Friday, May 13, 2016

Watch for Giant Ragweed Emerging in Early Planted Soybeans

May 13, 2016

Southeastern Minnesota has been busy planting corn and soybeans over the past 4 weeks.  Much of the corn was in before the end of April and several fields of soybeans were planted before the calendar was changed to May as well.  Once May came, there were many good planting days, so much of the soybean crop is now in the ground, too.  It is great to have the crop in the field, isn't it.  For a number of folks, there were also windows of opportunity to apply preemergence herbicides on both crops.

Applied or not, the next job on the list is to watch the emergence of your crop and weeds.  An early planting favors giant ragweed, one of our first troublesome weeds out of the ground.  We already have seedling giant ragweed in our corn herbicide trials (planted April 25).  The plots with no preemergence herbicides or ones with herbicides weak on giant ragweed have a nice carpet of seedlings in them.  The message, this weed grows quickly and can pass the 2-3 inch mark very quickly.  Once past the 2-3 inch mark, it becomes more challenging to control, especially if it is herbicide resistant. There are several good options for post emergence control in corn, but not so in soybeans.

If you are dealing with glyphosate resistant giant ragweed in RR soybean, you need to target them at 2 inches with a SOA-2 or SOA-14 herbicide.  Examples include FirstRate, Flexstar GT, or Cobra.  If you also have SOA-2 resistant giant ragweed, the FirstRate will not work for you.  If you planted LibertyLink soybean and did not get a preemergence herbicide on, you need to target the first glufosinate (Liberty) application when the ragweed is small and include a residual that will help with the giant ragweed and other weeds.  See results from our 2015 herbicide trial near Rochester on SOA-2 and SOA-9 resistant giant ragweed for more details. 
Weed Control Programs for SOA-2 & SOA-9 Resistant Giant Ragweed

To view more herbicide trial results in corn and soybeans check out our complete report at:
2015 Southern Minnesota Crops Research Report

Seedling giant ragweed

Lisa Behnken, Extension Educator, Crops (lbehnken@umn.edu)  

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