Waterhemp Resistance to Group 15 Herbicides

The continual evolution of weed species and populations resistant to herbicides from one or more site-of-action groups represents one of the most daunting challenges facing weed management practitioners.  Waterhemp has evolved resistance to herbicides from more site-of-action groups than any other Illinois weed species, including resistance to inhibitors of acetolactate synthase (ALS), photosystem II (PSII), protoporphyrinogen oxidase (PPO), enolpyruvyl shikimate-3-phosphate synthase (EPSPS), hydroxyphenyl pyruvate dioxygenase (HPPD), and synthetic auxins.  The University of Illinois weed science program recently announced confirmation of waterhemp populations resistant to Group 15 herbicides (Table 1), the first such confirmation of resistance in a dicot species to herbicides from this group.  Not every individual waterhemp plant is resistant to one or more herbicides, but the majority of field-level waterhemp populations contain one or more types of herbicide resistance.  Perhaps even more daunting is the occurrence of multiple herbicide resistances within individual plants and/or fields.  Waterhemp plants and populations demonstrating resistance to multiple herbicides are becoming increasingly common and greatly reduce the number of effective herbicide options.

Table 1.  Examples of Group 15 herbicides commonly used in Illinois.

Trade name Active ingredient
Dual Magnum S-metolachlor
Stalwart metolachlor
Outlook dimethenamid
Zidua pyroxasulfone
Harness, Warrant acetochlor

Integrated weed management programs offer the greatest potential for long-term, sustainable solutions for weed populations demonstrating resistance to herbicides from multiple groups.  Soil-residual herbicides are components of an integrated weed management program that provide several benefits, including reducing the intensity of selection for resistance to foliar-applied herbicides.  However, the recent discovery of resistance to Group 15 herbicides is yet another example of how waterhemp continues to challenge herbicide-only management programs.  There exists an urgent need for integrated weed management programs that return zero weed seed to the soil seedbank.

When a Group 15 herbicide is applied to the soil in a field with a resistant waterhemp population, the initial level of waterhemp control sometimes appears to be comparable to that of a susceptible population.  So, is there actually resistance to Group 15 herbicides?  A description of how the magnitude of resistance is determined and application rates of soil-residual herbicides might be helpful to answer this question.

Weed scientists characterize the magnitude of resistance (i.e., how resistant the plants are to the herbicide) by conducting a dose-response experiment in which a range of herbicide rates (often 8 to 10 rates, some more than and some less than a typical field use rate) is applied to the suspect-resistant population and to one or more populations known to be sensitive.  Dose-response experiments are most commonly conducted by spraying a foliar-applied herbicide directly onto plant foliage, but these experiments also can be conducted with soil-residual herbicides applied to soil containing seeds of the populations of interest.  At some time after application (often 14 or 21 days), a measure of plant response (percent injury, mortality, plant dry weight, etc.) is made for both populations, and a statistical equation is used to determine the herbicide rate that reduced the measured parameter by some value (frequently 50% is used for comparison).  The rate derived from the resistant population is divided by the rate derived from the sensitive population, and the quotient is referred to as the resistance ratio; the higher the resistance ratio, the greater the magnitude of resistance to that particular herbicide.  Table 2 presents actual resistance ratios for two Illinois waterhemp populations resistant to Group 15 herbicides.  The R/S ratios indicate the populations demonstrate the greatest resistance to S-metolachlor, followed by dimethenamid and pyroxasulfone/acetochlor.

What about application rates of soil-residual herbicides?  The application rates of most foliar-applied herbicides are usually selected to control only the weeds present when the application is made; in other words, most foliar-applied herbicides do not provide several weeks of residual weed control following application.  In contrast, application rates of soil-applied herbicides are selected to provide several weeks of residual weed control.  These rates are much greater than the rate needed to control germinating weeds at the time of application.  Furthermore, labeled application rates are not determined based on one or two weed species; rather, the labeled rates are those that control a broad spectrum of weed species for several weeks.  So then, what rate of S-metolachlor is needed to control a sensitive waterhemp population that germinates the day S-metolachlor is applied?  Is this rate greater than or less than the actual field application rate?  To answer these questions, we will use an illustration from our greenhouse research with two Group 15-resistant waterhemp populations.

Figure 1 shows the results of a greenhouse dose-response experiment 21 days after S-metolachlor was applied the same day waterhemp seeds were planted.  There are four waterhemp populations (CHR-M6 and MCR-NH 40 are resistant to Group 15 herbicides, WUS and ACR are sensitive) aligned in rows that were treated with various rates (0.0078–7.87 pints per acre) of S-metolachlor.  The pots in the far left column were not treated, while pots in the far right column were treated with the highest dose (7.87 pints) of S-metolachlor.  If we assume 2.5 pints per acre is the label recommend rate for this soil, the actual rate of S-metolachlor needed to control the sensitive populations is only 0.25 pints of S-metolachlor, which in this greenhouse experiment actually controlled these populations for 21 days after application.  In contrast, some resistant plants emerged and survived 7.87 pints of S-metolachlor.

One might be tempted to argue this discussion is irrelevant since field-scale applications of soil-residual herbicides are not made at rates low enough to discriminate between resistant and sensitive plants.  A portion of that argument is valid, but keep in mind that once a herbicide enters the soil environment it begins the process of degradation.  At some point during the course of its degradation, the amount of herbicide remaining in the soil will correspond to these discriminating rates.  The amount of time required for a particular herbicide’s degradation process to reach these discriminating rates depends upon many soil- and environmental-related factors (such as soil texture, organic matter content, moisture, pH, etc.).  At that discriminating dose, only resistant plants will emerge.

Compared with resistance to foliar-applied herbicides, resistance to soil-applied herbicides generally is more difficult to detect in the field. Resistance to foliar-applied herbicides is manifest as treated plants (assuming appropriate application rate and timing) that are not controlled, whereas resistance to soil-applied herbicides is manifest as a reduced duration of residual control.  It’s not always possible to predict if residual control is reduced 2 days, 8 days, 14 days, etc., as populations vary in their response to individual Group 15 herbicides. This does, however, emphasize the necessity of applying full label-recommended rates instead of reduced rates, as reduced rates will further curtail the duration of residual control.

Group 15 herbicides, whether applied at planting or with a postemergence herbicide after crop emergence, will continue to be important weed management tools.  The evolution of resistance to this important class of herbicides should serve as another warning that herbicide stewardship is as important as herbicide/trait selection.  Selection for herbicide resistance occurs each time a herbicide is applied, regardless of the herbicide or whether it is applied to the soil or plant foliage.  However, the overall intensity of selection for resistance to any particular herbicide or site-of-action group is reduced when multiple and different tactics are used to control the weed population.


Table 2.  Resistance ratios for two Illinois waterhemp populations resistant to Group 15 herbicides.  LD50 values represent the rates required to reduce waterhemp emergence/survival by 50 percent.

Resistant populations Sensitive populations
Herbicide (CHR-M6 and MCR-NH40) (ACR and WUS) R/S ratio
…………….…………….LD50 (g ai ha-1)………………………….
S-metolachlor 1808–3360 53–101 18–64
dimethenamid 729–1463 26–35 21–56
pyroxasulfone 65–153 9–10 7–17
acetochlor 178–226 13–40 5–18



Figure 1. Dose-response experiment of four waterhemp populations treated with soil-applied S-metolachlor.



Goal to reduce off-target movement of dicamba products

SPRINGFIELD, IL – The Illinois Department of Agriculture (IDOA) announced today it will require Special Local Needs labels, including new restrictions, for the use of the herbicide dicamba on soybeans in Illinois for the 2019 growing season. Dicamba is primarily used on soybeans to control post-emergence broadleaf weeds.
On February 15, IDOA notified the manufacturers of the three dicamba-containing products approved for over-the-top application to dicamba-tolerant (DT) soybeans that additional application restrictions will be required for the 2019 growing season. The affected formulations of dicamba are Engenia by BASF, XtendiMax with Vapor Grip Technology by Bayer, and FeXapan plus Vapor Grip Technology by DuPont/Corteva. The additional restrictions beyond federally-approved labels are:
1. The implementation of a cutoff date of June 30, 2019, for application to DT soybeans.
2. Prohibiting application when the wind is blowing toward adjacent residential areas.
3. Required consultation of the FieldWatch sensitive crop registry before application, as well as compliance with all associated record keeping label requirements.
4. Maintaining the label-specified downwind buffer between the last treated row and the nearest downfield edge of any Illinois Nature Preserves Commission site.
5. Recommendation to apply product when the wind is blowing away from sensitive areas, which include but are not limited to bodies of water and non-residential, uncultivated areas that may harbor sensitive plant species.

The intent of these additional restrictions is to reduce the potential for off-target movement of these products, thereby reducing the potential for possible adverse impacts to dicamba-sensitive crops/areas. The decision to pursue state-specific Special Local Needs (SLN) labels was made in response to the record number of misuse complaints IDOA received during the past two years. In 2017, IDOA received 430 total complaints, 246 of which were related to the use of dicamba on soybeans. Those numbers rose to 546 total complaints, including 330 dicamba-related complaints, in calendar year 2018. Prior to the 2017 introduction of these new formulations of dicamba for use on tolerant soybean varieties, total pesticide misuse complaints average 110 per year from 1989 to 2016.

Because of this significant increase in alleged pesticide misuse complaints, IDOA reviewed SLNs currently in place in other soybean-production states and worked with several Illinois stakeholder organizations before making the decision to require state-specific labels for Illinois.
“We now have two years of data showing how dicamba has the potential to drift off target,” said Acting Director John M. Sullivan. “It’s obvious measures need to be put in place so farmers can continue to effectively use these products, while also protecting surrounding property and crops.”

“Illinois Farm Bureau supports the Illinois Department of Agriculture (IDOA) in their administration of pesticide rules that they deem necessary to limit adverse effects to the environment,” said Richard Guebert, Jr., Illinois Farm Bureau President. “Dicamba-based products are useful and necessary tools in the fight against problematic weeds, helping farmers to remain productive and profitable. Illinois Farm Bureau will continue to work with IDOA and other partners into the future to find workable solutions for crop protection products.”

“The Illinois Corn Growers Association supports on-label use of crop protection products, along with farmer or applicator adherence to any additional label requirements issued by the Illinois Department of Agriculture. We know that Acting Director Sullivan takes seriously his obligation to protect the interests of many stakeholder groups, along with the preservation of public trust and transparency. We understand how the department came to this conclusion. It will no doubt cause difficulty for some farmers in certain areas and we are sensitive to that issue but encourage full compliance as per the 24(c) labels,” said Ted Mottaz, Illinois Corn Growers Association President.

“Co-existence is paramount when it comes to pesticide use,” said Jean Payne, President of the Illinois Fertilizer and Chemical Association (IFCA). “This proactive step demonstrates Illinois agriculture’s commitment to stewardship, and IFCA will educate our commercial applicator members regarding these pesticide label changes to ensure the continued legal and judicious use of this soybean production tool.”

“Volatilization and drift of pesticides are environmental issues that can impact our natural areas, water, and soil as well as Illinois’ growing specialty crop industry,” said Jennifer Walling, Executive Director of the Illinois Environmental Council. “I appreciate the efforts by the Illinois Department of Agriculture and industry stakeholders to reduce drift from dicamba. These rules are a step forward to address these issues. We are looking forward to working with stakeholders to research and monitor the results of the new labels.”

The three product registrants – BASF, Bayer, and DuPont/Corteva – have each submitted formal SLN labels for their respective dicamba-containing products to IDOA, which include the additional restrictions noted above. IDOA has submitted the resulting 24(c) registration packages for each product to the U.S. Environmental Protection Agency. The SLN labels will be distributed in addition to the already federally-approved labels with all Engenia, XtendiMax, and FeXapan product sold for use in the State of Illinois for the 2019 growing season.

Dicamba Buffers, Training and Licensing: What to Know for 2019

The United States Environmental Protection Agency (EPA) renewed the labels of three dicamba-containing products used in dicamba-resistant soybean varieties on October 31, 2018.  These renewed labels also contain new restrictions and requirements that did not appear on the original labels.  Each application must completely satisfy all label requirements and restrictions, but the following three new requirements might necessitate additional forethought and planning.

Additional in-field buffers

Fields that exist in counties that might harbor endangered terrestrial dicot plant species must have an in-field, 57-foot omnidirectional buffer. The new 57-foot buffer will occur on three sides of the field and be in addition to the required 110-foot downwind buffer.  Non-sensitive areas, as defined in the renewed labels, can be included in the omnidirectional buffer calculation.  This new buffer requirement includes fields in at least 29 Illinois counties (Figure 1).

Training requirement

Approximately 11,000 individuals in Illinois completed the label-mandated auxin training prior to applying these dicamba products in 2018.  Some have mistakenly assumed this particular training had to be completed only once, but the dicamba training must be completed every year.  Prior to the 2018 application season, dozens of face-to-face training sessions were held around the state, but it appears there will be fewer of these offered prior to the 2019 application season.  An option to complete the training via on-line modules likely will become available.

Licensing requirements

EPA mandates that only certified applicators (not operators) are allowed to purchase and apply these dicamba-containing products.  This requirement applies to both private and commercial applicators.  Historically, licensed operators working under the direct supervision of a licensed applicator have done the majority of pesticide applications in Illinois but this no longer permissible with the dicamba formulations applied to dicamba-resistant soybean.  According to the Illinois Pesticide Act for commercial applicators: “A person may make application to the Director to become licensed as a licensed commercial applicator…only after successfully demonstrating comprehension of the general competency standards and one or more of the technical category areas of pesticide use.”  The Illinois Department of Agriculture requires Field Crops as the technical category for application of dicamba to dicamba-resistant soybean.   Additionally for private applicators: “A person may become certified or recertified as a private applicator by: 1) attending a training session conducted by the University of Illinois Cooperative Extension Service which has been approved by or is in cooperation with the Department and by successful completion of a written, closed book, competency examination; or 2) in lieu of attendance at a training session, successfully complete a written closed book examination.  The closed book examination will consist of questions pertinent to general competency standards for which a correct answer is to be selected for each question from multiple choice answers.”


Figure 1.

Precautions for Dicamba use in Xtend Soybean

The extension weed science programs at The Ohio State University, Purdue University, and the University of Illinois recently collaborated to revise suggestions and precautions for use of dicamba in dicamba-resistant soybean.  The United States Environmental Protection Agency renewed labels of Xtendimax, Engenia, and FeXapan last October, and this updated extension weed science publication offers additional suggestions to help further reduce off-target dicamba movement.

Dicamba Precautions_2018 Update

Looking for Palmer amaranth Populations

The weed science programs at Southern Illinois University and the University of Illinois are participating in a research project sponsored by the United Soybean Board to survey and monitor the potential for the evolution of weeds with resistance to glufosinate and dicamba.  Early detection of evolved resistance to these herbicides could alert soybean farmers of the imposing threat in time for changes in management strategies that avoid herbicide failures.  Researchers in Illinois will collect seed from at least two of our most challenging weed species, waterhemp and horseweed (a.k.a. marestail).  We also would like to collect seed from Palmer amaranth and could benefit from your assistance.

If you know of Palmer amaranth populations in soybean fields around the state from which we could collect seed, please let us know.  Contact Dr. Karla Gage at Southern Illinois University (618-453-7679, or kgage@siu.edu) for Palmer amaranth populations south of Interstate 70, or Dr. Aaron Hager at the University of Illinois (217-333-9646, hager@illinois.edu) for populations north of Interstate 70 and we’ll make arrangements to collect seed prior to harvest.

Ewing Demonstration Center Celebrates 50 years of Continuous No-till Research at Agronomy Field Day on July 26

The University of Illinois Extension will host the Ewing Demonstration Center Agronomy Field Day on Thursday, July 26, 2018 at 9 a.m.  Every growing season presents challenges to production, and this year is no exception!  We are happy to host this summer field day to share with local growers current, ongoing agronomy research in southern Illinois, including cover crop trials on corn and soybeans, nitrogen management in corn, weed management in soybean, and our continuous no-till field, now in its 50th year of continuous no-till production.

We are highlighting our 50th year of continuous no-till production in our field day this year.  This no-till trial area was established in 1969 by George McKibben, the “Father of No-Till”, long-time agronomist and researcher at the Dixon Springs Ag Center in southern Illinois.  This plot has been cropped utilizing no-till production of corn and soybeans ever since.  The “zero-till” system as it was first called, was researched to “save the soil” that was lost over the many years of intensive tillage required to raise grain crops on the sloping hills of southern Illinois with the planting equipment available at the time.

In honor of this milestone, we will have the original “zero-till planter” on display.  This planter was modified and built in the early 1960s at the Dixon Springs Ag Center and used there and also at the Ewing Demonstration Center and other research sites.  The demonstrated success of this zero-till planter and production system was one of the inspirations that led companies like Allis-Chalmers and John Deere to start engineering and producing no-till planting equipment.  Also, joining us for the field day will be Donnie Morris, retired farm mechanic and engineer who built this planter, along with other retired Extension advisors and educators that worked at the Ewing Demonstration Center over the years.


The topics to be discussed at Field Day include:


Looking Back at 50 Years of Continuous No-till

  • Current and Retired Staff, University of Illinois

Insect Management in Corn and Soybean

  • Nick Seiter, Research Assistant Professor, University of Illinois

What We Have Learned After 48 Years of Continuous No-till

  • Ron Krausz, Manager SIU Belleville Research Center
  • Sarah Dintelmann, Undergraduate Assistant ,Weed Science, SIU

Managing Cover Crops in Corn and Soybean

  • Nathan Johanning, Extension Educator, University of Illinois

Intro to Corn Genetics:  Why is Sweet Corn Sweet?

  • Talon Becker, Extension Educator, University of Illinois


Please join us for Agronomy Field Day to help celebrate this milestone in crop production!  The field day is free and open to anyone interested, and lunch will be provided.  Certified Crop Advisor CEUs will also be offered (Soil & Water – 2.0; IPM – 0.5, Crop Management – 0.5).  The Ewing Demonstration Center is about 20 minutes south of Mt. Vernon located at 16132 N. Ewing Rd; Ewing, IL 62836, on the north edge of the village of Ewing, north of the Ewing Grade School on north Ewing Road.  Watch for signs.

To help us provide adequate lunch and materials, please RSVP to the University of Illinois Extension Office in Franklin County at 618-439-3178 by Tuesday, July 24.  For additional information on the field day, contact Marc Lamczyk at the Franklin County office or lamczyk@illinois.edu.

Reminder…University of Illinois Weed Science Field Research Tour

We would like to take this opportunity to once again extend the invitation to attend the 2018 University of Illinois Weed Science Field Day, to be held Wednesday, June 27th at the University of Illinois Crop Sciences Research and Education Center, located immediately south of the main campus on South Wright Street.  Coffee and refreshments will be available under the shade trees near the Seed House beginning at 8:00 a.m.  The tour will provide ample opportunity to look at research plots and interact with weed science faculty, staff, and graduate students. Participants can compare their favorite corn and soybean herbicide programs to other commercial programs.  The tour will conclude around noon with a catered lunch at the Seed House.  Cost for the Urbana weed science field tour is $10, which will help defray the cost of the field tour book, refreshments and lunch.

We look forward to visiting with you at the Urbana weed science field day on June 27th.  If you have any questions about the weed science field research tour, please feel free to call Charlie Mitsdarfer (217-621-7717) or Aaron Hager (217-621-8963).

Try togetherness: Study promotes cooperative weed management to curb herbicide resistance

The following article was written by Lauren Quinn, media specialist in the College of Agricultural, Consumer and Environmental Sciences at the University of Illinois.  The article describes recently publiched research, lead by Dr. Adam Davis, a research ecologist with USDA-ARS and adjunct professor in the Department of Crop Sciences, that examined cooperative weed management as a tool used against the challenges of herbicide-resistant waterhemp.  The following quote from Dr. Davis succinctly summarizes the research: “The crux of the story is that if you do good stuff and you aggregate it at larger spatial scales, it gets even better. If you do bad stuff and you aggregate it at large spatial scales, it gets even worse.”  I hope this information will be helpful and beneficial as you plan your future weed management programs.


In the fight against herbicide resistance, farmers are working with a shrinking toolkit. Waterhemp, a weedy nemesis of corn and soybean farmers, has developed resistance to multiple herbicide modes of action, often in the same plant. Even farmers using the latest recommendations for tank mixtures are fighting an uphill battle, with long-distance movement of pollen and seeds bringing the potential for new types of resistance into their fields each year.

In a study released this week, scientists at the University of Illinois and USDA’s Agricultural Research Service offer a new tool that is not only highly effective, it’s free. All it costs is a conversation.

“I think we’re at a point now where farmers are looking for new tools. This tool is free, but it requires that people talk to each other and work together as opposed to doing everything on their own,” says Adam Davis, research ecologist with USDA-ARS and adjunct professor in the Department of Crop Sciences at U of I.

The tool is cooperative weed management – in other words, making decisions about how to manage herbicide-resistant weeds in cooperation with neighboring farms. The more farms working together, and the larger area covered, the better.

Davis and his team tested the efficacy of farmer cooperation using a computer simulation of waterhemp resistance evolution through time and space. They ran the simulation using real numbers and management practices from the past, starting in 1987, to arrive at a realistic representation of herbicide resistance in waterhemp in 2015. Then they forecast 35 years into the future to determine how resistance might change under different management and cooperation scenarios.

“The crux of the story is that if you do good stuff and you aggregate it at larger spatial scales, it gets even better. If you do bad stuff and you aggregate it at large spatial scales, it gets even worse,” Davis says.

The “bad stuff,” according to the simulation, is using a single herbicide mode of action year after year. Resistance to a single chemical evolved and spread very quickly throughout the simulated landscape, especially if everyone was spraying the same one every year.

“If you take the cheap route, you’ll save some money in the short term on your herbicide costs, but in the long term, you’ll have a much greater likelihood of developing resistance,” Davis notes.

But if farmers invested in tank mixtures of herbicides representing three or four modes of action, the evolution and spread of resistance was delayed, and the delay got longer with increasing levels of cooperation.

“The message is not to use the most expensive herbicide program possible; the message is to use the available tools to manage your weeds better,” Davis says. “If you do that on your own farm, certainly it’s going to help. If you do it on a bunch of adjoining farms, it’s going to help even more. You can buy a couple of decades of time, in terms of delaying herbicide resistance evolution, by aggregating the best practices at large spatial scales.”

The simulation looked at management on individual farms, cooperatives of 10 neighboring farms, and cooperative weed management areas, comprising 10 neighboring farmer cooperatives. Davis says the specific number of farms making collective weed management decisions isn’t as important as the spatial scale they cover. He suggests forming weed management areas at the township scale and above.

The concept is simple, but farmers treasure their independence. How will it work?

Davis points to existing regional farm associations, such as drainage districts or commodity groups, as possible models for how weed management cooperatives might operate. He also suggests involving custom applicators in decision-making and implementation, since they’re already out there servicing multiple farms in a region.

The researchers are asking additional questions of the simulation, adding non-chemical control options like cover crops, crop rotation, and the Harrington Seed Destructor, to see how much more effective they get at larger scales. They’re also trying to quantify how much non-compliance a cooperative weed management area can withstand before its effectiveness falls apart.

But for now, the study suggests preserving the effectiveness of existing herbicides is worth the trouble of making nice with the neighbors.

The article, “Confronting herbicide resistance with cooperative management,” is published in Pest Management Science [DOI: 10.1002/ps.5105]. Co-authors include Jeffrey Evans, Alwyn Williams, Aaron Hager, Steven Mirsky, Patrick Tranel, and Adam Davis. The research was supported by USDA NIFA AFRI Award 2012-67013-19343, and is part of the USDA-ARS Area-Wide Pest Management Project.

Residual Soybean Herbicides Applied Postemergence

Soil-residual herbicides are important components of integrated weed management programs.  Reducing the number of weeds exposed to foliar-applied herbicides helps reduce the selection intensity for weeds resistant to foliar-applied herbicides.  Residual herbicides applied with postemergence soybean herbicides also can reduce the need for a second postemergence application.  However, simply applying a soil-residual herbicide does not guarantee the product will provide the desired level or duration of weed control.  Many edaphic and environmental factors influence the level of weed control achieved by soil-residual herbicides.

Soil-residual herbicides applied with postemergence herbicides require precipitation to move them into the soil solution where they are available for uptake. Herbicide effectiveness is reduced when a soil-residual herbicide is sprayed on a dry soil surface with no precipitation for several days following application. Residual herbicides generally require 0.5 to 1.0 inch of precipitation within 7 to10 days after application for optimal incorporation. Soil condition, soil moisture content, residue cover, and the chemical properties of the herbicide influence how much and how soon after application precipitation is needed for optimal activity.

Also, keep in mind the labels of soil-residual products include a maximum soybean growth stage or time interval prior to harvest beyond which an application cannot be made.  A sample of these growth stages and preharvest intervals is presented in Table 1.


Table 1.  Maximum soybean growth stage or preharvest interval for foliar application of soil-residual herbicides.

Herbicide Maximum soybean growth stage for broadcast application
Anthem Maxx Through the third trifoliate leaf stage
Dual Magnum/EverpreX Do not apply within 90 days of harvest
Dual II Magnum Through the third trifoliate leaf stage
FirstRate Before soybean reach growth stage R2
Outlook Fifth trifoliate
Prefix Do not apply within 90 days of harvest
Sequencea Do not apply within 90 days of harvest
Warrant/Warrant Ultra Before soybean reach growth stage R2
Zidua Third trifoliate stage

aApply postemergence only to glyphosate-resistant soybean varieties.

Corn Growth Stage and Postemergence Herbicides

The labels of most postemergence corn herbicides allow applications at various crop growth stages, but almost all product labels indicate a maximum growth stage beyond which broadcast applications should not be made, and a few even a state minimum growth stage before which applications should not be made.  These growth stages are usually indicated as a particular plant height or leaf stage; sometimes both of these are listed.  For product labels that indicate a specific corn height and growth state, be sure to follow the more restrictive of the two.  Application restrictions exist for several reasons, but of particular importance is the increased likelihood of crop injury if applications are made outside a specified growth stage or range.  The following table lists the maximum corn growth stage for broadcast application of several postemergence corn herbicides.  Be sure to constult the respective product label for additional precautions or restrictions.


Table 1.  Postemergence herbicide application timings based on corn growth stage(s).

Herbicide Maximum corn heights and/or growth stagesa
2,4-D Broadcast before corn exceeds 8” tall; use drop nozzles when corn is taller than 8”.
Accent Q Broadcast up to 20” tall or through the V6 stage.  Apply with drop nozzles when corn is 20–36” tall or before the V10 stage.
Anthem Maxx Apply from corn emergence through the V4 (visible fourth leaf collar) stage.
Armezon Pro Apply from corn emergence to the 8-leaf stage or 30” tall.  Use directed applications when corn is 12–30” tall.
Atrazine Apply before corn exceeds 12” tall.
Basagran No height specified on label.
Basis Blend Apply to corn from spike through 2 collar stage. Do not apply to corn having 3 fully emerged collars or over 6” tall.
Cadet Apply until corn is 48″ tall or prior to tasseling.
Callisto/Callisto GT May be applied to corn up to 30” tall or up to the 8-leaf stage.
Callisto Xtra Apply before corn exceeds 12” tall
Capreno Broadcast applications may be made to corn from the 1-leaf collar stage up to 20” tall.  Do not apply if corn is more than 20” tall or exhibiting 7 or more leaf collars.
Clarity or Banvel Apply between corn emergence and the 5-leaf stage or 8” tall; apply 0.5 pt/A rate when corn is 8 to 36” or if 6th leaf is emerging, or if 15 days prior to tassel emergence.  Do not apply when soybean are growing nearby if: 1) corn is more than 24” tall, 2) soybean are more than 10” tall, 3) soybean have begun to bloom.
DiFlexx Apply broadcast to corn from spike through V10 growth stage or 36” tall, whichever occurs first.
DiFlexx Duo Apply broadcast to corn from emergence up to, but not including, V7 or 30” tall, whichever occurs first.  Can be applied as a directed spray from V7 through V10, up to 36” tall corn, or up to 15 days prior to tassel, whichever occurs first.
Enlist One/Duo Apply when corn is no larger than V8 or 30” tall, whichever is more restrictive.  Directed applications can be made to corn up to 48” tall.
Glyphosate (glyphosate-resistant corn) Apply broadcast through the V8 stage or until corn reaches 30” tall.  Use drop nozzles for applications to corn 30–48” tall.
Halex GT (glyphosate-resistant corn) Apply to corn up to 30″ tall or the 8-leaf stage.
Harmony SG Apply to 2–6 leaf corn with 1–5 collars or up to 16” tall.
Harness Max Apply until corn reaches 11” tall.
Hornet WDG Apply broadcast until corn reaches 20” tall or V6 stage.  Apply with drop nozzles to corn up to 36” tall.
Impact/Armezon Can be applied up to 45 days before harvest.  Do not apply Armezon past the V8 growth stage.
Impact Z Apply before corn exceeds 12” tall.
Laudis Apply up to the V8 growth stage.
Liberty (glufosinate-resistant corn) Broadcast until corn at the V6 growth stage.  Use drop nozzles for up to 36” tall.
Marksman Apply between corn emergence and the 5-leaf or 8” height stage.
Moxy Apply prior to tassel emergence.
NorthStar Broadcast applications are made when corn is between 4 –20” tall (V2–V6).  Use directed applications when corn is 20–36” tall.
Permit Can be applied from spike through layby.
Realm Q May be broadcast applied to corn up to 20” tall or exhibiting 6 leaf collars.
Require Q Apply to corn 4–20” tall.  Do not apply to corn exhibiting 7 or more leaf collars.
Resolve Q Do not apply to corn taller than 20” or exhibiting 7 or more leaf collars.
Resource Apply to corn from the 2-leaf through 10-leaf stage.
Revulin Q Do not apply to corn taller than 30” or that exhibits 8 or more collars.
Solstice May be applied broadcast up to the V8 growth stage or 30” tall.
Starane Ultra Apply broadcast to corn with up to 5 fully exposed leaf collars (V5).
Status Can be applied to corn between 4” (V2) and 36” (V8) tall.
Steadfast Q Apply to corn up to 20” tall and exhibiting up to 6 leaf collars.
Stinger Apply to corn from emergence through 24” tall.
Yukon Apply broadcast or with drop nozzles to corn from spike to 36” tall.  Drop nozzles are recommended when corn exceeds 20”.
Zemax May be applied after corn emergence until plants reach 30” tall or up to the V8 stage.

a When maximum application timings are indicated by two corn growth stages, follow the most restrictive of the two.