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Plan B on Postemergence Weed Control in Xtend Soybean (06/04/20)

We have received many questions about what to spray for postemergence weed control on Xtend soybean acres in light of the 9th Circuit Court’s decision on June 3rd to immediately vacate the labels for Xtendimax, Engenia, and FeXapan.

We have received many questions about what to spray for postemergence weed control on Xtend soybean acres in light of the 9th Circuit Court’s decision on June 3rd to immediately vacate the labels for Xtendimax, Engenia, and FeXapan. The situation will likely become clearer in the coming days, but what are the options if our dicamba spray season is now over? Below are options for some of our problematic weeds that will likely show up in Xtend soybean fields:

Waterhemp – It is safe to assume that any waterhemp is resistant to ALS-inhibiting (Group 2) herbicides. Glyphosate-resistance is also present on most acres, though not all plants will be resistant. Glyphosate is most effective on waterhemp up to two leaves when applied at labeled rates with adjuvants. The best remaining options would be PPO-inhibiting (Group 14) herbicides. Flexstar (fomesafen), Cobra, or Ultra Blazer could all be used on small waterhemp. The addition of oil adjuvants will be important for weed control. Flexstar cannot be applied after June 20th west of Highway 281.

Common lambsquarters – Glyphosate has historically provided variable control of common lambsquarters. Harmony (thifensulfuron) will be one of our best options left for the Xtend acres.

Kochia – Glyphosate is the best option for those who do not have glyphosate-resistant kochia. For the acres with glyphosate-resistance, Flexstar is one of the few remaining options and must be applied to small plants. Flexstar will work best by maximizing spray coverage and using full rates of oil adjuvants.

Common ragweed – Glyphosate, FirstRate, and Flexstar are the best remaining options for common ragweed control. We do have several populations resistant to glyphosate and FirstRate, so do not expect control with either product on those populations.

Horseweed/marestail – The safe assumption for horseweed is that it is glyphosate-resistant. This leaves FirstRate as the best remaining option. However, we do have some populations that are also resistant to FirstRate. Unfortunately, we are left with no effective postemergence options in Xtend soybean for horseweed populations that are resistant to both glyphosate and FirstRate. 

It is important to remember some of the best practices for applying these alternate options. For instance, Group 14 herbicides are contact herbicides that work better with higher carrier volumes and smaller droplets. Flexstar can also have carryover issues for rotational crops like corn and sugarbeet. Basagran is another herbicide option that can help control these weeds. However, we must reset weed control expectations compared to dicamba and target weeds smaller than 1 inch. Many weeds we are hoping to control may already be larger than 1 inch, so inconsistent control could be expected. Now is also a good time to reinforce the use of tank-mixing Group 15 herbicides with your postemergence applications for waterhemp control. The Group 15 herbicides will not control any emerged plants, but will help control later emerging waterhemp.

 

 

Joe Ikley

Extension Weed Specialist

 

Tom Peters

 Extension Sugarbeet Agronomist

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