Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/3/2008
Publication Date: 8/26/2008
Citation: Bukin, B., Gaines, T., Nissen, S., Westra, P., Shaner, D.L., Sleugh, B.B., Peterson, V.F. 2008. Aminopyralid and Clopyralid Absorption and Translocation Differences in Canada Thistle (Cirsium arvense). Weed Science. Interpretive Summary: Canada thistle is an invasive weed in agricultural and natural situations. Management of this weed is difficult. Aminopyralid is a new herbicide that has been registered for the management of Canada thistle in natural situations. Aminopyralid is an analog of chlorpyralid but is used at a lower rate. The purpose of this research was to determine if the higher efficacy of aminopyralid compared to chlorpyralid is due to differences in absorption, translocation or metabolism. This research shows that the higher efficacy of aminopyralid is not due to increased absorption, translocation or metabolism compared to chlorpyralid. The results suggest that the increased activity of aminopyralid is due to greater inhibition of the target site for these herbicides.
Technical Abstract: Aminopyralid (4-amino-3,6-dichloro-2-pyridinecarboxylic acid) is a new auxinic herbicide that provides Canada thistle control at lower use rates than clopyralid (3,6-dichloro-2-pyridinecarboxylic acid). Studies were conducted to determine if differences in absorption, translocation, or metabolism account for aminopyralid's greater biological activity. Radiolabeled aminopyralid and clopyralid were applied to individual leaves of rosette-stage Canada thistle plants. Non-ionic surfactant (NIS) was used for the absorption studies because it provided higher aminopyralid absorption than methylated seed oil or crop oil concentrate. Clopyralid was absorbed very rapidly reaching 72% 24 hours after treatment (HAT) and remaining near or above 80% over a 192 h time course, while aminopyralid absorption increased from 34% to 60% over the same time period. Clopyralid translocation out of the treated leaf was significantly higher than aminopyralid, 39% compared to 17% respectively, 192 HAT. More applied clopyralid translocated to above ground tissue 192 HAT (27%) than to roots (12%), while aminopyralid translocation was similar to above ground tissue (10%) and roots (7%) 192 HAT. Neither aminopyralid nor clopyralid were metabolized 192 HAT. Although aminopyralid is effective at lower use rates, clopyralid has higher absorption and translocation in Canada thistle. These results suggest that aminopyralid's chemical structure may provide greater biological activity at the target site than clopyralid.