Postemergence Herbicides in Corn

The 2017 Illinois corn crop currently is at various stages of development.  Applications of postemergence corn herbicides continue to be made across areas of Illinois, although the recent precipitation has delayed applications in some areas.  Even though applications may be delayed, adequate soil moisture coupled with warm temperatures will certainly promote rapid growth of emerged weeds.

Properly timing the application of the postemergence herbicide is critical toward achieving the goal of removing weed interference from the corn crop before the weeds adversely impact (i.e., reduce) corn grain yield.  Unfortunately, it’s not possible to accurately predict the specific day after planting or emergence when weed interference begins to reduce corn yield.  This interval is influenced by many factors and can vary based upon the weed spectrum, the density of certain species, available soil moisture, etc. Weed scientists generally suggest an interval, based either on weed size (in inches) or days after crop/weed emergence, during which postemergence herbicides should be applied to avoid crop yield loss via weed interference.  In corn, it is often recommended to remove weeds before they exceed 2 inches tall.  The longer weeds are allowed to remain with the crop the greater the likelihood of crop yield loss.

It’s important to remember that 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.

As mentioned, corn plant height is commonly used on many herbicide labels but plant height may not always provide an accurate indication of the plant’s true physiological maturity.  Determining plant height may seem relatively straightforward, but using different benchmarks for measurement can lead to different plant heights.  Generally, corn plant height is determined by measuring from the soil surface to the arch of the uppermost leaf that is at least 50% emerged from the whorl.  Be sure to measure several plants in a given field and average the numbers.  Plant height is obviously influenced by many factors, including genetics and the growing environment.  Adverse environmental conditions, such as cool air/soil temperatures, hail, etc., can greatly retard plant height and result in corn plants that are physiologically older than their height suggests.

Many agronomists agree that leaf number is a more accurate measurement of corn developmental stage.  Counting leaves and counting leaf collars are the two primary techniques used.  Leaf counting begins with the short first leaf (the one with a rounded tip) and ends with the leaf that is at least 40–50% emerged from the whorl.  Counting leaf collars also begins with the short first leaf, but includes only leaves with a visible collar (the light-colored band where the leaf joins the stem).  Leaves in the whorl or those without a fully developed collar are not counted.  The leaf collar method quite often stages a corn plant at one leaf less than the leaf counting method.

Corn plants under stress conditions may be more prone to injury from postemergence herbicides. Stress can arise from a number of factors, including cool temperatures and wet soils.  Be sure to consult the product label when selecting spray additives to include with postemergence herbicides.  Many labels suggest changing from one type of additive to another type when the corn crop is under stressful growing conditions.  Attempting to save a trip across the field by applying a postemergence corn herbicide with a liquid nitrogen fertilizer solution (such as 28% UAN) as the carrier is not advisable.  While applying high rates of UAN by itself can cause corn injury, adding a postemergence herbicide can greatly increase corn injury.

Labels of several postemergence corn herbicides (most commonly ALS-inhibiting herbicides but also some HPPD-inhibiting herbicides) include restrictions with respect to applying the product to corn previously treated with certain soil insecticides.  Be sure to consult the respective herbicide label for other restrictions and limitations.

Corn Replanting and Herbicide Considerations

Following the recent and excessive precipitation, some corn replanting likely will occur when soil conditions are conducive.  We hope that replanting occurs only in small areas of a given field, but in some situations entire fields may have to be replanted.  While there are many agronomic considerations associated with replanting, some weed control/herbicide issues also should be considered.

Herbicide-resistance traits in the replanted hybrids should be taken into account.  For example, if you initially planted a glyphosate-resistant corn hybrid and have areas that need to be replanted, either replant these areas with a similar glyphosate-resistant hybrid or take special precautions to reduce drift with any postemergence glyphosate application if you replant with a non-glyphosate resistant hybrid as these plants will be extremely sensitive to glyphosate.

Is there an interval between when a herbicide was applied and corn replanting?  For soil-applied corn herbicides, replanting can proceed whenever field conditions are feasible.  However, for some postemergence corn herbicides, there are intervals between application and replanting.  For example, if a corn field previously treated with Spirit, NorthStar, Permit, or Yukon is lost due to excessive precipitation and must be replanted, there is a 4-week, 14-day, 1-month, and 1-month, respectively, interval that must elapse between the herbicide application and corn replanting.

While most soil-applied herbicides allow more than one application per season, a few can be applied only once per season.  For example, the Acuron and Resicore labels indicate not to reapply if corn is to be replanted.  In instances where small areas of a field will be replanted, some may elect to simply replant without applying any additional residual herbicide.  If, however, you elect to make a second application of a particular corn herbicide, keep in mind that many product labels indicate a maximum per acre rate that can be applied during one growing season.

If corn plants from the first planting remain, what are some options to control them prior to replanting?  Tillage is very effective and consistent at removing existing corn plants, and the replanting operation can proceed at any time afterward.  However, tillage might not always be an option.  Several herbicide options are available that can be applied to control existing corn plants (Table 1), but careful attention must be given to what, if any, herbicide resistance trait(s) the existing corn plants contain.


Table 1.  Options to control existing corn plants prior to replanting.


Corn trait
































Glyphosate is very effective for controlling existing stands of corn sensitive to glyphosate.  Corn replanting can occur immediately after application, but control of existing corn plants might be improved if at least 24 hours elapses between application and replanting.  Glyphosate also would control sensitive weeds that might have emerged with the initial stand of corn.  Be very cautious to avoid drift when spraying glyphosate, especially if spraying around wet holes.

Other herbicides to control emerged corn include paraquat and glufosinate (only hybrids sensitive to glufosinate), although previous research with these herbicides has demonstrated that complete control is not always achieved.  Performance of these produces is often enhanced when applied in combination with atrazine or metribuzin.  Paraquat and glufosinate would also control a broad spectrum of emerged weeds.

Corn hybrids resistant to glyphosate, glufosinate, or both can be controlled with Select Max prior to replanting field corn.  The label specifies to apply 6 fluid ounces per acre to control glyphosate-resistant field corn up to 12 inches tall.  Applications should include NIS and AMS (do not use a COC or MSO in this particular use), and care must be taken to avoid in-field overlaps or excessive injury to the replanted corn might occur.  Glyphosate can be tankmixed with the Select Max to control emerged broadleaf weed species.  DO NOT replant fields treated in this way sooner than six days after application or severe injury to the replanted corn can occur.

The product labels of Poast, Poast Plus, Fusion, Fusilade, Select, and Assure II include an interval that must elapse between application and rotation to or replanting with grass crops such as corn.  These intervals range from 30 (Poast, Poast Plus, Select) to 60 (Fusion, Fusliade), to as many as 120 (Assure II) days, making these products unlikely choices for this particular use.  Severe injury to replanted corn can occur if soil residues of any of the ACCase-inhibiting herbicides described herein are taken up by the emerging corn plants (Figure 1).


Replanted corn injured by soil residues of an ACCase-inhibiting herbicide.

Replanted corn injured by soil residues of an ACCase-inhibiting herbicide.

University of Illinois Weed Science Field Research Tour

The weed science program at the University of Illinois invites all weed management practitioners to our annual weed science field tour, which will be held on Wednesday, June 28 at the Crop Sciences Research and Education Center (a.k.a. South Farm), located south of campus on Wright Street extended.  Registration will begin at 8:00 a.m. and refreshments (coffee, juice, and doughnuts) will be available.  Preregistration is not required, but please let us know in advance if you will be bringing a large group of participants so we can plan accordingly for meals.

Similar to past years, we will car pool to the fields where participants can join in a guided (but informal) tour format. 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 and get an early look at a few new products that soon will be on the market. 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 will apply for 2 hours of CCA credit under the IPM category.  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).

Dry Soils and Residual Herbicides

Decades ago it was very common for the majority of corn and soybean acres in Illinois to be treated with one or more soil-residual herbicides before crop/weed emergence.  During the 1980s, commercialization of broad-spectrum, postemergence herbicides began the shift away from widespread use of soil-residual herbicides; products such as Basagran, Classic, Accent and Pursuit contributed to the early adoption of postemergence weed control programs.  The era of total postemergence weed control reached its zenith following the widespread adoption of glyphosate-resistant crops and the concomitant use of glyphosate.  However, the evolution of glyphosate resistance in several weed species has heralded a shift back to the use of soil-residual herbicides, especially in soybean.

Soil-residual herbicides can provide many weed management benefits, but several factors influence their effectiveness.  Factors such as product selection, application rate, and when the herbicide is applied in relation to crop planting are largely under the control of the farmer, whereas soil moisture content at the time of application and the interval between application and the first precipitation event are factors largely beyond the farmer’s control.

In order for a soil-applied herbicide to be effective, the herbicide needs to be available for uptake by the weed seedling (usually before the seedling emerges, but some soil-applied herbicides can control small emerged weeds under certain conditions).  Soil-applied herbicides have an Achilles heel: when applied to the soil surface they require mechanical incorporation or precipitation to move them into the soil solution.  Herbicide effectiveness can be significantly reduced when a soil-applied herbicide is sprayed on a dry soil surface with no incorporation (mechanical or by precipitation) for several days following application.  How much rainfall is required to move the herbicide into the soil and how soon after application precipitation is needed are difficult to define and can vary by herbicide, but surface-applied herbicides generally require 0.5 to 1.0 inch of precipitation within 7 to10 days after application for optimal incorporation.  Factors such as 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.  If no precipitation is received between application and planting, mechanical incorporation, where feasible, can still help move the herbicide into the soil solution.

Stakeholders needed: Herbicide resistance listening session at Commodity Classic

Are you an Illinois farmer?


How about an Illinois Ag chemical retailer, seed dealer, crop consultant, machinery/implement dealer, pesticide manufacture or public landowner?


Are you planning on attending the Commodity Classic in San Antonio in March?


Would you be willing to share your experiences regarding herbicide resistance management?


If you answered yes……..

Two weed scientists, Dr. Jeff Gonsolus (University of Minnesota) and Dr. Christy Sprague (Michigan State University), are working on behalf of the Weed Science Society of America to convene a Herbicide Resistance Management (HRM) Listening Session to be held at the Commodity Classic in San Antonio, Texas on the afternoon of Saturday, March 4th.


This listening session will be facilitated by professional facilitators and the primary objective is for the scientists to listen to (not talk to) key stakeholders about challenges and barriers to practicing herbicide resistance management and to explore innovative approaches for circumventing these challenges and barriers.


The session will be held in the Henry B. Gonzales Convention Center Room 007 C&D on March 4th beginning with lunch at noon and ending around 5 pm.


To reserve your seat at the table, contact either Dr. Sprague (; 517-353-0224) or Dr. Gunsolus (; 612-625-8130) by February 15th.


For more information regarding the intended outcomes from this and other listening sessions to be held across the country this year, click here.

2016 University of Illinois Plant Clinic Herbicide Resistance Report

Glyphosate and PPO inhibitor Summary: 593 field samples representing approximately 2,000 waterhemp or palmer amaranth plants were tested for herbicide resistance at the University of Illinois Plant Clinic in the 2016 season. The Plant Clinic started offering herbicide resistance testing of waterhemp for resistance to two groups of herbicides (glyphosate and PPO inhibitors) in 2015. We added palmer amaranth testing in 2016.  Almost twice as many whole fields were tested 2016 compared to last year, 593 vs. 338.  The tests use qPCR protocols to determine if the most common site of action for resistance to these two groups of herbicide is present in the plants.

Samples from 10 states across the Midwest were submitted in 2016. The following chart details the number of field samples from each state, along with the number of fields that were positive for glyphosate resistance and PPO inhibitor resistance.  Fields with plants that are positive for both glyphosate and PPO inhibitor resistance are of particular concern due to the limited possibilities for control of these weeds.

2016 University of Illinois Plant Clinic Herbicide resistance testing results.
State No. of Field samples No. of Glyphosate resistant fields No. of PPO Inhibitor resistant fields No. of Fields Positive for both Glyphosate and PPO Inhibitor resistance % of Fields Positive for both Glyphosate and PPO Inhibitor resistance  
IL 378 280 244 182 48.1 %
IA 87 77 70 65 74.7 %
IN 9 9 6 6 66.6 %
KS 1 1 0 0 0 %
KY 3 1 1 0 0 %
MI 1 0 1 0 0 %
MN 78 58 34 27 34.6 %
MO 11 10 10 9 81.8 %
NE 8 5 1 0 0 %
WI 17 15 4 2 11.8 %

In Illinois, we received samples from 52 counties that had at least one sampled field that had waterhemp or palmer amaranth plants that tested resistant to both glyphosate and PPO inhibitors.


Palmer amaranth issues: Until the 2016 season, palmer amaranth in Illinois was not known to be resistant to PPO inhibitors.  However, several samples from southwestern Illinois were confirmed to be PPO inhibitor resistant (3 from Madison,  and 1 from St. Clair counties) in our testing.

Due to difficulties in positively identifying related amaranth species, and concern regarding possible contamination of seed with amaranth weed seeds including palmer amaranth, the Plant Clinic is now offering a molecular identification service to positively identify palmer amaranth. This protocol was adapted and tested in fall of 2016, and will be offered to the public starting in 2017.   Find our sample forms for this testing on the Plant Clinic website.

Authors: Diane Plewa and Suzanne Bissonnette

Helpful Reminders about Applying Dicamba in Dicamba-Resistant Soybean

Proper application stewardship of dicamba in dicamba-resistant soybean increases the likelihood of good weed control while concomitantly decreasing the potential for off-target herbicide movement.  The labels of two commercially available dicamba formulations approved for application in dicamba-resistant soybean (Engenia from BASF and XtendiMax from Monsanto) include many application guidelines and restrictions that all applicators must follow.  The Illinois Fertilizer and Chemical Association (IFCA) recently published a helpful checklist of “do’s and don’ts” for applying dicamba in dicamba-resistant soybean.  We strongly encourage all who are considering growing a dicamba-resistant soybean variety to carefully review the entire respective product label well before the beginning of the 2017 growing season to ensure they can meet all application guidelines and restrictions.  The IFCA checklist is an excellent resource for review and can easily be printed and posted in your office or mixing area and referenced throughout the growing season.

IFCA Dicamba Stewardship Bulletin 2017 pdf

IFCA Dicamba Stewardship Bulletin 2017 Word

Extension Bi-State Crops Conferences in and near Western Illinois

Newer and longer-term partnerships between personnel in Illinois and personnel in Missouri and Iowa have resulted in several bi-state crops conferences to be held during January 2017 in Western Illinois or Eastern Iowa.


Friday, January 6, 2017: Bi-State Crop Advantage Conference, Burlington, IA, 8:30 AM – 4:00 PM

Location: Comfort Suites, 1708 Stonegate Center Drive, Burlington, IA.

Hosts: Iowa State University and University of Illinois Extension

More Information: Click here to access the flier.

Online Registration: Click here to register


Friday, January 27, 2017: Bi-State Crop Advantage Conference, Davenport, IA, 8:30 AM – 4:00 PM

Location: Rhythm City Casino Resort, 7077 Elmore Ave., Davenport, IA

Hosts: Iowa State University and University of Illinois Extension

More Information: Click here to access the flier.

Online Registration: Click here to register.


Friday, January 27, 2017: Western Illinois-Northeastern Missouri No-till Crop Management Conference, Quincy, IL, 8:45 AM – 2:00 PM

Location: John Wood Community College, 1301 S. 48th St., Quincy, IL

Hosts: University of Illinois and University of Missouri Extension, Illinois and Missouri NRCS

More Information: Click here to access the flier.

Online Registration: Click here to register.

Dicamba and Soybean: What to Expect in 2017

The long-awaited label allowing dicamba use in dicamba-resistant soybean was granted November 9, 2016, by the United States Environmental Protection Agency (EPA), although only one commercial product received that label.  Many Illinois farmers anticipate this technology will provide a much-needed solution to challenges caused by weed populations resistant to herbicides from multiple site-of-action groups and other difficult-to-control species.  Without question, there are instances and scenarios in which dicamba will improve control of certain weed species, but dicamba will not bring back the “good ol’ days” of POST-only weed control programs in soybean.  Current expectations of what this technology can accomplish tend to be a bit more optimistic than what the technology actually will be able to deliver.

Where it fits:

Horseweed control. Many farmers experienced significant challenges attempting to control horseweed (i.e., marestail) before planting no-till soybean in 2016.  The “traditional” burndown tankmix of glyphosate and 2,4-D was not as effective on horseweed as many have come to expect.  Anecdotally, we suspect resistance to glyphosate is widespread in Illinois horseweed populations, and that this likely contributed to many control failures with burndown herbicides.  Horseweed control with spring-applied 2,4-D can be quite variable, and poor control is common when a glyphosate plus 2,4-D tankmix is applied to a glyphosate-resistant horseweed population.  Dicamba can provide some remedies because the recently approved dicamba label allows up to 1 lb dicamba acid equivalent to be applied prior to planting dicamba-resistant soybean.  This can provide better and more consistent control of glyphosate-resistant horseweed compared with 0.5 lb acid equivalent 2,4-D.  Note: Although the new label allows soybean to be planted immediately after dicamba application, it’s still advisable to wait a few days following application before injuring the weeds with the planting operation.

Annual morningglory species are a well-known bane of glyphosate.  Control of species such as tall and ivyleaf morningglory with glyphosate is rarely adequate, but dicamba is much more effective on these species.  Add to this list common and giant ragweed, two other species that have evolved resistance to glyphosate and ALS-inhibiting herbicides.  Dicamba certainly can provide better control of herbicide-resistant ragweeds than can glyphosate or ALS inhibitors.

Where it maybe fits:

Waterhemp.  Before the widespread evolution of glyphosate-resistance in waterhemp, glyphosate was considered by many weed scientists to be an excellent herbicide against waterhemp.  That same adjective is not ascribed to dicamba.  Most university weed control guides list dicamba as good or very good on waterhemp, but not excellent.  Dicamba can improve control of pigweed species, but it will never be as effective as glyphosate once was.  Illinois farmers have made great strides toward utilizing more diverse herbicide programs for waterhemp control than they were using a decade ago.  We suggest that dicamba should be used in a way that does not reduce this diversity.  It is imperative to maintain a diverse weed management approach to prolong the effective utility of dicamba.  Illinois waterhemp populations have evolved resistance to herbicides from six site-of-action groups, and resistance to dicamba is not a question of “if”, but “when.”

Here are some realities:

What are some limitations of utilizing dicamba for weed control in soybean?  The current label contains several mandates related to the actual spray application procedure that are somewhat unique.  For instance, there are limitations on boom height, sprayer speed, and nozzle type that applicators must follow.  An infield, downwind buffer of 110 or 220 feet (depending on application rate) must be maintained.  One of the most significant limitations is the inability to tankmix with other herbicides.  There is an avenue by which other herbicides can be approved for application with dicamba, but if the current label remains unchanged during the 2017 growing season, applicators will be required to apply dicamba alone.  In other words, farmers will make a separate application of dicamba and another application of other needed herbicides.

Many claims have been made about yield potential of dicamba-resistant soybean varieties and the lower volatility potential of the recently labeled dicamba formulation.  We believe that our clientele should be aware that the University of Illinois weed science program has not had the opportunity to evaluate yield or volatility potential.  We do not have data that dispute these claims, but neither do we have data to support them.  University variety testing programs are now releasing results from yield trials that include dicamba-resistant varieties.  Dr. Shawn Conley, soybean and wheat extension specialist at the University of Wisconsin, recently published an article summarizing his research with dicamba-resistant soybean varieties in 2016.

Of particular concern is the apparent confusion about particle drift and volatilization.  While the newly labeled formulation is reportedly less likely to volatilize after application, there is absolutely nothing unique about the formulation that will reduce physical drift during application.  Off-target movement of dicamba is of particular concern due to the number of sensitive dicot species grown in Illinois.  The new formulation of dicamba is no more likely to drift than any other herbicide formulation, but the symptoms that drift did occur can be induced at extremely low concentrations of dicamba. Several years ago, we were able to induce soybean leaves to “cup” with as little as 1/10,000 pint of dicamba.  Many media reports suggest that use of older, more volatile dicamba formulations was largely responsible for the widespread off-target injury that occurred in areas of the mid-south during 2016, but volatility is generally a minor component of off-target movement when compared with actual physical drift during application.

One last item to consider is that this new registration for dicamba use in dicamba-resistant soybean will expire on November 9, 2018.  EPA documents indicate the registration will automatically expire “…unless the EPA determines before that date that off-site incidents are not occurring at unacceptable frequencies or levels.”  In other words, the continued availability to utilize dicamba in dicamba-resistant soybean is very much dependent on those who use it.

Much more undoubtedly will be shared about dicamba use in dicamba-resistant soybean from a variety of sources.  We do believe dicamba can provide a solution to unique weed management challenges, but we also believe not all weed management challenges can be met with dicamba.  Other herbicide-resistant crop technologies, such as Liberty Link and Enlist (once all import approvals are received), also can provide solutions and remain viable options for soybean producers.  Proper stewardship of all technologies only helps prolong their effective utilization.

Fall-Applied Herbicides: Which Weed Species Should be the Target?

Herbicides applied in the fall often can provide improved control of many winter annual weed species compared with similar applications made in the spring.  Marestail is one example of a weed species that is often better controlled with herbicides applied in the fall compared with the spring.  An increasing frequency of marestail populations in Illinois are resistant to glyphosate, and recently we confirmed resistance to ALS-inhibiting herbicides also is present in Illinois populations.  Targeting emerged marestail with higher application rates of products such as 2,4-D in the fall almost always results in better control at planting compared with targeting overwintered and often larger plants with lower rates of 2,4-D in the spring.

One question typically posed is whether or not a fall application needs to include one or more herbicides that provide residual control of winter annual weed species.  Typically, the earlier the fall application is made (say, early October) the more benefit a soil-residual herbicide can provide since emergence of winter annual weeds is often not complete.  However, delaying the herbicide application until later in the fall (say, mid-November) often diminishes the necessity of a soil-residual herbicide since most of the winter annual weeds have emerged and can be controlled with non-residual herbicides.  Applying a soil-residual herbicide late in the fall in hopes of having a clean field prior to planting is akin to gambling on the weather.  Cold winter conditions can reduce herbicide degradation in the soil and increase herbicide persistence.  This might not always be favorable since, depending on the residual herbicide, increased persistence also can cause injury to the following crop.  A more moderate winter and early spring warming will increase herbicide degradation, which could result in the need for a burndown herbicide to control existing vegetation before planting.

We recommend fall-applied herbicides target fall-emerging winter annual species, biennials and perennials.  We do not recommend fall application of residual herbicides for control of any spring-emerging annual weed species.  We are aware some products have 2(ee) recommendations that suggest the product will control certain summer annual weed species following application in the fall.  Particularly concerning to us is that “pigweed species” are listed on at least one product label.  The extension weed science program at the University of Illinois does not recommend fall-application of residual herbicides to control Amaranthus species the next spring for the following reasons:

  • Inconsistent performance: as previously described, the performance consistency of soil-residual herbicides applied in the fall is greatly dependent on weather and soil conditions after application. Our data suggest the greatest and most consistent control of Amaranthus species either at planting or several weeks after planting was achieved when residual herbicides were applied in the spring, not in the fall.
  • Increased selection for herbicide-resistant biotypes: soil-applied herbicides are not immune from selection for herbicide-resistant biotypes (please see the April 16, 2013 article titled: “Herbicide Resistance: Are Soil-Applied Herbicides Immune?”). Following a fall application, the concentration of herbicide remaining in the spring when Amaranthus species begin to germinate will be much lower compared with the same product rate applied closer to planting.

Populations of several summer annual broadleaf weed species in Illinois demonstrate resistance to herbicides from more than one site-of-action herbicide class.  Their effective management requires an integrated approach that often includes soil-residual herbicides.  Applying these herbicides when they will be most effective against these challenging summer annual species is a critical component of an integrated management program.