University of Minnesota Extension
www.extension.umn.edu
612-624-1222
Menu Menu

Extension > Minnesota Crop News > Herbicides alone cannot adequately manage herbicide-resistant weeds -- Thoughts regarding planning for next year's soybean crop

Wednesday, September 20, 2017

Herbicides alone cannot adequately manage herbicide-resistant weeds -- Thoughts regarding planning for next year's soybean crop

Jeff Gunsolus, Lisa Behnken and Fritz Breitenbach

As we enter the fall harvest, many will be evaluating what soybean variety to select for next year. No longer is the focus solely on yield and tolerances to disease, iron chlorosis, and nematodes. This fall, farmers, consultants and advisors will be asking questions regarding how label modifications, if any, to the newly introduced dicamba formulations of Xtendimax, FeXapan, and Engenia might affect their variety selection decisions. The conundrum is that discussions at EPA and State Departments of Agriculture assessing the impact of this year’s off-target events on next year’s label will likely extend well into the fall. As you strategize future weed management plans, we would like for all of us to rethink the dicamba issue. Think about what brought on this issue in the first place: weed resistance to multiple groups of herbicides.

As herbicide resistance increases and spreads, farmers are compelled to shoulder added risk and the conflicts that ensue


Increasing reports of tall waterhemp, giant and common ragweed and kochia resistance to multiple-groups of herbicides are a big driver for this renewed interest in dicamba. Dicamba has historically been used in grass-crops such as small grains and corn and known to elicit easily recognizable injury symptoms in broadleaf crops such as soybean. If you’re old enough, you may remember that the rapid adoption of the HPPD-herbicides (e.g. Callisto, Impact, and Laudis) was, in part, due to the complications of using dicamba in corn – off target movement and tank contamination. Is the focus on managing the off-target movement of dicamba clouding our perspective on the broader issue of weed management?

As we read the various national and regional agricultural media reports, certain emerging themes concerned us and highlight the conflict of using the dicamba- technology in its current form.

Five themes and our responses follow:

Theme 1: Off-target injury has damaged personal relationships - "neighbor against neighbor."

Our response: The dilemma of reporting your neighbor for off-target movement of dicamba is true.


The Dicamba Damage Survey was initiated by the MN Dept. of Agriculture (MDA) in response to numerous off-target incidents. The idea was to create a reporting option without the threat of legal action, thereby maximizing the collection of valuable information, such as crop affected, crop stage, calendar date at application and environmental conditions, that may correlate with off-target injury. The MDA survey closed on September 15, 2017 with 249 dicamba-related cases. Fifty-five of these cases were initiated as misuse reports requiring investigation, thus about 200 of these may not have been reported if enforcement was required. Normally, MDA receives ~100 pesticide misuse investigations in a year. There were 47 of the 87 MN counties reporting at least one dicamba complaint, with a concentration of complaints in south central MN.

Theme 2: Farmers are considering purchasing Xtend soybean seed as a defensive strategy.

Our response: We have heard this solution to the problem from many sources; however, planting Xtend soybean as a defense against dicamba injury condones chemical trespass.


Although this seems like a logical solution to addressing the uncertainties of dicamba’s off-target movement potential it concerns us on two different levels.

First, we find this solution disrespectful of your neighbor’s freedom to farm as they choose. Consider for example, dicamba’s potential impact on food-grade beans, sunflowers, organic production, and fresh market crops.

Second, historically, Roundup Ready corn was also viewed by many as a solution to off-target drift of glyphosate from Roundup Ready soybean fields. However, once you purchase the seed technology, the temptation to use the associated herbicide is great and over-reliance on a single herbicide increases the potential for rapidly selecting for herbicide resistance to the newly introduced technology.

Theme 3: Farmers desperately need this new technology to control herbicide-resistant weeds.

Our response: Farmers do have other weed management options available, so our response depends on the specifics of the situation.


The Xtend technology, if dicamba could be contained, is effective at addressing some of our more difficult to manage weeds. However, it is not a stand-alone herbicide nor is it the only herbicide technology capable of managing tall waterhemp and giant and common ragweed.

One alternative would be to consider using the LibertyLink technology. At this time, no broadleaf weeds have been reported to be resistant to the Group 10, glufosinate herbicide. However, a stacked Xtend/LibertyLink trait is not currently available, so a deliberate decision to plant LibertyLink seed is necessary. Earlier Crop News and research report postings by Lisa Behnken and Fritz Breitenbach provide excellent resources as you explore the LibertyLink soybean option:

Regardless of herbicide-resistant crop technology, a full label rate of a preemergence herbicide must be used.


As noted in the Crop News reports released on April 18 and May 24, 2017 Lisa Behnken, Fritz Breitenbach, and I note the benefits of preemergence (PRE) herbicides, regardless of soybean variety:

Choosing the right PRE for the weeds on the farm builds a foundation that helps:
  1. control primary weed species
  2. decrease the density of weeds
  3. provide a uniform weed size for postemergence (POST) herbicide applications
  4. increase or widen the window of time for the POST herbicide application
  5. increase the number of herbicide Groups used to control weeds
  6. complement successful row cultivation
  7. extend the duration of preemergence activity for late-emerging tall waterhemp

Herbicides alone cannot adequately manage herbicide-resistant weeds - Integration with mechanical and cultural weed control is essential and beats hand-pulling weeds


In the Crop News posted on April 26, 2017, Lizabeth Stahl, Jared Goplen and Lisa Behnken provide sound weed biology-based insights as to why non-chemical weed control options need to be integrated with existing herbicide technologies if we are to develop weed management programs less susceptible to selecting for herbicide-resistant weeds.
In this article the authors note the importance that weed emergence, agronomics, row spacing, and cultivation play in developing a more robust weed management plan.

Theme 4: Commercial and farmer applicators noted that the label is either too complicated, confusing, or vague to understand or implement.

Our response: We believe this label is one of the most detailed yet challenging labels to implement under the varying field and environmental conditions commonly experienced in a growing season.


Reports from commercial and farmer applicators have commented that they did their very best to apply the new dicamba herbicide formulations properly. The new formulations and labels were developed to contain dicamba's wayward ways via particle drift, tank contamination and volatilization. Reports vary as to the percentages of cases where dicamba was not contained. However, as I (Jeff Gunsolus) complete my 32nd summer in Minnesota, I have never experienced any herbicide-induced injury problem as extensive nor as consistent, on both the state and national scale, in expressing symptoms, as my experiences this summer.

The bottom-line challenge in regard to dicamba is that non-Xtend soybeans are extremely sensitive to dicamba. Therefore, reduced volatility does not mean NO volatility. This fall we would like to explore in more detail the probabilities for suitable field working days available to apply dicamba (based on label requirements). In addition, suitable field working days may not occur in synchrony with the soybean growth stage where many applicators would like to apply the product. Our current working hypothesis to reduce risk of off-target dicamba movement is to end applications by mid-June. To be clear, this statement is not based on data but based on experience with dicamba’s behavior in corn and extrapolation from existing research and the known chemical properties of dicamba.

Theme 5: Farmer perceptions on the future role of herbicides: The agrichemical industry will develop the solution.

Our response: We do not believe this is a reasonable expectation to ask of the agrichemical industry. Herbicides alone cannot adequately manage herbicide-resistant weeds.


A Sociological perspective: Between February and May of 2015, farmer focus groups were conducted in Arkansas (2 groups), Iowa (4 groups), Minnesota (2 groups) and North Carolina (2 groups) by sociologists from Michigan State University Groups were kept small, between 6-10 participants and were recruited through university extension employees with cooperation from crop consultants. We were a part of the Minnesota effort.

The key message from these interviews was that farmers who have experienced fewer herbicide-resistant weed issues expressed greater optimism regarding the potential for herbicides alone to solve the herbicide resistance problem and this sentiment was expressed more frequently in Iowa and Minnesota.

Farmers from Arkansas and North Carolina who have experienced herbicide-resistant weeds for several years were more likely to express doubt in the ability of herbicides alone to manage herbicide resistant weeds but still consider herbicides alone, as their only option.

Moving away from the previously simple, cheap and easier means of chemical weed management makes change difficult and all farmers would really appreciate a new herbicide technology that could rival the glyphosate / Roundup Ready era.

The problem is that none of the new herbicide resistant trait technologies can fill that void, nor is the next glyphosate anywhere in view. Herbicides alone cannot adequately manage herbicide-resistant weeds; there is no point in waiting for the next glyphosate, and it is time to Take Control and start planning for a more integrated approach to weed management, because several weed species already have a head start.

No comments:

Post a Comment

  • © Regents of the University of Minnesota. All rights reserved.
  • The University of Minnesota is an equal opportunity educator and employer. Privacy