Photo 1. Giant ragweed in corn herbicide trial at Rochester, MN, May 20,2016. Plot was planted April 25, 2016.
Photo 2. Giant ragweed in corn herbicide trial at Rochester, MN, May 20, 2016. Plot was planted April 25, 2016.
Currently, giant ragweed is moving into the exponential growth stage, and will pass the ideal 2-4 inch weed height for good control very soon. Once past this ideal size it becomes more challenging to control, especially if the biotype is herbicide resistant. In corn, there are several good options for post emergence control, but application needs to occur soon. Unfortunately, the wet fields may give the advantage to the giant ragweed. In soybeans, be vigilant. There are not as many options for post emergence control of giant ragweed, plus it may become difficult to hit the optimum herbicide timing for ideal control. This is especially true if you have herbicide resistant giant ragweed.
Weed control options in soybean
If you are dealing with glyphosate resistant giant ragweed in Roundup Ready soybean, those seedlings need to be targeted at 2-3 inches with a SOA-2 or SOA-14 herbicide. Examples include FirstRate (SOA-2), Flexstar GT, or Cobra (SOA-14). However, if the giant ragweed is also SOA-2 resistant, FirstRate will not work and SOA-14 herbicides become the best option. See PRE and POST Herbicide Diversification Options for Glyphosate-Resistant Corn and Soybean for additional information.
If you planted LibertyLink soybeans, and there wasn’t an opportunity to get a preemergence herbicide on, the first glufosinate (Liberty) application should be targeted when the ragweed is small. Include a residual, such as Reflex or Flexstar (NOT FlexstarGT) with the glufosinate to help control the last flushes of giant ragweed and other weeds, such as waterhemp, that will become troublesome as the season progresses. Note, adding a Group 15 herbicide such as Dual II Magnum, will assist with later season waterhemp control.
For more information
See results from the 2015 herbicide trial near Rochester on SOA-2 and SOA-9 resistant giant ragweed for more details, Glufosinate (LibertyLink) and glyphosate (Roundup Ready/GT) weed control programs for SOA-2 & SOA-9 resistant giant ragweed in soybean.
Also see the Crop News article, Herbicide resistant giant ragweed control: Alternative herbicide options are limited and the video series, Herbicide resistant giant ragweed, on the U of M Crops YouTube channel: https://www.youtube.com/user/UMNCrops.
Additional information is available on the University of Minnesota Extension Weeds website: http://z.umn.edu/weeds.