Absorption, Translocation, and Metabolism of Imazethapyr in Common Ragweed (Ambrosia artemisiifolia) and Giant Ragweed (Ambrosia trifida)

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Common and giant ragweed are important weeds of soybeans in Indiana. These two weeds respond differently to imazethapyr POST treatments with common ragweed demonstrating more tolerance than giant ragweed. Both plants show initial susceptibility to imazethapyr, but common ragweed can regrow 10 to 14 days following herbicide application. Laboratory studies were conducted to determine the factors that contribute to the differential control of common and giant ragweed with imazethapyr. Differential absorption was observed at 72 h, with common ragweed absorbing 52% of the applied14C-imazethapyr and giant ragweed absorbing 39%. The absorption of radioactivity was the same for both species by 672 h. Imazethapyr exhibited both xylem and phloem mobility by translocating both acropetally and basipetally from a treated leaf in giant and common ragweed. A higher percentage of the absorbed radioactivity accumulated in the lower foliage and roots of giant ragweed than common ragweed by 336 h. The rate of imazethapyr metabolism in common ragweed was greater than in giant ragweed. At 336 h, 81 and 68% of the identified radioactivity in the treated leaf was imazethapyr metabolites in common and giant ragweed, respectively. A higher level of the inactive glucose conjugate metabolite was found in the lower plant and root tissues of common ragweed than in giant ragweed. The differential control of common and giant ragweed with foliar applications of imazethapyr was attributed to differences in both translocation and metabolism.