"Slobbering Horse Syndrome," "slobbers," and "excessive drooling" are all terms for a disorder that results in the spring and summer when horses eat legume forages, particularly clover, which have been infected by the fungus 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.
The fungus most commonly affects clovers (red, white, and alsike) and alfalfa, and increases when these forages become drought-stressed or are exposed to prolonged wet conditions. It forms a black patch which may be visible on the leaves of the plant, hence the common name "black patch." Pastures can vary in species composition from year to year due to environmental conditions, grazing pressure and management. White clover will increase in a pasture when conditions are favorable. If pastures are overgrazed, and the more productive cool season grasses such as timothy, brome, and orchardgrass are eliminated, white clover becomes very competitive. White clover will also increase if adequate soil fertility levels are not maintained and if warm temperatures in spring restrict the growth of cool season grasses. Change in pasture management (i.e. fertilizing, resting, and rotating pastures) can reduce the amount of white clover in a pasture. Change in environmental conditions, cool spring temperatures and drier summers with adequate rainfall will reduce the growth of the fungus on the plants. This is not a problem that occurs in all pastures every year.
Hay made from contaminated forages is also suspect, and the fungus can maintain toxicity while baled for several years. Your veterinarian or Extension agent can confirm the presence of "black patch" in your pasture or hay. Horses are most commonly affected, although cattle, sheep, goats and swine are also susceptible. Along with excessive salivation, symptoms include tearing, difficulty breathing, increased urination and feed refusal. In severe cases, diarrhea may also occur, though horses do not usually dehydrate or develop more serious health problems. It is not uncommon for some horses in a pasture to be more severely affected than others. This might be related to whether or not they prefer clover over the other forages available in the pasture or have different levels of sensitivity to the toxin. Some horses may get ulcerations and scabs on the face and areas of the legs that come in close contact with the infected clover, as an allergic reaction to the fungus in the clover.
To help eliminate salivation and skin lesions, remove horses from pastures with high clover concentrations, and begin mowing pastures until the Rhizoctonia is no longer observed on the leaves. Most cases don't require treatment, and most animals recover within a couple of days after the fungus is removed. Severe cases that do not clear up within a day or two should be seen by your veterinarian.
Clover can be suppressed by using a variety of herbicides such as Ally1. Ally is relatively safe and has no grazing restrictions. Fertilizing pastures in spring, summer (if conditions warrant pasture growth), and in fall with 40-50 pounds of nitrogen per acre will provide the grasses with a competitive advantage over the clover. If pasture grasses do not receive nitrogen fertilizer, the competitive advantage shifts to the clover, which is able to fix nitrogen and quickly take over, especially if the pastures are overgrazed.