When I moved my Arab from CA to MD, she had a reaction to the clover, same thing, excessive drooling, like really excessive drooling, and there was definitely clover in the pasture. None of the other horses had a problem, and the vet said that some horses are just more sensitive to other, and my Arab having come from CA where there is no pasture, needed to get used to everything. She also had a really bad reaction to the gnats on her belly, neck, chest, and belly were all scabbed up, itchy and irritating. The next summer she had no problem with either thing, but I was really worried for a while. I would walk around the pasture, see where the clover may be, and possibly get rid of it. Our horses were rotated between 3 different large pastures, so it was impossible to actually get rid of the clover, otherwise I would have.