Photosensitisation caused by St John's Wort

Leanne Polsen - District Veterinarian, Northern Tablelands Local Land Services and Karl Andersson - Agronomist, Northern Tablelands Local Land Services

It is not just our pastures and livestock benefiting from this season’s favourable conditions………. Weeds are also thriving in the current climate. St John's Wort has taken a firm foothold in pastures across the region with grazing animals already suffering the effects of the plant’s toxin, hypericin.

The whole St John’s Wort plant is poisonous and affects all grazing animals including horses. With repeated grazing, animals become sensitive to light and develop sunburn. Skin damage is observed on pale or non-pigmented skin or areas with less wool such as the nose, face and ears of sheep. Animals also experience weight loss, agitation, reduced productivity, depression and sometimes death.

St John’s Wort is an erect, woody perennial herb. Leaves and branches are always opposite one another on the stem. The leaves are 1.5 to 3cm in length with an ovoid shape, when held up to the light oil glands appear as perforations.

Flowering in late spring to summer, the 5 petals are golden yellow, with black dots along the margins. Seed capsules form and ripen from green to brown in late summer/early autumn. These sticky capsules easily adhere to animal fur or clothing and are dispersed far and wide.

Two varieties of St John’s Wort may be found – a narrow leaf (7-9 mm at the sixth node of a flowering stem) and a wide leaf (10-12 mm). The broad leaf variety is more compact, growing to around 60 cm compared to 90 cm, has thicker stems (6 mm compared to 4.5 mm), and larger seed pods than the narrow leaf variety. The broad leaf variety also has fewer oil glands and so lower levels of hypericin.

Bright sunlight is required to activate the toxin in the bloodstream before signs of poisoning develop, but photosensitisation may occur within five hours. If you suspect St John’s Wort is causing photosensitisation in your animals, move them to another paddock free from St John’s Wort with good shade cover. Secondary infections may require treatment from your veterinarian.

The best strategies to manage St John’s Wort are to reduce its ability to establish and spread by maintaining good perennial pasture cover and preventing invasion through good property biosecurity.

If St John’s Wort is already established, management strategies include; maintaining buffer zones around infestations, utilising grazing practices that maintain good ground cover and promote persistence of desirable species, grazing off early in spring with more tolerant classes of livestock such as darker skinned cattle and older sheep carrying four or more months of wool.

Grazing of the narrow leaf variety is suitable from early July to mid-September, with a longer window from early May to mid-October for the broad leaf variety.

Three modes of biological control are available - two beetles (Chrysolina hypericin and C. quadrigemina) and a mite (Aculus hypericin). Like many bio-controls, there is a boom-bust pattern, and the beetles in particular, are still recovering from the drought. Beetle re-establishment can be aided by collecting and transferring them to new infestations (see the Biocontrol hub app).

The beetles effectively defoliate plants in winter (larvae) and spring (adults) which can set the weed back by decreasing flowering and seed production. The beetles are more effective in unshaded areas and when active over consecutive years.

The mite is active year-round, but only attacks the narrow leaf variety. The mite feeds on growing tips and can kill a plant over 2-3 years by depleting root reserves.

Chemical control through selective herbicides (ideally November to January) can also be effective and is best used in conjunction with establishing and managing a competitive pasture.

For further information visit:

The Biocontrol Hub app is also available for download on your Apple or Android device.

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