Are blueberries poisonous to horses?

At my house, we have a 2 acre blueberry field, and I want to fence it and put a horse in. I, of course, would give him tons of hay, and take him out each day to our small patch of gr and graze him for an hour. But if he ever did eat a good amount of blueberries, or the bushes, would it hurt him at all? Please only people with horse and blueberry experience answer. :D