Absorption, Translocation, and Foliar Activity of Clopyralid and Chlorsulfuron in Canada Thistle (Cirsium arvense) and Perennial Sowthistle (Sonchus arvensis)

Both 14 C-clopyralid (3,6-dichloropicolinic acid) and 14 C-chlorsulfuron {2-chloro- N -[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]benzensulfonamide} were readily absorbed by Canada thistle [ Cirsium arvense (L.) Scop. ♯ CIRAR] leaves, with 99 and 75%, respectively, of the applied doses absorbed 144 h after application. Absorbed 14 C-clopyralid was rapidly exported from the treated leaves, whereas 14 C-chlorsulfuron was translocated much more slowly. After 144 h, 29% of the applied 14 C-clopyralid and 5% of the applied 14 C-chlorsulfuron were recovered in the roots and developing root buds of Canada thistle plants. Smaller amounts of the two herbicides were absorbed and translocated in perennial sowthistle ( Sonchus arvensis L. ♯ SONAR) than in Canada thistle. More 14 C-clopyralid than 14 C-chlorsulfuron was absorbed and translocated out of treated leaves of perennial sowthistle, but equal amounts, 3 to 4% of the applied doses, were recovered in the roots and root buds 144 h after application. Foliar applications of clopyralid, followed by removal of the treated shoot 24, 72, or 144 h after application, markedly reduced shoot regrowth in both Canada thistle and perennial sowthistle. Similar treatment with chlorsulfuron did not prevent shoot regrowth in either species.