Quantity
A few rhubarb leaves can be enough to cause clinical illness in a horse. The oxalate concentration varies by variety and growing conditions, but treat any amount of leaf consumption as potentially dangerous.
Notes
Garden waste is the number one exposure source. Rhubarb doesn't commonly grow wild in horse pastures, but it's extremely common in vegetable gardens, and the leaves are often discarded after harvesting the stalks. Those discarded leaves tossed on a compost pile near a paddock are an invitation for a curious horse to investigate. The stalks contain lower levels of oxalates and are less dangerous, but are still not safe to feed intentionally.
Negative Signs
* Drooling and mouth irritation (the oxalates burn on contact)
* Colic signs — restlessness, pawing, sweating
* Muscle tremors (from calcium depletion)
* Diarrhea, possibly bloody
* Weakness and staggering
* Decreased urination or dark urine (kidney damage)
* Irregular heartbeat (severe calcium depletion)
FAQ
Q: My horse ate a rhubarb stalk but not the leaves. Is that dangerous?
A: Rhubarb stalks contain much lower levels of oxalates than the leaves and are unlikely to cause acute poisoning from a single stalk. Monitor for colic signs and drooling, and call your vet if anything seems off. The leaves are the real danger.
Alternatives
Keep rhubarb plants fenced off from horses and never discard rhubarb leaves where a horse can reach them. Compost rhubarb waste in an enclosed compost bin, not an open pile near the paddock.
Risks & Disclaimer
If your horse ate rhubarb leaves, call your vet. The combination of kidney damage and calcium depletion can be life-threatening. If you saw the horse eat the leaves, note roughly how much and how long ago — this helps your vet plan treatment.
