Weeds are a hazard that requires diligence in soybean fields, Extension specialists say.

In the above photo, soybeans are shown in a bag and overflowing with credit to pnmralex at Pixabay.

Kevin Bradley, a state Extension weed scientist and education director for the University of Missouri, said in his state many corn growers are also soybean producers, so they are well aware of the weed problems each crop faces.

“Our No. 1 worst weed in the Midwest is waterhemp,” he said. “It has been for some time and likely will be for some time to come. In the South, that changes to Palmer amaranth. In the Western corn belt, kochia is also a concern.”

Regardless of the crop, combating weeds is necessary.

“There are some obvious problems that may be present in fields that were intended for corn and had a fall herbicide application that only allows for corn in the rotation the next year,” he said.

Bradley writes for Mizzou Crop and Post News about the topics. As thinks about “soybean on soybean rotation” one concern is it puts a greater pressure on post-emergence herbicide and greater likelihood of resistance development.

Sarah Lancaster, an assistant professor and Extension specialist in weed science at Kansas State University, said growers should first think about what herbicides had been applied in the summer or fall of 2023 and if carryover injury might be a concern.

“The second thing to think about would be which herbicide-resistant trait to choose,” she said. “The most common ones in the marketplace are Enlist E3 or XtendFlex. I’m starting to hear some concerns about the availability of over-the-top dicamba formulations for the XtendFlex system, so I would encourage anyone interested in that system to contact their input providers as soon as practical.”

Treatments to consider

Bradley believes growers must rely on effective soil residual herbicide programs as much as possible. Currently soybean growers rely mostly on three active ingredients for post-emergence control of pigweed species—glufosinate, 2,4-D and dicamba.

If the Environmental Protection Agency does not allow dicamba application in 2025 it could make a difference on how soybean acres are allocated.

“I believe it would change the percentage of acres that get planted with XtendFlex soybeans quite a bit,” he said. “I would expect much more Enlist soybeans would get planted, and much more glufosinate and 2,4-D would get sprayed.”

Lancaster said one of the most important herbicide considerations for soybeans or corn is to use an effective and aggressive residual herbicide program. That starts with the burndown application. With the assumption that most soybean are going to be in a reduced tillage system, an early application to burndown emerged weeds should include herbicides like glyphosate, 2-4-D and dicamba.

A residual product like flumioxazin (Valor and similar products) or metribuzin (Tricor and similar products) can be helpful to prevent the emergence of new weeds, she said. Adding a contact herbicide like Sharpen or Reviton can help control difficult weeds like marestail, also known as horseweed.

Planting time

At planting, apply residual herbicides from at least two herbicide groups at full rates, Lancaster said. For Palmer amaranth and waterhemp, options include Group 15 herbicides like S-metolacher (Dual), acetocholor (Warrant), dimethenamid-P (Outlook) or pyroxasulfone (Zidua and Anthem) and Group 14 herbicides like flumioxazin (Valor) or sulfantrazone (Spart). Metribuzin (Group 5) is also an option.

“Be aware of maximum annual rates for products that are used in both the burndown and at planting application,” Lancaster said.

In season, the options depend on the herbicide-resistant trait that is chosen, she said. Regardless of the herbicide, timely application is the key to success. In the Enlist system, some of the best postemergence treatments for pigweed control include Enlist One in combination with Liberty. Glyphosate can be added for grass control, but avoid a Group 1 herbicide like clethodim (Select Max) or quizalofop (Assure II) because of concerns with angagonism.

In the XtendFlex system, Liberty remains a useful product; however, it cannot be tank mixed with XtendiMax, Engenina or Tavium, she said. Group 14 herbicides such as fomesafen (Reflex) can be mixed with dicamba products, but it is important that they be applied to small weeds (less than 4 inches) for a successful application.