St. Johnswort, 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 dangerous 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.

Where and When It Grows
St. Johnswort is a perennial that grows along roadsides and in meadows, pastures, rangelands, and waste places. In the Pacific Coast states, it may reach a height of 2 meters; in other areas, it is generally about 0.5 meters tall. It is a smooth-branched, erect plant.  It usually is found on dry, gravelly, or sandy soils in full sunshine. It may grow in dense patches or mixed among other plants. It is considered a noxious weed in many states.  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.

How It Affects Livestock
When an animal eats St. Johnswort, the poisonous compound in the plant, hypericin, reaches the skin from an internal route (stomach to blood to skin). Here it sensitizes the skin to sunlight. Pigments in the skin shield colored skin from sunrays so that only white or unpigmented areas are affected. Photosensitized lesions itch, become red, swollen, and sore, and the skin may peel or come off in large sheets. White-skinned cattle are more susceptible to St. Johnswort poisoning than white-skinned sheep.

Cattle are poisoned by St. Johnswort if they eat an amount equal to approximately one percent of their body weight and are then exposed to direct sunshine for 2 to 5 days. In experimental feedings, sheep were fed 5 percent of their body weight to cause symptoms. Signs of clinical poisoning usually appear 2 to 21 days after animals begin to have access to St. Johnswort. The delay probably is dependent on the time required for hypericin to build to a critical concentration in the skin.

Signs and Lesions of Poisoning