I wouldn't put any on a plot unless it had been seasoned for a couple of years. You'll have whatever the horses have been eating growing all over your plot if you put fresh down. My Brother-in-law has some high dollar cowboy shooting mounts that he stall feeds every day and when he does turn them out they leave nice little oat plots wherever they leave a pile.
As welder said horse manure is going to leave a lot of seed of whatever the horse has previously eaten. It's not as good as cow manure or chicken litter and generally breaks down slower due to it being more grainy. (Horses only have a two chamber stomach while cows have four chambers hence why they chew cud and have such nicer poo while the horse's allows seeds to go through relatively intact)
Now that being said. If you were looking to do a fall plot and do a burndown beforehand and horse crap was readily and cheaply available it probably would work pretty well. Add your real manure and plant buckwheat for your green manure and come fall you should have a pile of organic matter and micronutrients into your soil.
horse manure
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