Clover itself doesn’t make horses drool. The real culprit is a toxin called slaframine, produced by a fungus that grows on clover leaves. When horses eat infected clover, either fresh in pasture or baled into hay, they can develop dramatic, almost alarming levels of drooling within hours. The condition is so common it has its own nickname: “the slobbers.”
The Fungus Behind the Slobbers
A fungus called Rhizoctonia leguminicola, commonly known as “black patch,” colonizes clover plants and produces slaframine as it grows. The fungus gets its nickname from the dark, irregular lesions it leaves on infected leaves and stems. Red clover is the most commonly affected plant, though the fungus can also infect white clover and alfalfa.
Black patch thrives in warm, humid conditions. Outbreaks are most common in midsummer, particularly during the second cutting of clover when higher temperatures encourage more extensive fungal growth. One documented outbreak among horses in Brazil was traced to alfalfa hay harvested at 78% humidity. Wet seasons consistently correlate with worse infections. The fungus doesn’t produce spores, so it spreads by sending thread-like growths directly from one plant to its neighbors, making dense stands of clover especially vulnerable.
The fungus overwinters poorly in prolonged cold (it dies after about two months at freezing temperatures), but it can survive on stored seed for at least two years and on dried plant stems for a year at room temperature. Even in regions with dry summers, moisture from morning dew can be enough to keep it alive. Small amounts of fungus survive winter in the crown tissue of clover plants, then spread rapidly once warm, humid weather returns.
How Slaframine Triggers Drooling
Slaframine is an alkaloid compound that mimics one of the body’s natural chemical signals. After a horse eats contaminated clover, the toxin travels to the liver, where it gets converted into an active form. This activated compound then binds to receptors on the salivary glands, the same receptors the nervous system normally uses to control saliva production. The result is a flood of thin, watery saliva far beyond what a horse would normally produce.
The effect isn’t limited to saliva. Slaframine stimulates the same type of receptor throughout the digestive tract. Horses may also develop watery eyes, diarrhea, mild bloating, and more frequent urination. But the excessive drooling is by far the most visible and characteristic sign.
What the Slobbers Look Like
Profuse salivation typically develops within hours of the first time a horse eats contaminated forage. The drooling can be startling: long strings of saliva dripping from the mouth, a soaked chest, and wet ground beneath the horse’s head. Some owners initially mistake it for choking or a dental problem. The key difference is that horses with the slobbers continue eating and drinking normally. They don’t show signs of pain, distress, or difficulty swallowing.
Beyond the drooling, watch for mild tearing from the eyes and loose stool. These secondary signs are less dramatic but confirm that the issue is systemic rather than something stuck in the mouth.
Is It Dangerous?
The slobbers look alarming but are almost always harmless. There are no well-documented cases of slaframine poisoning being lethal in horses. The primary risk is dehydration from the sheer volume of saliva lost, so making sure your horse has constant access to fresh water matters more than usual during an episode. In rare cases, prolonged diarrhea could compound the fluid loss, but this resolves once the contaminated feed is removed.
Slaframine levels in freshly harvested toxic hay have been measured at 50 to 100 parts per million. That concentration drops roughly tenfold over 10 months of storage, falling to about 7 parts per million. So while contaminated hay can still cause drooling months after baling, the effect weakens significantly over time.
How Quickly Horses Recover
Recovery is fast once you remove the source. When horses are pulled off infected pasture or switched away from contaminated hay, the drooling stops completely within 24 to 48 hours. No veterinary treatment is typically needed. The toxin doesn’t accumulate in the body or cause lasting damage, so horses return to normal as soon as they stop eating the contaminated forage.
Reducing Clover Problems in Your Pasture
The fungus hits clover hardest when the plants are already stressed. Continuous grazing, drought, and extreme heat all weaken clover and make it more susceptible to black patch infection. White clover in particular tolerates heavy grazing pressure, which is why it often dominates overgrazed pastures and becomes the primary source of slobbers outbreaks.
If your horses are drooling every summer, the pasture likely has too much clover relative to grass. A few management changes can shift that balance:
- Fertilize with nitrogen. Clover fixes its own nitrogen from the air and doesn’t benefit from nitrogen fertilizer, but grasses do. Applying nitrogen encourages grass growth, which gradually crowds out clover.
- Overseed with grasses. Introducing competitive grass species fills gaps in the stand that clover would otherwise colonize.
- Rotate and rest pastures. Giving pastures recovery time prevents the overgrazing that favors white clover. Rotational grazing keeps grass competitive and reduces the dense clover mats where the fungus spreads most easily.
You don’t need to eliminate clover entirely. It’s a valuable forage plant that adds nitrogen to soil and provides good nutrition. The goal is to prevent it from dominating, especially during the warm, humid months when black patch is most active. If you’re buying hay and notice dark patches on clover leaves or stems, that’s a visual clue the fungus may be present. Older hay is less likely to cause problems than freshly cut hay from the same field, since slaframine degrades substantially during storage.