Common St. Johnswort (Hypericum perforatum L.), or Klamath weed, is a range weed that causes animals to be highly sensitive to sunlight (photosensitivity). Animals that eat St. Johnswort and then are exposed to direct sunshine develop severe sunburns that are seen as skin irritations in non-haired or white areas. Young cattle and sheep are most often affected, but almost all white-skinned cattle, sheep, and horses react to eating the plant. Severe lesions often develop in the udders and teats of affected cows. This causes them to quit lactating and wean their calves. Recently sheared sheep are especially susceptible. Although St. Johnswort seldom kills, it causes severe economic losses.
St. Johnswort is a dangerous perennial weed at all stages of growth. Young tender shoots may attract animals in the spring. Normally, cattle and sheep will not eat mature St. Johnswort if they have other forage. Hay containing dry St. Johnswort can cause poisoning in the winter.
St. Johnswort grows along roadsides and in meadows, pastures, rangelands, and waste places. It grows as high as 2 meters. It is a smooth-branched, erect plant and is usually is found on dry, gravelly, or sandy soils in full sunshine. The leaves are covered with clear, small dots that contain the toxic substances (hypericin). Five-petaled flowers grow in clusters; they are orange-yellow with occasional black dots along the edges. After maturity, flowers wilt and the entire plant turns brown.
Signs of livestock poisoning are:
Scratching head with hind legs and rubbing head against solid objects
Seeking shade or standing in water
Rapid pulse, increased temperature
Redness and swelling of white-skinned areas (sunburn)
Swollen eyelids, clouded eyes; possibly blindness
Peeling or sloughing of affected skin
At the first signs of poisoning, move affected animals to shady or dark quarters. Treat affected skin areas with healing oil. Give animals plenty of fresh water and feed. Chemical or biological control with the Klamath beetle is recommended for extensive infestations.