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Nightmare in the making: Marestail, giant ragweed poking through canopy

LOST CAUSE? Can these giant ragweed escapes be tamed? CCAs say it may depend on what type of tolerance is present in the soybeans. That determines what herbicide you can apply.
Soybean Bug Beat: Pests like weeds send off alarm bells when they show up in young soybeans.

Your soybeans are up and growing. More marestail escapes are showing up in a couple of soybean fields than you like. In another field, giant ragweeds are poking through. There are too many to ignore. What should you do?

The Indiana certified crop advisers panel took a stab at this question. The panel includes Traci Bultemeier, an accounts manager for DuPont Pioneer, Fort Wayne; Gene Flaningam, Flaningam Ag Consulting LLC, Vincennes; and Bryan Overstreet, Purdue University Extension ag educator in Jasper County.

Bultemeier: Contact your local retailer for application options that exist within their service area. If either species is thought to be glyphosate tolerant, and the soybean crop is glyphosate tolerant only (no tolerance to Liberty, which contains the active ingredient glufosinate), find some summer student help to walk fields and pull or hoe plants before seed sets.

Then, for the following year or years, consider a solid fall herbicide application, followed by a spring herbicide program that includes a strong residual. These programs can be costly, but the money saved by excellent weed control will far outweigh the initial cost. I recommend a fall and spring program ahead of non-GMO, LibertyLink, Roundup Ready, Xtend or any other type of soybean.

Flaningam: Marestail in Roundup Ready soybeans are almost impossible to treat once they have escaped your preemerge program. Giant ragweed escapes can be controlled or suppressed with different modes of action, such as PPO-inhibitor herbicides.

The Roundup Ready Xtend and LibertyLink soybeans are two different programs to consider. Evaluate your fields this year. Once soybeans get into the reproductive stage, yield losses may occur from postemergence herbicides. If it goes that far, you need to weigh out your options of yield gains vs. possible losses from injury in this situation.

Overstreet: First, answer this question: What, if any, herbicide-resistant traits do you have in the soybeans in those fields where you have weed escape issues? If you have LibertyLink soybeans with tolerance to glufosinate and marestail plants are less than 6 inches tall, you may be able to control them. Another option, if you have Xtend soybeans, is to try controlling them with XtendiMax or other herbicides approved and labeled for that system. If you have non-GMO or regular Roundup Ready soybeans, I am afraid you really have no good options to control marestail.

The answer may be very similar with giant ragweed. However, in most places in Indiana, we can still use PPO herbicides such as Flexstar or Cobra if the weeds are not too tall for these herbicides to control.

As it gets later in the season, you may want to shy away from Flexstar as a possible choice for going after weed escapes. That’s because Flexstar can have carryover issues into next year’s crop if applied late in the season. Refer to the herbicide label for Flexstar for specific information about plant-back restrictions.

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