Sounds like Clover! http://pubs.caes.uga.edu/caespubs/pubcd/B1224.htm
"Horses grazing red clover occasionally salivate excessively, a condition commonly known as the "slobbers." This condition is due to the presence of a fungus (Rhizoctonia leguminicola) and is normally aesthetic, but pregnant mares have reportedly aborted after eating infected red clover for extended periods of time. The slobbers generally occur when more than half the dry matter present in a pasture is red clover. Grass-legume mixtures seldom contain this much red clover, so problems with horses grazing red clover are rare." http://ohioline.osu.edu/b762/b762_7.html
"Red clover may have a fungus that grows on it that will cause the horse to salivate excessively. This fungus is most common in clover raised under wet conditions. Feeding horses red clover hay is not recommended if the hay is causing this excessive slobbering. If the horse is taken off the clover hay and fed other hay for a few days, the slobbering will stop without special treatment. This problem occurs also with the white clover common in Ohio pastures during wet seasons. If the pasture is clipped short and allowed to dry out, new growth is usually safe to graze." http://www.omafra.gov.on.ca/english/...o_slobbers.htm http://chathamchatlist.com/highlight...orse-slobbers/
"If the clover was sprayed and killed a month ago I would look more toward another factor causing it rather than the fungus. This slobbering could be caused by mechanical or chemical irritations."