The Dicamba Dilemma in Illinois: Facts and Speculations

Only a short time ago, many agricultural professionals were optimistic Illinois would somehow be “spared” the incidents of off-target damage caused by dicamba that continue to plague several states to our south.  The recent preponderance of evidence (observations made traveling the state, stories on social media, an increasing number of pesticide misuse complaints filed with the Illinois Department of Agriculture (IDOA), etc.), suggests otherwise.  Instances of soybean demonstrating symptoms of exposure to dicamba have greatly increased over the past two weeks and it’s nearly certain the number of affected acres will continue to rise.  To estimate the extent of this using the number of complaints filed with the IDOA as the sole metric would be to grossly underestimate the current reality.

Some might be surprised to learn that instances of soybean exposure to dicamba have been an annual occurrence in Illinois since dicamba was first commercialized almost 50 years ago.  One of the first experiments that described soybean’s sensitivity to dicamba was conducted by Dr. Loyd Wax at the University of Illinois in 1966–19671.  The stated objective of the experiments “…was to determine the response of soybeans to soil and foliar applications of dicamba, picloram, and 2,4-D to assess the potential hazard of using these herbicides in crops in rotation with soybean and in areas adjacent to soybean fields.”  The symptoms of soybean exposure to dicamba described by these researchers 50 years ago are nearly identical to those currently being observed.

The widespread adoption of glyphosate-resistant corn hybrids in Illinois during the first decade of the 21st century was accompanied by a decrease in dicamba use in corn, resulting in relatively few complaints of soybean exposure during the last 10 years.  With more dicamba currently being applied, it’s not surprising the instances of soybean exposure have increased. Whether applied in corn or dicamba-resistant soybean, the fact remains that few dicot species in the Illinois landscape are more sensitive to dicamba than soybean.

Symptoms of exposure:

There appears to be some confusion about symptoms of exposure to dicamba compared with leaf symptoms caused by non-dicamba factors.  Dr. Wax and his colleagues described the effects of dicamba on soybean leaves as “…cupped and crinkled,” which are terms still commonly used today.  Other factors can cause leaf distortions, but I am not aware of anything other than dicamba that causes the following symptoms collectively:

1) ) extreme cupping of trifoliolate leaves, most pronounced on the upper trifoliolates (Figure 1)

2) veins of affected leaves tend to assume a parallel orientation instead of the usual net venation pattern (Figure 2)

3) tips of cupped leaves with parallel veins are often brown or cream-colored

4) plants are stunted as compared to plants not demonstrating the aforementioned symptoms; these plants may sometimes remain stunted the remainder of the season

5) depending on time and dose of exposure, pod development can be adversely affected

While the symptoms of exposure to dicamba are apparent, identifying how the exposure occurred is not always obvious.  Speculations and “explanations” from some industry personnel have included almost everything except the Russians (stay tuned…that one might be next).  Soybean’s extreme sensitivity to dicamba sometimes complicates accurately identifying the source of exposure.  Recent research published by Dr. Kevin Bradley, weed scientist at the University of Missouri, indicated symptoms of exposure to dicamba could be induced at 1/20,000 of a 1x (0.5 lb ae/acre) field use rate.  Additionally, symptoms generally do not develop immediately after exposure; we have observed instances where 21 days elapsed between exposure and symptom development.

Possible routes of exposure:

1) Physical drift of spray particles during the actual application.

This route of exposure might be the easiest to identify based on field observations.  Symptoms are usually most pronounced along the edge of the field adjacent to the drift source, and lessen as the distance from the source increases (Figure 3).  Remember, the symptoms of exposure to dicamba depend largely on the dose.  Symptoms are different on soybean directly sprayed with dicamba (often dead plants) compared with soybean exposed to a very low concentration (leaf cupping, etc.) farther from the source.  Exposure from physical drift has been observed this year, but it does not appear to account for the majority of off-target exposure instances to date.

2) Residues remaining in/on the spray equipment from previous applications are applied at low concentrations with the POST soybean herbicide.

These symptoms are often most pronounced around the perimeter of the field and along the edge where the applicator began spraying the remainder of the field.  Symptoms often become less pronounced as the sprayer moves farther across the field away from the side where the application began (Figure 4).  Contamination has been touted by some as an explanation for cupping of Liberty Link varieties, but it seems odd that it hasn’t been mentioned much as an explanation for cupping of Roundup Ready varieties.

3) Herbicide vapors on the plant or soil surface move out of the treated field (vapor drift).

The volatility of a herbicide (i.e., tendency to change from a liquid to a gas) is a function of several factors related to the formulation of the herbicide and to prevailing environmental conditions.  Vapor pressure is a measure of the tendency of a herbicide to volatilize.  As the vapor pressure of a herbicide increases, the potential for volatility also increases. Volatility tends to increase as soil moisture and temperature increase.  As soil moisture decreases, the amount of herbicide adsorbed to soil colloids can increase and reduce the amount of herbicide available to volatilize.  All commercially-available formulations of dicamba have the potential to volatilize.

4) Applications made during temperature inversion conditions.

Small droplets can remain suspended in the air when pesticides are applied during temperature inversion conditions.  These particles then move out of the target area when winds begin to move the following morning.  Where and how far they move depends primarily on wind direction and speed.  Labels of dicamba-containing products approved for in-crop application to dicamba-resistant soybean restrict applications during temperature inversions.  Some have speculated applications made at night (when inversions occur) have been responsible for off-target damage, but does anyone have actual data on how many acres are treated when headlights are needed on the applicators?

Leaf distortions:

There has been much conversation about leaf symptoms that likely were not caused by exposure to dicamba.  As mentioned previously, symptoms of dicamba exposure can vary according to the dose of exposure and stage of soybean development.  However, the symptoms of low-dose exposure tend to be fairly consistent.  Can other herbicides cause leaf distortions?  Yes, but these symptoms are different from those caused by dicamba.  Foliar-applied PPO inhibitors can cause leaf distortions, but the degree of “cupping” is generally much less than that caused by dicamba and the symptoms appear on leaves treated with the application (Figure 5).  In contrast, cupping caused by dicamba is generally seen on leaves that emerge after the exposure occurred.  We also have observed distorted leaves following POST application of soil-residual herbicides (Figure 6), but these symptoms are very different from those caused by dicamba.

Effects on soybean yield:

If cupped soybean plants were actually exposed to dicamba, will yield be adversely effected?   The answer is that it is absolutely NOT possible at this point of the season to predict whether or not yield will be impacted.  Published literature suggests this injury does not always result in soybean yield loss, but several factors are involved in determining if yield loss will occur.  In particular, soybean growth stage at the time of exposure, dosage of exposure, and growing conditions for the remainder of the growing season are important factors that determine if yield loss does or does not occur.  Much of the available literature suggests that if minor exposure occurs during early vegetative development, yield loss is less likely to occur than if exposure occurs when soybean have entered reproductive development.  However, there are no data that describe yield effects on soybean exposed to dicamba more than once.

Comments heard from the field and industry:

I have attempted to describe the current situation in Illinois as accurately as possible, and to provide data-derived information related to soybean exposure to dicamba.  As mentioned previously, soybean injury from dicamba has occurred each year in Illinois since the product was first commercialized.  However, the response of some individuals from companies who market formulations approved for use in dicamba-resistant varieties has been unlike anything I’ve experienced during my 24-year tenure at the University of Illinois.  Some comments heard from the field, social media, and industry are, in my opinion, quite troubling.

“Only a negligible percentage of soybean acres are affected”

I doubt anyone has completely accurate data on the actual number of soybean acres that have been impacted by dicamba.  Even if those data support the aforementioned statement, I haven’t spoken with many farmers who consider themselves or their acres as “negligible.”  Merely counting official reports filed with the IDOA does not accurately reflect the extent of acres impacted.

“Thoroughly investigate before drawing conclusions”

Excellent advice, especially when followed.  Without question, there have been instances of symptom misidentification.  I attempted to describe some of these in this article.  However, it seems that other factors are repeatedly being mentioned as able to cause leaf cupping.  Environmental conditions are frequently mentioned as inducing leaf cupping, yet I cannot find any peer-reviewed literature that specify or describe these conditions.  If these conditions exist, one would speculate they could be replicated under controlled conditions to confirm their impact on symptom development.  Also curious to me is that I have yet to see or have anyone report cupping of dicamba-resistant varieties.  Are these varieties somehow immune to these environmental conditions?

“The instances of volatility likely are due to applying older, non-approved formulations”

Again I ask, where are the data that indicate older formulations are being applied?  If we should “thoroughly investigate before drawing conclusions,” it seems premature to me to conclude the instances of volatility are wholly attributable to older dicamba formulations.  Much discussion has been made about the newer formulations that are purportedly lower volatility formulations.  These statements will have to be taken at face value, as I am aware of only one university that has evaluated volatility of only one commercial formulation.  Please keep in mind that low volatility is not the same as no volatility.  The new formulations are still volatile, albeit less volatile than older formulations.  Symptoms in many affected fields do NOT follow patterns associated with physical drift or contaminated application equipment, and exposure though volatility remains a very possible source of exposure.

“It is unlikely yield will be reduced.  You might even see a yield increase.”

This is perhaps the most troubling statement I have heard.  In my opinion, statements similar to these are unprofessional and unethical.  These individuals do NOT have the necessary data to make such bold predictions, which includes:

1) when the exposure occurred

2) the dose of the exposure

3) what the growing conditions will be like the remainder of the season

When dicamba is applied in a state that grows soybean, the occurrence of off-target symptoms is not a question of “if,”, but rather “scale.”  Some suggest the solution is to plant all soybean acres to dicamba-resistant varieties.  That might solve issues associated with soybean, but would likely increase the incidents of damage to other dicot species across the Illinois landscape.

Literature cited:

 Wax LM, Knuth LA, Slife FW (1969)  Response of soybean to 2,4-D, dicamba, and picloram.  Weed Sci 17:388–393.

 

Figure 1.  Cupping of young trifoliolate leaves following exposure to dicamba.

Figure 1. Cupping of young trifoliolate leaves following exposure to dicamba.

 

Figure 2.  Veins of young, affected leaves assume a parallel orientation following exposure to dicamba.

Figure 2. Veins of young, affected leaves assume a parallel orientation following exposure to dicamba.

 

Figure 3.  Physical spray drift from the soybean field to the left onto the soybean field to the right.

Figure 3. Physical spray drift from the soybean field to the left onto the soybean field to the right.

 

Figure 4.  Damage to soybean from dicamba residues dislodged from spray equipment.

Figure 4. Damage to soybean from dicamba residues dislodged from spray equipment.

 

Figure 5.  Leaf distortion caused by a foliar-applied PPO-inhibiting herbicide.

Figure 5. Leaf distortion caused by a foliar-applied PPO-inhibiting herbicide.

 

Figure 6.  Leaf distortion following  the POST application of a soil-residual herbicide.

Figure 6. Leaf distortion following the POST application of a soil-residual herbicide.


Join us for the Ewing Agronomy Field Day on Thursday, July 27, 2017

The University of Illinois Extension will host the Ewing Demonstration Center Agronomy Field Day on Thursday, July 27, 2017 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 49th year of continuous no-till production.

 

The topics to be discussed at Field Day include:

 

Managing Nitrogen for Corn & 2017 Growing Season Overview

  • Emerson Nafziger, Extension Crop Specialist, University of Illinois

Management Strategies for PPO-resistance

  • Karla Gage, Assistant Professor—Weed Science, Southern Illinois University

Southern Rust Management in Corn

  • Talon Becker, Extension Educator, University of Illinois

Insect Headlines in 2017

  • Kelly Estes, State Survey Coordinator, Illinois Cooperative Agriculture Pest Survey Program

Cover Crops:  The Good, The Bad, and The Practical

  • Nathan Johanning, Extension Educator, University of Illinois

 

The field day is free and open to anyone interested, and lunch will be provided.  Certified Crop Advisor CEUs will also be offered.  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 Monday, July 24.  For additional information on the field day, contact Marc Lamczyk at the number above or lamczyk@illinois.edu.


Palmer Amaranth ID of seed or plant tissue and Herbicide Resistance Testing at the University of Illinois Plant Clinic

Are you having trouble with Palmer amaranth? We have something new for you.

NEW Palmer ID test available this year: We spent a long winter optimizing a new assay in collaboration with Dr. Pat Tranel’s lab to aide in Palmer amaranth ID. Contamination of seed mixes with Palmer amaranth became a wide-spread issue last year. Efforts to determine if a seed mixture is contaminated can be hampered by low germination rates and slow grow outs in greenhouse tests. A new molecular assay to confirm the presence or absence of Palmer amaranth in sample seed or leaf tissue is now being offered by the University of Illinois Plant Clinic. Up to 100 Amaranthus spp. seeds, or 5 plants, can be tested as a single mixed sample. To submit plant tissue, remove the top two inches of young, healthy, newly-emerged leaves from suspect Palmer amaranth plants. Only Amaranthus spp. seeds are accepted for testing; if you have a seed sample containing forbes, grasses, etc., please contact the Illinois Crop Improvement Association at (217) 356-4053 for assistance in separating the Amaranthus seed.

Figure 1:

Figure 1:  Waterhemp leaf extract prior to DNA extraction. University of Illinois Plant Clinic photo

Herbicide Resistance testing: The University of Illinois Plant Clinic also continues to offer herbicide resistance testing in waterhemp and Palmer amaranth in 2017, Figure 1. This is the third year the service has been offered at the Plant Clinic. Samples are tested using molecular assays for common modes of action for both glyphosate and PPO-inhibitor herbicide resistance. Currently, only waterhemp and Palmer amaranth are accepted for this type of testing. To sample, remove the top two inches of plants that have survived an application of glyphosate and/or PPO-inhibitor herbicides. Young, healthy, newly-emerged leaves are ideal for submissions. We recommend submitting five plants per field. Results are given on a per-field basis.

For results from last year’s tests, please see: http://bulletin.ipm.illinois.edu/?p=3821

The fee for each service is $50 per sample. For additional information about how to sample, please see our flyer: http://web.extension.illinois.edu/plantclinic/downloads/herbicide.pdf and the molecular sample submission form: http://web.extension.illinois.edu/plantclinic/downloads/molecular.pdf

Authors: Diane Plewa and Suzanne Bissonnette


What to do if you suspect herbicide drift

Each year, the Illinois Department of Agriculture (IDOA) receives approximately 120 pesticide misuse complaints, of which 60% are pesticide drift complaints.  Neighborly discussions before pesticides are applied are important so applicators understand if sensitive plants are growing near the application site.  In the unfortunate case that drift has occurred, it’s a good idea to know the basics of the complaint process and what resources are available to you.

Before doing anything, both parties should make an effort to discuss the suspected drift incident and rule out other possible causes of the damage.  In cases where the cause of the damage remains unclear or where the parties will not work together, a formal complaint may be necessary.

The IDOA and University of Illinois Extension have important but different roles in assisting citizens of Illinois in dealing with pesticides. These roles are based on the IDOA’s responsibilities to administer and enforce the laws related to the use of pesticides and University of Illinois Extension’s responsibilities to educate and solve problems.

You may send affected plant samples to the University of Illinois Plant Clinic. For information on how to do so, go to http://web.extension.illinois.edu/plantclinic/. Be sure to include as much relevant information as possible. Keep in mind that the Plant Clinic does not perform pesticide residue tests, and without such tests, the cause of a symptom cannot be attributed to pesticide drift with 100% certainty. However, it is possible for Clinic staff and specialists to rule out other possible causes and establish whether the likely cause is drift.

The IDOA has three roles that impact its handling of pesticide-drift complaints. These roles are (1) education and licensing of applicators and operators via the Pesticide Safety Education Program, (2) investigation of complaints, and (3) enforcement of pesticide laws. The roles of IDOA are determined by laws and statutes passed by the Illinois legislature or the federal government.

If you choose to file a complaint with IDOA, time is of the essence. The pesticide drift complaint process is started by filling out a complaint form which can be found at: https://www.agr.state.il.us/pdf/pesticidemisusecomplaintform.pdf or by calling IDOA’s Bureau of Environmental Programs at 1-800-641-3934 (voice and TDD) or 217-785-2427.  Additional information on pesticide uses and misuses can be found on the agency’s website at: https://www.agr.state.il.us/pesticides-uses-misuses.

Complaint forms must be received by IDOA within 30 days of the incident or within 30 days of when the damage was first noticed. Complaints filed after that will be kept on record, but no administrative action can be taken.

The complaint process

Once a complaint is filed with the department, a field inspector is assigned the case. In most cases, the inspector will interview the complainant and inspect the site. Various types of samples, such as plants, water, or soil, may be collected for analysis at an approved laboratory.

The inspector may also interview applicators in the area, examine pesticide records and collect weather data in an attempt to determine the nature and cause of the damage. The field investigator will then submit a report to the Department for review.

Both parties will receive written notification if the Department finds a violation and takes an enforcement action. Penalties range from advisory or warning letters to monetary penalties of $750 to $10,000, depending on the type and severity of the violation. Penalties are determined through a point system defined in the Illinois Pesticide Act.

Even if a violation of the Illinois Pesticide Act cannot be substantiated, both the complainant and the alleged violator will be notified in writing of the complaint’s status. Remember, the Department’s role in pesticide misuse incidents is limited to determining whether a violation has occurred. IDOA cannot help complainants recover damages.

Certainly, it is easiest and best to prevent herbicide drift from occurring.  Drift can be extremely expensive and often results in poor neighbor relations.

Additional information for use when handling potential drift injury

A useful resource that includes information and helpful tips on preventing and dealing with the off-target movement of herbicide applications is an online module titled,

“Herbicide Tolerant Crop Stewardship”.   Especially useful would be chapter 5, “Avoiding/Handling Injury.”  While it was created with producers in mind, it would also be beneficial to homeowners, gardeners, and anyone who grows plants and it’s free.  It can be found at:  http://web.extension.illinois.edu/psep/articulate/htcs/.

Aaron Hager and Michelle Wiesbrook


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

Tillage

SelectMax

Glyphosate

Glufosinate

Paraquat

Non-GMO

Yes

Yes

Yes

Yes

Yes

RR

Yes

Yes

No

Yes

Yes

LL

Yes

Yes

Yes

No

Yes

RR/LL

Yes

Yes

No

No

Yes

 

 

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 (sprague1@msu.edu; 517-353-0224) or Dr. Gunsolus (gunso001@umn.edu; 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