Herbicide resistant volunteer corn control in soybean continues to be a challenge for farmers in Minnesota. Volunteer corn (VC) can directly impact soybean production by competing with soybean and reducing yield, VC can create harvest challenges, and lead to issues with foreign material in harvested soybean grain. In addition, VC in soybeans can have implications for effective corn rootworm management, and corn rootworm is one of the biggest threats to corn production which impacts many soybean growers.
Further complicating the management of VC is the adoption of Growth Regulator Tolerant (GR) soybeans. It has been well documented that herbicide tank mixes with growth regulators (e.g. Enlist and Xtend) and graminicides (e.g. Select Max and Assure II) can reduce grass and VC control in soybean. In a 2021 winter survey of 649 Minnesota soybean growers 52% indicated they would plant E3 enlist soybeans, 19% would plant XtendFlex soybeans, and 32% would plant Xtend soybeans. The survey question was about soybean herbicide resistance trait selection for 2022 planting intentions, so growers may have selected more than one technology, but what remains clear is that there has been widescale adoption of these growth regulator technologies. After the 2022 season it is expected that the 2,4-D tolerant soybean acres will continue to increase significantly.
These challenges in controlling VC in 2,4-D tolerant soybeans have been realized by many soybean growers. Conversations with farmers and retail agronomists have led me to believe we have more questions than answers on how to best control volunteer corn. Initial results from the project completed in 2022 have helped answer some of these questions and second year will help confirm guidelines for the best and most cost-effective approaches to managing VC.

1. Determine the best and most cost-effective herbicide program for weed and volunteer corn control in 2,4-D choline tolerant soybeans.
2. Evaluate the potential for herbicide antagonism between 2,4-D and graminicides (i.e. controls grasses and volunteer corn) herbicides.
3. Demonstrate a layered residual herbicide and evaluate any impact it has on volunteer corn control.
4. Communicate these results with Minnesota farmers and Ag professionals working with these farmers.

1. At each site, the project will be incorporated into public field days for farmers and ag professionals. This project will also be highlighted during small group tours that are available to be scheduled upon request of small groups of farmers, retailers, or industry representatives.
2. In-season updates will be provided via extension crop news blog, twitter posts, and on the strategic farming webinar series.
3. At season’s end the results will be summarized and published in a research report, to be posted on the web.
4. Results will be disseminated via traditional extension activities including winter meetings, workshops, webinars, and newsletters/blogs.
5. We will post a short video on the crops YouTube channel.
6. We will cover this project/subject on the Gopher Coffee Shop Podcast
7. Results will be incorporated into a “Best Practices” segment on the Farm Connections television program. The Farm Connections program airs on KSMQ out of Austin, MN and Pioneer TV out of Granite Falls, MN.
8. We will explore opportunities to collaborate with some of the Northern Soybean Marketing efforts on foreign material in soybean grain and export implications.

Update:
Research sites were established in Waseca, MN at the Southern Research and Outreach Center and at the Extension research and demonstration site in Rochester, MN. Plots were planted, sprayed, and evaluated season long. Plot yields will be taken in the coming weeks. Data is being summarized and a cumulative report will be published online.

2022 results were shared at a field day in Rochester with 40 participants, participants also had the opportunity to see the 2023 research plots. Plots and results were also shared with two industry groups and a group of Brazilian farmers and ag professionals.

Update:
Research sites were established in Waseca, MN at the Southern Research and Outreach Center and at the Extension research and demonstration site in Rochester, MN. Plots were planted, sprayed, and evaluated season long. Plots have been harvested. Data is being summarized and a cumulative report will be published online.

2022 results were shared at a field day in Rochester with 40 participants, participants also had the opportunity to see the 2023 research plots. Plots and results were also shared with two industry groups and a group of Brazilian farmers and ag professionals.

This project was featured in an article in Source Magazine: https://extension.umn.edu/news/when-corn-becomes-weed
Results have been published on the following website too: z.umn.edu/antagonismstudy

Results are going to be shared in an article that is in the queue for the December edition on the Soybean Research and Information Network. The project will be part of the discussion on the February 7th Strategic Farming Let’s Talk Crops Session.

Results will also be shared in PowerPoint presentations at various meetings this winter.

Update:
Project data has been collect and is being summarized for use in an academic article and an extension fact sheet.

Results were shared during our winter meeting series, applicator training sessions, and ag expo. I would estimate that well over 1,000 farmers and ag professionals have seen results from this study.

We will continue to share results this summer at field events.

Updated June 3, 2024:
We are in the process of writing a final extension publication and academic paper on this project. The intention is to make physical copies of the extension publication available at meetings this winter.
Results have been share as previously mentioned and published in several location so far.

https://soybeanresearchinfo.com/research-highlight/looking-for-better-herbicide-management-of-volunteer-corn-in-soybean-fields/

https://extension.umn.edu/news/when-corn-becomes-weed

https://extension.umn.edu/regional-research-trials/managing-volunteer-corn-24-d-tolerant-soybeans

View uploaded report

View uploaded report 2

Soybean varieties tolerant to 2,4-D-choline, glyphosate, and glufosinate have been widely adopted by Minnesota soybean growers. While 2,4-D tolerant soybeans provide growers with another site of action to manage glyphosate-resistant weed populations, there has also been difficulty in achieving adequate control of volunteer corn in this system. The ACCase-inhibiting herbicides (e.g. Assure II and Select Max) when tank mixed with auxinic herbicides (e.g. 2,4-D) showed antagonism and resulted in reduced control of grassy weeds. Growers relying on previously effective herbicide rates and application strategies (i.e. tank-mixes) are often surprised when they do not achieve adequate volunteer corn control. The objective of this research was to evaluate the interaction between ACCase-inhibiting herbicides clethodim (select max) and quizalofop-ethyl (Assure II)) and 2,4-D choline alone or tank-mixed with glyphosate or S-metolachlor (Dual II Magnum) for glyphosate-resistant volunteer corn control in 2,4-D tolerant soybean. In 2022 and 2023, a randomized complete block experiment was designed and implemented at two field locations in Southern Minnesota. Volunteer corn seeds were collected from grain that was harvested in the previous year from a field that was planted with a glyphosate resistant hybrid. To get a consistent stand of volunteer corn, corn grain was planted 1.5 inches deep at a density of 25,000 plants per acre, in 30 inch rows planted perpendicular to the soybean rows. An initial application of 1.33 pints of Dual II Magnum (S-metolachlor) was sprayed PRE to keep weed pressure down without affecting volunteer corn growth. Clethodim (select max) was applied at 6 or 9 oz per acre, and quizalofop-ethyl (Assure II) was applied at 4 or 12 oz per acre. The high and low doses of each graminicide were POST applied in tank mix combinations with 2,4-D choline alone, 2,4-D choline plus glyphosate, 2,4-D choline plus S-metolachlor, and 2,4-D choline plus glyphosate plus S-metolachlor. In addition, the low rate of each graminicide was applied sequentially following a POST application of 2,4-D choline plus glyphosate. Appropriate adjuvants were added to each tank mix combination and all treatments were made at 4 MPH with a tractor-mounted sprayer delivering 15 GPA at 40 PSI using 110015 AIXR nozzles. Volunteer corn and other weed control were visually recorded at 14, 21, 28, and 35 days after POST herbicide application (DAP). Generally, lower rates of either graminicide resulted in reduced volunteer corn and grass weed control, although reduced control was more pronounced with Assure II (quizalofop-ethyl) treatments. Higher graminicide rates helped overcome the antagonism between ACCase-inhibiting herbicides and 2,4-D choline and could be a useful strategy for managing volunteer corn. Utilizing sequential applications also provided better control of volunteer corn. Glyphosate did not appear to cause any antagonism.

The research project will help find the best and most cost-effective herbicide programs for controlling volunteer corn in a 2,4-D tolerant soybean herbicide program. The project is also designed to identify herbicide program options for VC management. Options are important, in recent years herbicide supply shortages have made it imperative that growers have multiple options for when their first choice is not available or becomes unavailable. In addition, we have designed the project to help address future VC control issues that are likely to arise. Enlist corn with FOP herbicide (e.g. Assure II) tolerance, is likely to enter the Minnesota marketplace in the near future, and we have included the Select Max herbicide option for controlling FOP tolerant volunteer corn. The outreach plan highlighted in the deliverable’s sections will reach many growers and ag professionals.