According to Wikipedia:
Tansy (Tanacetum vulgare) is a perennial, herbaceous flowering plant of the aster family, native to temperate Europe and Asia. It has been introduced to other parts of the world including North America, and in some areas has become invasive.
The seeds of this invasive and poisonous plant float in on the breeze and travel the waterway depositing themselves in the rich loam from high water and in the fields.
Horses in particular are susceptible to the toxic properties of this plant. Cows and people can be affected also. If left to multiply, the weed could take over the hay field and the hay would then be toxic to the herd.
The plant has to be dug out with the roots intact in order to keep it from re-growing. Once the plant is out of the ground we have to take it with us away from the fields and river so the seeds from the drying plant do not drop to the ground for future plants.