Research Highlights

Research Highlights
Looking for Better Herbicide Management of Volunteer Corn in Soybean Fields

Volunteer corn growing in soybean fields is a nuisance and can be difficult to control. The photos taken July 19, 2022, show evidence of tank mix antagonism on the left. The split-application plot (right) shows no antagonism. Photos: Ryan Miller

By Carol Brown

Weed management is crucial for crop farmers everywhere. But when a soybean field contains volunteer corn from the previous season, dealing with weeds becomes more complicated. With herbicide-tolerant corn varieties, termination of volunteer corn can be a difficult situation.

A research project in Minnesota focuses on successfully managing volunteer corn and weeds. Ryan Miller, a crops educator at the University of Minnesota Extension regional office in Rochester, is leading a study that compares numerous herbicides and combinations to find the best approaches for volunteer corn termination. The project is supported by the Minnesota Soybean Research and Promotion Council. 

“Volunteer corn is tricky,” Miller says. “Year to year, farmers are going to have different amounts of corn loss from field to field. We are learning the best strategies for managing volunteer corn with herbicides. We designed this project to look at different rates and tank mixes with various combinations of products.”

Miller and his research team conducted the project on research plots at Rochester and Waseca in 2022 and 2023, for a total of four site years of data. The team compared 19 herbicide treatments at each site, along with an untreated control plot.

Herbicide rates: Enlist One, 2 pt/a; glyphosate as PowerMax 3, 30 fl oz/ac; Select Max (low rate), 6 fl oz/ac; Select Max (high rate), 9 fl oz/ac; Assure II (low rate), 4 fl oz/ac; Assure II (high rate), 12 fl oz/ac; Dual II Magnum, 1 pt/ac.
Additives: AMS as Amos, 5% v/v and 2.5% v/v; NIS as Preference, 0.25% v/v; COC as SuperHC, 1 qt/ac
1 = lower rate of AMS used with the initial post-applications in the sequential treatments
2 = sequential applications 7 days following first post-treatment

There are several reasons why volunteer corn in a soybean field is undesirable. In addition to increased issues at harvest with corn grain mixed in with the soybeans, Miller’s project showed reduced soybean yields of 10 to 20% at the Rochester site. Within the soybean rotation, volunteer corn can serve as a host for the corn rootworm beetle. If the corn is left in the field too long, the eggs can hatch, allowing the beetle to complete its lifecycle. There is also a chance that the volunteer corn will recruit beetles from other fields for egg laying late in the season, Miller says, which diminishes the benefit of having a crop rotation.

“Getting better management of weeds such as herbicide-resistant waterhemp has been a major driver in the adoption of the E3 soybean platform, which are Enlist, glufosinate- and glyphosate-tolerant soybeans,” he says. “But when grass-controlling herbicides, or graminicides, are tank-mixed with growth regulator herbicides, there can be antagonism and ultimately reduced grass control, which includes volunteer corn.”

Split applications work well for volunteer corn control when using reduced rates of grass-controlling herbicides. In the project, Miller applied the second herbicide treatment seven days after the first application. The follow-up application avoids the antagonism that occurs with tank mixes. But a split application includes additional costs that aren’t offset by the reduced herbicide costs.

“We considered factors such as increased herbicide rates and split applications and the economics behind them,” Miller says. “Split applications aren’t popular with a farmer or an applicator, so there are tendencies to use tank mixes to save some money.”

With tank mixes comes the possibility of antagonism between products, and the team is looking to find strategies that work around these issues. There are theories about where and how the antagonism is happening. Miller says researchers have demonstrated that it is a chemistry issue, but other researchers found evidence that it may be a plant response. Whatever the reason, Miller is looking for better weed and volunteer corn control through herbicide management. 

“We tested two products from different families of grass-controlling herbicides: Assure 2 (quizalofop) and Select Max (clethodim). Both products worked at higher application rates,” he says. “Our treatment structure included each graminicide applied in tank mixes with 2,4-D. We wanted to tease out whether the Roundup was having an impact or causing antagonism, too.”

Miller’s treatment structure included high and low rates each of Assure 2 and Select Max tank-mixed with:

  • 2,4-D alone
  • 2,4-D plus glyphosate
  • 2,4-D plus S-metolachlor
  • 2,4-D plus glyphosate and S-metolachlor. 

The team is also looking at a residual herbicide’s impact on volunteer corn control. Several treatments included the addition of a S-metolachlor product, as Miller and the research team have encouraged the use of overlapping residuals for weed control, but it did not have an impact on volunteer corn control in this project.

If farmers are growing 2,4-D tolerant soybeans, Miller and his team recommend increasing the rates of graminicides if they are tank-mixed with a 2,4-D herbicide to overcome any antagonism. An alternative strategy would be a sequential application of a grass-controlling herbicide seven days after an application of 2,4-D and glyphosate. 

Additional Resources

Managing Volunteer Corn in 2,4-D Tolerant Soybeans – University of Minnesota Extension fact sheet

Published: Dec 11, 2023

The materials on SRIN were funded with checkoff dollars from United Soybean Board and the North Central Soybean Research Program. To find checkoff funded research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.