Some horses really like to eat clover and in the summer, some clover can become infected with a type of fungus that produces a chemical called slaframine. This chemical stimulates the production of a tremendous amount of saliva. This fungus, rhizoctonia leguminicola, can also infect other types of legumes such as alfalfa and can be found on red and white clover. It thrives under conditions of high rainfall or high humidity.
While this condition is aesthetically unpleasant and messy, there are rarely any actual health concerns for the horse. Symptoms of slaframine consumption can begin as soon as one hour after ingestion of the contaminated plants and may last up to three days after the source is removed.