The most common reason horses slobber excessively is a fungal toxin called slaframine found in clover, especially red clover. But heavy drooling can also signal dental problems, a lodged obstruction in the esophagus, toxic plants, or a viral infection. The cause usually becomes clear once you look at the timing, what your horse has been eating, and whether any other symptoms are present.
Clover Slobbers: The Most Likely Cause
If your horse has been grazing pasture that contains red clover or eating clover-heavy hay, the culprit is almost certainly a condition called “slobbers.” A fungus called Rhizoctonia leguminicola grows on red clover and produces a toxin, slaframine, that directly stimulates the salivary glands. The result is dramatic: long strings of saliva pouring from the mouth, often alarming enough to send owners into a panic.
Symptoms appear within an hour of eating contaminated clover. The fungus thrives during warm, humid weather, particularly at temperatures between 75°F and 85°F, and is most common on second-cutting red clover hay. It can overwinter on infected plants and survive at least two years on infected seed, so a pasture that caused problems one summer may cause them again the next.
The good news is that clover slobbers is mostly a nuisance, not a medical emergency. Once you remove access to the contaminated pasture or hay, the excessive salivation typically stops. In more severe cases, horses may also develop diarrhea, excessive tearing, frequent urination, or loss of appetite, so keep a close eye on any horse that’s been affected. If symptoms go beyond just drooling, a veterinary exam is worthwhile to rule out other causes.
How to Manage Clover in Your Pasture
You don’t necessarily need to eliminate all clover from your fields, but a few steps can reduce the risk of slobbers. The fungus favors warm, wet conditions and slightly acidic soil (pH around 5.9 to 6.5), so pastures in humid climates during summer are the highest risk. Second-cutting red clover hay is the single biggest source of contaminated feed, so inspect it carefully before purchasing or feeding. Look for dark, irregular patches on the clover leaves, a sign of blackpatch disease caused by the same fungus.
If your pasture has a heavy red clover population and your horses are drooling every summer, consider reseeding with grass-dominant mixes or clover cultivars that may be more resistant to blackpatch. During peak risk periods (warm, rainy stretches in mid to late summer), pulling horses off clover-heavy areas can prevent the problem entirely.
Dental Problems
Horses’ teeth grow continuously throughout their lives, and uneven wear creates sharp enamel points on the edges of the molars. These points develop on the cheek side of the upper teeth and the tongue side of the lower teeth, cutting into soft tissue and causing pain that leads to excess salivation. In severe cases, called “shear mouth,” the chewing surfaces develop such a steep angle that the horse can barely grind food at all.
Other dental issues that cause drooling include fractured teeth, gum disease from food trapped in gaps between teeth, and impacted teeth. A classic sign of dental trouble is “quidding,” where the horse forms a ball of partially chewed food and then drops it from the mouth. You might also notice feed packed between the teeth and cheek. If your horse is drooling and also dropping food or losing weight, a dental exam is the logical next step. Most horses benefit from having their teeth floated (smoothed) by a veterinarian or equine dentist every one to two years.
Choke (Esophageal Obstruction)
Choke in horses is not the same as choking in humans. It refers to a blockage in the esophagus, not the airway, so the horse can still breathe. But it causes unmistakable symptoms: a green, frothy discharge from the nostrils containing saliva and bits of food, along with coughing, gulping, restlessness, and extension of the head and neck. Excessive salivation is one of the earliest signs.
Choke typically happens when a horse eats too quickly, doesn’t chew dry feed thoroughly, or swallows a chunk of apple or carrot that gets lodged. This is a situation that needs veterinary attention promptly. Most cases resolve with sedation and gentle flushing, but a prolonged obstruction can damage the esophageal lining or lead to aspiration pneumonia from material entering the lungs.
Toxic Plants Beyond Clover
Several pasture plants irritate the mouth and trigger drooling. Buttercups are one of the most common offenders. Their leaves and flowers contain compounds that irritate the tissues of the mouth and throat, causing salivation along with possible colic and diarrhea. Buttercups are low in toxicity and horses usually avoid them when other forage is available, but overgrazed pastures force horses to eat what’s there.
Horsenettle is a more serious concern. All parts of the plant are toxic (especially the berries), and it remains dangerous even when dried in hay. Along with salivation, horsenettle can cause muscle tremors, weakness, and depression. Foxtail grass awns and other sharp plant material in hay can also lodge in the gums or cheek and create ulcers that make the horse drool.
Vesicular Stomatitis
Vesicular stomatitis is a viral disease that causes blisters inside the mouth, on the lips, around the nose, and sometimes near the hooves. Excessive salivation or frothing is often the first sign, appearing two to eight days after exposure. If you look inside the mouth, you’ll see raised, whitish blisters on the gums, tongue, or inner lips. These blisters eventually rupture, leaving painful ulcers that make the horse reluctant to eat or drink.
Fever often develops at the same time as the blisters. If lesions form around the coronary band (where the hoof meets the skin), lameness follows. Vesicular stomatitis is a reportable disease in the United States, meaning your veterinarian is required to notify state and federal animal health officials if it’s suspected. Outbreaks tend to occur in the western U.S. during warm months and are spread by biting insects. While most horses recover, the disease can spread to other livestock and occasionally to humans.
Other Causes Worth Knowing
A few less common possibilities round out the list. Botulism can cause drooling because the toxin interferes with the nerves controlling swallowing. Horses with botulism look progressively weaker, often struggling to hold their head up or stand. Exposure to certain insecticides, particularly organophosphates, can also trigger profuse salivation as part of a broader toxicity syndrome. Any injury or inflammation inside the mouth, from a bit injury to a splinter from chewing wood fencing, can produce localized pain and excess saliva.
Narrowing Down the Cause
A few observations help you figure out what’s going on. If the drooling started during summer while your horse is on clover-heavy pasture and there are no other symptoms, clover slobbers is the overwhelming favorite. Remove the clover source and see if it stops.
If drooling is accompanied by green, frothy nasal discharge, think choke. If you see blisters or ulcers inside the mouth, vesicular stomatitis or a foreign body irritation is more likely. If the horse is quidding or losing weight gradually, dental problems are the prime suspect. And if the drooling comes with progressive weakness, difficulty swallowing, or fever, the situation is more urgent and warrants a same-day veterinary call.