Horse Photosensitisation Plant Management

Identifying and removing photosensitising plants from horse grazing areas prevents primary photosensitisation, which can cause severe skin damage on unpigmented areas.

Key Facts

Plants causing primary photosensitisation include St John wort, buckwheat, and certain clovers
Photodynamic molecules in these plants accumulate in unpigmented skin and react with UV light causing burns
St John wort can colonise overgrazed or disturbed pasture rapidly and may not be recognised by owners
Buckwheat is occasionally grown as a cover crop adjacent to grazing land creating ingestion risk
Secondary photosensitisation from liver damage requires identifying and treating the underlying liver pathology
Horses with extensive white markings are most vulnerable to photosensitisation effects

Welfare Considerations

Photosensitisation from plant ingestion causes acute welfare harm through painful skin inflammation and ulceration that can progress to full-thickness necrosis in severe cases. The preventable nature of primary photosensitisation makes plant management a welfare responsibility for horse owners and land managers. Regular pasture inspection and prompt removal of identified photosensitising plants eliminates the exposure risk. White-marked horses that develop any unexplained skin lesions on unpigmented areas should be assessed for photosensitisation as a potential cause, and the pasture inspected for photosensitising plant species.

What You Can Do

Learn to identify St John wort and other photosensitising plants and inspect pastures regularly
Remove photosensitising plants from grazing areas before they can be ingested by horses
Stable white-faced or pink-skinned horses during peak UV hours if photosensitisation is suspected
Seek veterinary liver function assessment if photosensitisation occurs without identified plant exposure
Consult your vet or a pasture management specialist about effective photosensitising plant control methods