It is easily recognised by the yellow flowers from October to January and leaves that appear perforated when held up to the light.

The sticky seed capsules adhere to animals, both domestic and native, hence it’s spread along roads and animal tracks. Seeds are also carried in the digestive tracts of animals and seedlings have been observed in cattle dung. The weed’s seed also adds to the vegetable fault in wool. It competes with beneficial pasture species to reduce available feed and can reduce property values where it has taken over large areas of pasture.

The problem

St John’s wort contains the toxin hypericum which causes photosensitisation in sheep, cattle, horses and goats. 

The skin damage leads to weight loss, reduced productivity and, in extreme cases, death. 

The hypericum levels vary with the season. In spring, levels rise rapidly with the new season flower and continue to rise to reach a maximum when the plant is in full flower. Levels decrease as the flowers are lost. In late autumn, only low to moderate levels of hypericum are present with minimum levels produced in the winter.

Effects on livestock

Stock will only eat St John’s wort when pasture is scarce; however it is quite poisonous, particularly to animals not accustomed to it.

Once ingested, hypericum is activated by bright sunlight, altering its structure and making the compound potentially poisonous. On sunny days, livestock grazing pastures heavily infested with flowering wort can develop clinical signs in less than five hours. Early symptoms include agitation, head-rubbing, intermittent hind limb weakness with knuckling over, panting, confusion and depression. Some animals may develop mild diarrhoea.

This is followed by inflammation and swelling of the skin around the forehead, ears and eyes. Affected animals also have a high temperature. If affected animals continue to graze wort, affected livestock will rub their irritated heads and ears against anything available leaving raw, weeping, bleeding areas of skin which dry to form crusty scabs.

Pregnant and lactating animals should always be removed from wort-infested pasture. Hypericum can cross from the mother to foetus or suckling offspring.

Production losses that follow include weightloss and failure to thrive due to a reduction in eating; less wool produced; less milk produced from cows leading to smaller calves; fewer lambs and calves born alive and fewer ewes or cows that are sufficiently healthy.

Treatment

Removed from pasture and kept in full shade for a minimum of four days but up to seven days. Severe cases may need antibiotics to cover for secondary bacterial infections.

Tolerance in animals

Since hypericum only becomes poisonous after it has been activated by sunlight, an animal’s tolerance is affected by the amount of skin protection it has.

Factors such as skin pigmentation, dense wool, tough skin and adequate shade will increase tolerance.

Control

Grazing management can be used in conjunction with chemical control by following a few simple guidelines.

* Superfine or fine-wool adult merino wethers or dry, non-pregnant ewes with minimum of four months wool–graze from early July to mid September

* Fully pigmented red or black cattle can graze about six weeks earlier and six weeks later than sheep

* Several shorter periods using high-stocking rates will be more effective

* Fence off worst areas to facilitate repeated heavy grazing

* Where possible, avoid using pregnant, lactating or young stock to graze infested areas

* Always provide adequate shade.