horse manure

I can't say for fresh horse manure. I use ROTTED horse manure in my daylily beds; It increases the organic matter content in the soil which helps plants thrive in seemingly hopeless conditions such as the drought that we have had this summer.
I am pretty sure cow manure would be a better bet. My dad raised horses and mules and I noticed when in a small area they tended to use the same area for a bathroom and wither nothing grew there or noxious weeds only grew there In retrospect when he had cattle where they dropped the grass was much greener... Not saying horse manure won't work - just noting my observations...
I dunno about horse, I have a giant cattle manure pile thats been sitting for years and gets added to from time to time that i've been spreading in small amounts around my trees.
Horse Manure needs to be composted before it can be applied to trees or plants. If the manure doesn't have time to break down it will "burn" the tree through dehydration because it is too Nitrogen rich. Cow manure is better but it too needs some time to break down. Depending on the tree you are looking to fertilize, ( chestnuts / oaks / pear / apple ) I'd look at other organic fertilizers.
Once it breaks down it will grow anything. Best garden i ever had was in a 12" of horse compost.