Wort can be identified by its yellow flowers seen from October to January. Its sticky seed capsules adhere to domestic and native animals – hence its spread. Seeds can be carried in the digestive tract of animals and have been observed in cattle dung.

Wort contains the toxin hypericum, which causes photosensitisation in sheep, cattle, horses and goats. This results in skin damage and leads to livestock weight loss, reduced productivity and, in extreme cases, death.

Hypericum levels vary seasonally. Levels rise rapidly in spring once the new season flower shoots reach 5-10 cm.  Levels continue to rise as the flower head develops, reaching a maximum when in full flower.  Levels decrease as flowers are lost.  In late autumn only low to moderate levels of hypericum are present, with minimum levels of hypericum produced in winter.

Stock will only eat wort when pasture is scarce. It is poisonous, particularly to animals not accustomed to it. When ingested, hypericum passes from the stomach to the bloodstream.  Once in the skin vessels 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 an abnormally high temperature.

If animals continue to graze wort, their condition will worsen. They may rub their irritated heads and ears against anything available leaving raw, weeping, bleeding areas of skin.  

Production losses include: weight loss and failure to thrive due to a reduction in eating ability and interest in eating; less wool produced in sheep; less milk produced from cows leading to smaller calves; fewer lambs and calves born alive and less surviving to weaning; and fewer ewes or cows that are sufficiently heavy or healthy to conceive.

Wort seed adds significantly to the vegetable fault in wool. Wort competes with beneficial pasture species to reduce available feed and can reduce property values where it has taken over large areas of pasture.

Livestock affected by wort should be 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 secondary bacterial infections.

Pregnant and lactating animals should be removed from wort-infested pasture. Hypericum can cross from mother to foetus or suckling offspring resulting in the birth of weak or dead progeny and poor performance in suckling young.

Grazing management can be used in conjunction with chemical control by following these 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 – graze cattle 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 and;

- Provide adequate shade.