Small Farms Program Supervisor
Weed control is important this time of year no matter what part of your land you're working with. Spare time for digging up weeds is always limited, so focus on controlling weeds that are poisonous or nuisance weeds like thistles.
In western Washington, one weed that causes damage to stock is tansy ragwort. Tansy ragwort is a member of the sunflower family. The botanical name for this large family of plants is Compositae or Asteraceae. There are 19,000 members and include; artichokes, marigolds, mums, dandelions, daisies knapweeds, thistles, zinnias, and common groundsel, which is another toxic weed common in this area.
Tansy spreads mainly by the production of seed. Allowing the weed to set seed in overgrazed pastures enables tansy to become established. Since tansy easily invades areas where the soil is exposed, it helps to move animals off of any wet areas before they break through the sod layer. Tansy cannot compete with a dense grass stand. You should look at controlling tansy in non-pasture areas as well. Make your neighbors aware of the problems associated with this weed and stress the importance of control. Since it is a biennial, you don't have to worry about reinfestation from the underground roots. Don't allow it to reseed itself.
The problem with tansy is that it contains alkaloids that in the presence of certain liver enzymes, are converted to substances called pyrroles. These can cause irreversible liver damage to horses, cattle, and to a lesser extent goats. The damage has a cumulative effect and can eventually kill the animal without obvious symptoms.
Tansy ragwort is often confused with common tansy. Common tansy is a weed that is closely related but is not dangerous to livestock. It can be differentiated from tansy ragwort by the differences in its leaves and seedhead. The leaves of tansy ragwort are usually dark green and deeply veined. Common tansy has small, individual leaves that are serrated, light green and look somewhat "fern-like".
The flower of tansy ragwort is yellow and has 10-13 petals. The yellow flower of common tansy is more of a "button" and never really opens up. Feel free to call the King Conservation District anytime for identification assistance.
Tansy digs up easily in most soils. Don't leave tansy in pastures after clipping or pulling it out. Wilted tansy actually becomes more palatable to grazing livestock. If you plan to burn the weeds after you have pulled them out, do it with great care. Certain literature warns that the smoke from burning tansy can be bad for your health. It is not recommended to compost the dead plants. The safest way to dispose of tansy is to put the dead plants into large leaf bags and put them out with your regular trash. Always wear gloves when handling this weed.
Herbicides may be used for control in heavily infested areas. To get control without using a lot of herbicide, clip the weeds first and spot spray or use a wick applicator to apply. Biological or natural controls are available too. The immature form of the cinnabar moth feeds and defoliates tansy plants. Another insect that feeds on tansy is the adult form of the ragwort flea beetle. Both of these insects can be used effectively as part of an integrated pest management system. Effective and complete control of tansy may take 2-4 years but is well worth the trouble compared to vet bills or loss of a family horse or show animal.
For more information refer to: PNW Cooperative Extension 210, Pasture Management for Control of Tansy Ragwort. reprinted 8-84.