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Agronomy Day 2008

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Tour C

Giant Ragweed: Improved Management Options for Fighting an Old Enemy in Corn

A.J. Woodyard
A.J. Woodyard
Graduate Student
Department of Crop Sciences
217-244-6882
awoodyar@illinois.edu
Dean Riechers
Dean Riechers
Associate Professor
Department of Crop Sciences
217-333-9655
riechers@illinois.edu
Severe infestation of giant ragweed
Fig. 1: Severe infestation of giant ragweed
in corn, east of Urbana.

Giant ragweed (Ambrosia trifida L.) has long been a highly competitive, summer annual weed species found across Illinois and throughout the Midwest. However, in recent years the distribution of giant ragweed has shifted from predominantly undisturbed areas like fencerows and drainage ditches towards much of the fertile cropland in the state. Giant ragweed has historically been an early germinating species, with much of its germination completed by May 1st. However, giant ragweed emergence has recently been stretching through June and sometimes into late July, creating control issues due to multiple emergence timings. Giant ragweed has been shown to reduce corn yields by 37% with a single plant per square meter, indicating that excellent control of giant ragweed is warranted for maximizing yields in corn.

Injury to giant ragweed from
Fig. 2: Injury to giant ragweed
from Callisto only.

Giant ragweed is prevalent in both no-tillage and conventional tillage systems, therefore chemical control is necessary to deal with multiple emergence timings. The widespread and rapid adoption of glyphosateresistant corn in recent years has resulted in the use of glyphosate as the primary control option for giant ragweed. However, glyphosate resistance continues to become an increasing issue; for example, glyphosate-resistant giant ragweed has been reported in both Indiana and Ohio. Due to the continued threat and development of glyphosate resistance in giant ragweed throughout the Corn Belt, other management options must be considered in both conventional and herbicide-tolerant corn programs.

Injury to giant ragweed from
Fig. 3: Injury to giant ragweed
from Callisto + atrazine.

Managing giant ragweed in future years will likely require a combination of both preemergence and postemergence herbicide applications with multiple modes of action, since utilizing glyphosate as the only mode of action has recently shown signs of faltering. There are several preemergence herbicides that are effective in reducing early season competition from giant ragweed in corn, with the most common being atrazine. Several postemergence herbicides were evaluated in our field research this summer, including glyphosate, pigment inhibitors (bleachers including HPPD inhibitors), growth regulating herbicides (dicamba), as well as the synergistic tank mix of photosynthetic inhibitors (atrazine or Buctril) with HPPD inhibitors. Current field and greenhouse studies are also investigating the potential of utilizing the residual activity from a preemergence application of atrazine with a subsequent postemergence application of a HPPD inhibitor to benefit from the synergistic interaction and enhance control of giant ragweed. Our field study also examined potential tank-mix partners and adjuvant combinations for postemergence applications with glyphosate, including atrazine and the pigment inhibitors, to broaden the weed control spectrum, obtain residual activity, and possibly control herbicideresistant weeds.

Energizing Agriculture