also. Heavy, heavy mulch with pinestraw and grown as understory plants in a grove of pine trees whose canopies were high enough to give them sunlight. We never had access to alfalfa meal but I bet if we had, she would have used that.