Its most likely from a fungus on the clover. Out horses had been doing it too that last couple weeks, the vet happened to be out when they were doing it and wasn't at all concerned about it. As long as they aren't doing anything else besides the drewling, I wouldn't worry about it.
I just looked it up the fungus is called Rhizoctonia leguminicola. "This fungus produces an alkaloid called slaframine, which is responsible for the excessive drooling and slobbering. Slaframine stimulates the salivary glands and smooth muscles."
If possible, you can move the horses to another pasture that might not have as much clover. Its also possible that the fungus is in your hay as well, although since you said it stopped when they were eating hay, it sounds like its just in the pasture.
Last edited by horseluver250; 09-01-2010 at 09:03 AM.