A horse with slobbers (courtesy Steven S. Nicholson, DVM , LSU)
Why Do Horses Drool?
Grazing animals can sometimes consume plants that have spines, burrs or sharp awns on the seeds that can injure the tissues of the mouth causing the horse to drool. When horse's drool profusely over long periods of time, the culprit is usually a fungus that grows on white and red clover and less frequently on alfalfa.
Take another look at your pasture
White clover and red clover are perennial plants. White clover can tolerate intense grazing and is often found in overgrazed pastures. When the clover plants are stressed by being continuously grazed or during periods of drought or extremely hot humid conditions, the Rhizoctonia fungus can grow rapidly on the plant leaves. The fungus produces a toxin called slaframine that stimulates the salivary glands of horses causing them to drool - sometimes in alarming quantities. If the fungus is growing on red clover that is baled for hay, it will persist and can cause horses to drool when they consume the hay.
Will the fungus on the clover hurt the horse?
Removing the horse from the pasture will allow you to determine if the clover in the pasture is causing the horse to drool. The drooling is normally only a nuisance and will not harm the horse. It is not abnormal for some horses in the pasture to be more affected than others, since horses vary in their sensitivity to the toxin and preference for clover. If you have major concerns over the "slobbering" then you should contact your veterinarian.
Are there solutions for eliminating the "slobbers?"
To remedy the situation, you can attempt to reduce the clover concentration in the pasture by overseeding with grasses and practicing good management practices, including fertilizing, resting, and rotating your pastures. Clover fixes nitrogen and does not need to be fertilized with a nitrogen based fertilizer; it is grasses that typically need nitrogen. If you do not meet the fertility requirements of the grasses, the clover may take over the pasture. If the problem persists, you can also use a broadleaf herbicide, labeled for pasture use to remove existing clover plants.