Stack It Up

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Sprayer in corn field

Keeping track of fields, herbicide-resistant crop traits and widespread occurrence of herbicide-resistant weed challenges seems like a never-ending task to avoid severe misapplication mistakes. Yet, University of Nebraska weed scientist Amit Jhala says confusion about these trait stacks among growers continues as farmers raise questions at field days and winter meetings, and through e-mails and social media.

“Most growers now select corn and soybeans resistant to two or three herbicides because glyphosate-resistant waterhemp, Palmer amaranth, marestail and giant ragweed are widespread,” Jhala says. “These multiple herbicide-resistant trait stacks give growers the flexibility to use other herbicides like dicamba or 2,4-D choline and/or glufosinate when needed depending on trait planted.”

While there exist multiple herbicide-resistant trait options in corn, they are fewer and, to date, less complicated that the options for soybeans. There are currently soybean traits with resistance to glyphosate, glufosinate, isoxaflutole, 2,4-D choline, and dicamba.

“Spraying the wrong herbicide on the wrong trait is a whole different level of disaster,” Jhala says. “Communications with neighbors and custom applicators are critical to avoid misapplication and off-target drift. Good records are a must.” 

However, the limitations of these stacked traits happen when growers over-rely on herbicides for their weed management. “In the next few years, I think we will have a [weed] population which will be resistant to almost all postemergence herbicides tools,” Jhala says. “At some point Palmer will resist all four postemergence herbicides that we rely on in corn and soybeans.”

Fortunately, one concern seen in some hybrids and varieties from the early days of biotech seed breeding is gone. Thanks to improved genetic science, added traits stacked into corn and soybeans no longer cause yield penalties. Growers don’t have to choose between yield and trait stacks. Growers still have to contend with herbicide misapplication when it comes to yield loss in traited crops.

Complex Decisions

Preemergence residual herbicides are the weed management foundation of herbicide-resistant weeds in a corn/soybean cropping system. “As long as the herbicides are labeled for the crop and applied as directed before emergence, it doesn’t matter what trait stacks you use,” Jhala says. “This is because preemergence herbicides applied at crop planting are selective.”

However, the complexities stack up as farmers make trait buying decisions with postemergence herbicide applications. Careful planning is critical to apply only those herbicides in the herbicide-tolerant trait package you planted, Jhala says.

For example, residual herbicides like flumioxazin, sulfentrazone and other combo chemistries in the PPO-inhibiting group must be applied within two to three days of planting soybeans. What’s nice is these chemistries can also be used on Enlist E3® soybeans or any other type of soybean variety.

“Starting clean is our first recommendation, especially when managing herbicide-resistant waterhemp and Palmer amaranth that has a wide window of emergence,” Jhala says. Using preemergence herbicides with multiple modes of action can also delay postemergence herbicide application. Fewer weeds will be exposed to postemergence herbicides which can reduce the odds these weeds become resistant.

Volunteer Corn

More herbicide options will encourage farmers to use herbicide-tolerant stacked traits. As a result, the need for multi-year crop, trait, and herbicide use planning grows critical. “In corn, you must plan for controlling volunteer corn the following season,” Jhala says. “Whatever corn traits you planted this year means that volunteer corn will resist those herbicides next season.”

For example, if you planted Enlist® corn this year, then next year’s volunteer corn is resistant to four different herbicides (glyphosate, glufosinate, 2,4-D Choline, and FOP-based Group 1 herbicides). “So, then your options to control volunteer corn are rotating to soybeans and using DIM-based Group 1 herbicides like clethodim or sethoxydim,” Jhala says.

For farmers who grow corn-on-corn, using Enlist on second-year corn will control glyphosate and glufosinate-tolerant volunteer corn using quizalofop-based herbicides. Then, rotating to soybeans is a must.

Don’t Forget Adjuvants

Jhala stresses the importance of reading herbicide labels which spell out product use details. “Always use the recommended adjuvants like ammonium sulfate, crop oil or non-ionic surfactant,” he says. “The fewer the adjuvants the better so you don’t risk crop injury, especially with contact herbicides like lactofen and acifluorfen.”


Content provided by DTN/Progressive Farmer.

The transgenic soybean event in Enlist E3® soybeans is jointly developed and owned by Corteva Agriscience and M.S. Technologies L.L.C. Enlist Duo® and Enlist One® herbicides are the only 2,4-D products authorized for use with Enlist® crops. Consult Enlist herbicide labels for weed species controlled. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Always read and follow label directions .