Understanding aminocyclopyrachlor behavior in soil and plants (2007)

Many noxious and invasive weeds are perennial species that are inherently difficult to control. Canada thistle (Cirsium arvense) and field bindweed (Convolvulus arevensis) are two species of particular interest as they are capable of spreading quite rapidly through creeping underground reproductive structures and are able to continually regenerate from carbohydrate reserves stored in the roots. These weed species infest both cropland and non-cropland, including rangeland, pasture, natural areas, and rights-of-way, causing yield loss in crops from competition for soil resources and by harboring crop insect and disease pests as well as reducing ecosystem diversity in natural areas by displacing desirable or native vegetation with monocultures. Based on long-term weed control observed in the field with aminocyclopyrachlor (Lindenmayer et al. 2009), a better understanding of the herbicide's behavior in soil as well as within the plants was necessary. The objectives of this research were to (1) compare soil and foliar activity of aminocyclopyrachlor on Canada thistle to that of aminopyralid; (2) determine the dissipation rates of aminocyclopyrachlor, aminopyralid, and clopyralid under field conditions as well as evaluate their adsorption in six North American soils; and (3) evaluate aminocyclopyrachlor absorption, translocation, and metabolism in field bindweed. Results of the first study indicated that aminocyclopyrachlor was just as effective when applied to the soil as it was when applied to Canada thistle foliage and was similar to aminocyclopyrachlor for up to one year after treatment. The study also revealed that Canada thistle biomass was reduced to a far greater extent when either aminocyclopyrachlor or aminopyralid was absorbed via root tissue than by emerging shoot tissue. Overall, these results suggest that Canada thistle control can be achieved even through dormant season applications, reversing the tradition of spring or fall applied herbicides to actively growing foliage and that xylem mobility throughout Canada thistle plants from root absorption may contribute to more effective weed control. Results of the second study revealed that aminocyclopyrachlor, aminopyralid, and clopyralid all had similar dissipation rates under field conditions with soil half-lives of 32.5, 28.9, and 26.6 d, respectively. Mobility of aminocyclopyrachlor and aminopyralid was limited for the first 14 d with some downward movement after 28 d, while clopyralid had more significant leaching by 14 d. Adsorption in the six soils tested was greatest with aminocyclopyrachlor, followed by aminopyralid, and clopyralid had the least soil adsorption with average Kd values across the six soils of 0.503, 0.378, and 0.236 mL g-1, respectively. Adsorption was generally correlated with soil organic matter or texture, but not with pH. These results agreed with previously published information about aminopyralid and clopyralid and shed new light on aminocyclopyrachlor soil behavior. Results of the third study showed that aminocyclopyrachlor absorption in field bindweed was maximized at 48.3% of the applied radioactivity by 48 hours after treatment (HAT). A translocation pattern of movement out of the treated leaf into the other plant tissues was revealed, with nearly equivalent aminocyclopyrachlor distribution between the treated leaf, above-ground tissue, and below-ground tissue at 192 HAT. Over the 192 h, no soluble metabolites were observed, but an increasing portion of the radioactivity was found in the fraction bound to the plant tissue. These results indicate that aminocyclopyrachlor has greater translocation to below-ground tissue in field bindweed compared with other herbicides and other weed species and aminocyclopyrachlor is not rapidly metabolized in any field bindweed plant tissue.