Quantity
The toxic dose in horses is not precisely established, but prolonged access to onions at roughly 0.5% of body weight per day or more is likely to cause clinical hemolytic anemia. For a 500kg horse, that's about 2.5kg of onion. But the damage is cumulative — smaller daily amounts over weeks can be just as dangerous as a single large dose.
Notes
The most common scenario is a horse breaking into a garden or being fed kitchen scraps. Wild onion and wild garlic species that grow in some pastures can also cause toxicity if horses graze on them heavily. Onion powder is the most concentrated form and is found in many processed human foods. All forms — raw, cooked, dehydrated, powdered — are equally dangerous.
Negative Signs
* Weakness, lethargy, and exercise intolerance
* Pale or yellowish gums and mucous membranes
* Elevated heart rate and rapid breathing (compensating for anemia)
* Dark red or brown urine
* Loss of appetite
* Depression and reluctance to move
FAQ
Q: There's wild garlic growing in my horse's pasture. Is that the same as onion?
A: Yes — wild garlic, wild onion, and all allium species contain the same toxic compounds. If your horse is grazing on them, take action. Horses generally avoid them due to the strong taste, but in sparse pastures, they may eat what's available.
Alternatives
Never feed your horse kitchen scraps that might contain onion, and fence off vegetable gardens. If wild onion or garlic grows in your pasture, consider herbicide treatment or manual removal.
Risks & Disclaimer
If your horse ate a significant quantity of onion or has had repeated access to onion-containing scraps, call your vet for a blood test. Heinz body anemia can be detected on a blood smear before clinical signs become severe. Early detection means better outcomes.
