Spreading fresh manure on pasture?

We just got a five acre mini horse farm. We have 4 horses on it. My father keeps throwing the fresh horse manure around and then dragging the harrow over it. Is this going to be OK for the horses? My father does not want to pile it and let it compost as he sees this as too much work. But I worry. He has got it spread all over every inch of the pastures now and the horses are grazing in between it. We water constantly to keep things green so it is getting washed in.