St John's wort
St John's wort (Hypericum spp.) is a serious perennial weed of pastures, catchments, forests and national parks in the high rainfall areas of southern Australia.
Livestock grazing St John's wort can develop photosensitisation (light sensitivity) as a result of liver damage. This leads to low productivity and sometimes death.
Conditions when poisoning by St John's wort is likely to occur
St John's wort is present in large areas of New South Wales, Victoria and South Australia. It is most abundant on the central and southern tablelands of New South Wales and in north-eastern Victoria.
Poisoning with St John's wort is more common in spring and summer because production of the toxin, hypericin, is strongly associated with development of the plant flower stems.
The narrow-leaf strain of St John's wort contains more toxin than the broad-leaf strain.
Strategies to prevent poisoning by St John's wort
Seasonal management tactics, herbicides, safe grazing and biological control can reduce the impact of St John's wort.