Soybean Injury from Soil-Applied Herbicides

Integrated weed management programs offer the greatest potential for long-term, sustainable solutions to weed populations demonstrating resistance to herbicides from multiple families.  Soil-residual herbicides are important components of integrated weed management programs and provide several benefits, including reducing the intensity of selection for resistance to foliar-applied herbicides.  Recent survey data indicate the percentage of Illinois soybean acres treated with soil-residual herbicides has increased during the past few years.

In the vast majority of instances, soil-applied herbicides control target weed species with little to no adverse effect on the crop.  However, soybean plants sometimes are injured by these herbicides. Questions about soybean injury caused by soil-applied herbicides recently have been posed, so this article will review some of the factors that can contribute to herbicide-induced soybean injury.

Herbicides vary in their inherent potential to cause soybean injury.  Many university-generated herbicide effectiveness rating tables also provide estimates of soybean injury potential.  Some herbicide active ingredients, such as cloransulam and clomazone, are often rated as having very low potential to cause soybean injury, whereas other active ingredients are rated as having a greater inherent potential to cause injury.  The rate at which the herbicide is applied can influence the potential for soybean injury by increasing or decreasing the amount of herbicide in a given volume of soil.

Most many cultivars are not overly sensitive to any particular herbicide, but other soybean cultivars can vary in their sensitivity to certain herbicides.  Data in the scientific literature and company-generated variety trials demonstrate cultivar sensitivity differences to various soil-residual herbicides.  Some cultivars demonstrate sensitivity to one active ingredient, whereas other cultivars can be sensitive to more than one active ingredient.

The environment has a large influence on the severity of soybean injury caused by soil-applied herbicides.  Environment-induced crop stress, often caused by cool, wet soil conditions, can enhance soybean injury from soil-applied herbicides.  In most cases, herbicide selectivity arises from the soybean plant’s ability to rapidly metabolize the herbicide to a nonphytotoxic form before it causes much visible injury.  Soybean plants growing under favorable conditions are able to adequately metabolize the herbicide before any injury symptoms are expressed.  However, when the soybean plant is under stress, its ability to metabolize the herbicide can be sufficiently reduced to the point at which injury symptoms develop.

Soil physical properties can increase or decrease the potential for soybean injury by impacting how much herbicide is available for plant uptake.  Soils with higher amounts of clay and organic matter have a greater ability to adsorb more herbicide onto these soil colloids.  Herbicide bound to soil colloids is not available for plant uptake.  In contrast, coarse-textured soils have less adsorptive capacity so more herbicide remains available for plant uptake.  Labels of soil-applied herbicides often contain precautionary language about the increased potential for soybean injury when the product is applied to sandy soils or soils low in organic matter.

The application timing of soil-residual herbicides also can impact the potential for soybean injury.  Applications made immediately before or after soybean planting result in a high concentration of herbicide near the emerging soybean plants.  In contrast, a herbicide is often more widely distributed within the soil profile by the time of soybean emergence when applications are made several days or weeks prior to planting.

The soil-applied PPO-inhibiting herbicides, including saflufenacil, flumioxazin, and sulfentrazone, are very effective for control of Amaranthus species.  These herbicides (and many others) also can cause soybean injury.  Our first experience with soybean injury from soil-applied PPO inhibitors occurred in 1996 while evaluating sulfentrazone for control of herbicide-resistant waterhemp.  Soybean injury symptoms caused by these soil-applied herbicides can vary depending on the soybean developmental stage when exposure occurred.  The most commonly encountered injury symptoms occur on the hypocotyl and cotyledons (Figure 1), often indicating the plants were exposed to a high concentration of herbicide as they were emerging.

Figure 1. Injury symptoms evident on emerging soybean plant.

Symptoms include necrotic lesions on the soybean hypocotyl near the soil surface and reddish-colored spots or lesions on the hypocotyl and/or cotyledons (Figures 2 and 3).  Lesions on the hypocotyl may not always kill the young soybean plants, but can create an area of weakened tissue that may lead to stems breaking during rain or high wind.  In severe cases, plants may actually die following emergence of the cotyledons.

Figure 2. Reddish-colored hypocotyl tissue near the soil surface.

Figure 3. Emerged soybean with damage to hypocotyl and cotyledons.

Plants with damage only to cotyledons usually develop normally (Figure 4).

Figure 4. Injury to cotyledons.

Other symptoms can occur after soybean emergence if treated soil is splashed into the soybean meristem by heavy precipitation.

There likely is no solitary reason for the recent instances of soybean injury from soil-applied PPO-inhibiting herbicides.  As previously mentioned, our first experience with this type of soybean injury occurred almost 20 years ago and we have continued to observe this type of injury intermittently ever since.  These herbicides have become very popular choices for the management of herbicide-resistant Amaranthus populations, and widespread application of these herbicides increases the probability of encountering soybean cultivars that inherently are more sensitive to one or more of these herbicides.  In many instances of soybean injury, the herbicide was applied after soybean fields were planted and a precipitation event occurred within a few days of soybean emergence.  Cool air and soil temperatures during the same interval can further increase injury potential by slowing the rate of herbicide metabolism.  A crusted soil surface can slow soybean emergence, increasing the time the hypocotyl and cotyledons remain in the zone of high herbicide concentration.  Once the herbicide is moved deeper into the soil profile, the potential to cause additional injury is greatly reduced.

 


It’s not too early to scout for Palmer amaranth

Planting progress may have been slowed by the recent precipitation, but weed emergence continues despite wet field conditions.  Be aware that Palmer amaranth has begun to emerge. We scouted our Palmer amaranth field location west of Kankakee on May 8 and observed emerged Palmer amaranth plants (Figure 1).

Figure 1. Emerged Palmer amaranth, May 8 near Kankakee

 

We returned to the field on May 15 to find a very robust Palmer amaranth population (Figure 2).  The largest Palmer amaranth plants found on May 15 were approximately 2 inches tall with 6 to 8 true leaves.

Figure 2. Emerged Palmer amaranth, May 15 near Kankakee

 

Some leaf petioles on older Palmer amaranth plants already had elongated to approximately twice the length of the leaf blade, but we did not find any leaves with chevrons or watermarks.  Extension weed scientists in Iowa and Ohio also have reported finding emerged Palmer amaranth plants in their respective states.

Completing corn and soybean planting will assume top priority when field conditions are again suitable, but please do not overlook the need to control emerged Palmer amaranth in fields already planted.  The return of warm air temperatures coupled with ample soil moisture will help emerged Palmer amaranth plants grow very rapidly.  Postemergence herbicides should be applied before Palmer amaranth plants are taller than 4 inches.  It is NOT advisable to delay the application of postemergence herbicides until additional Palmer amaranth plants emerge.  Delayed application of postemergence herbicides, whether by choice or due to adverse weather conditions, can lead to disastrous outcomes.

Palmer amaranth continues to germinate throughout much of the growing season, making it altogether likely that additional Palmer amaranth plants will emerge following the initial postemergence herbicide application.  Consider including a soil-residual herbicide with the postemergence herbicide to control additional Palmer amaranth emergence and allow the crop to gain a competitive advantage over later-emerging weeds.

Palmer Amaranth 2014 HI


University of Illinois Weed Science Field Research Tours

We invite you to attend the 2014 University of Illinois Weed Science Field Day on Wednesday, June 25th at the University of Illinois Crop Sciences Research and Education Center, located immediately south of the main campus.  Coffee and refreshments will be available under the shade trees near the Seed House beginning at 8:00 a.m.

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 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.

We are continuing field research work at the Dekalb, Perry and Brownstown research centers.  There will not be formal weed science tours at these locations, but most of the weed science plots will be signed during the agronomy day field tours scheduled for these locations.  We are also planning to host a field tour at our Palmer amaranth field location west of Kankakee toward the later part of July.  We’ll provide additional information about this tour via the Bulletin when details have been finalized.

We look forward to visiting with you at the Urbana weed science field day on June 25th.  Please contact us at 217-333-4424 if you have any questions.


Visualizing the growth rate of Palmer amaranth

Recently, the weed science program at the University of Illinois published recommendations for the management of Palmer amaranth in Illinois agronomic crops.  One particular recommendation emphasizes the need for timely application of foliar-applied herbicides; in particular, foliar-applied herbicides must be applied before Palmer amaranth plants are taller than four inches.  Most weed management practitioners are very familiar with the growth rates of waterhemp and many other weed species, but perhaps are less familiar with the growth rate of Palmer amaranth.

Cody Evans, a weed science graduate student at the University of Illinois, initiated work in the greenhouse to compare the growth rate of waterhemp and Palmer amaranth.  This was not a replicated experiment, but simply a demonstration of the relative growth rate differences between these two species.  Both species emerged on the same day, and the plants were photographed every other day for a month.  As the photographs illustrate, Palmer amaranth plants reached a 4-inch height less than 10 days after emergence.

The growth rate comparison illustrated in the accompanying photographs was conducted under greenhouse conditions, but experience suggests a similar growth rate of Palmer amaranth should be expected under field conditions.  Timely applications of foliar-applied herbicides will require vigilant scouting throughout a large portion of the growing season.


Ready for Samples at the University of Illinois Plant Clinic

Welcome to another Diagnostics season! Samples have been steadily appearing this spring here at the Clinic in our 39th year of operation. On the field front, there have been concerns with virus disease diagnosis in wheat. On the home landscape front, there is a mountain of winter kill and windburn injury from the harsh winter just past.

The University of Illinois Plant Clinic began year-round operation in the fall of 2011. Our new location is in Jonathan Baldwin Turner Hall on the south end of the Urbana campus. During the winter, our hours are irregular due to trainings and winter meetings so call ahead. However, we resume regular business hours, 8am-12pm and 1pm-4:30pm, on Monday April 28th, 2014.

Example of a great sample: Sample form, symptomatic plant, protected root ball and payment

Plant Clinic services include plant and insect identification, diagnosis of disease, insect, weed and chemical injury observation (chemical injury on field crops only), nematode assays, and help with nutrient related problems, as well as management recommendations involving these diagnoses. Microscopic examinations, laboratory culturing, virus assays, and nematode assays are some of the techniques used in the clinic. Many samples can be diagnosed within a day or two. Should culturing be necessary, isolates may not be ready to make a final reading for as much as two weeks. Nematode processing also requires about 1-2 weeks depending on the procedure. We send your final diagnoses and invoices to you through both the US mail and email. If you provide your email address on the sample form you will get your information earlier.

Please refer to the Plant Clinic website for additional details on sampling, sample forms, fees and services offered. If you have questions about what, where, or how to sample call us at 217-333-0519. Whenever submitting a sample, provide as much information as possible on the pattern of injury in the planting, the pattern on individual affected plants, and details describing how symptoms have changed over time to cause you concern.

Our fees vary depending on the procedure necessary. General diagnosis including culturing is $15, ELISA and immunostrip testing is $25, Nematode analysis for SCN or PWN is $20, Specialty Nematode testing (such as corn) is $40.

Please include payment with the sample for diagnosis to be initiated. Checks should be made payable to the University of Illinois or to the Plant Clinic. Companies can setup an account, call and we will accommodate you. Call if uncertain of which test is needed.

Plant Clinic location, S-417 Turner Hall, 1102 S. Goodwin, Urbana IL 61801

Drop off a sample:

You can also drop off a sample at S-417 Turner Hall. Park in the metered lot F 28 on the east side of Turner or at the ACES library metered lot on the west side of Turner. Come in the South door. Take the elevator located in the SE corner of the building. Turn left when exiting the elevator; we are located along the SE corridor of the 4th floor. Please use the green drop box located just outside S-417 if we are temporarily out of the office.

Sending a sample thru US mail or delivery service address to:

University of Illinois Plant Clinic
1102 S. Goodwin, S-417 Turner Hall
Urbana, IL 61801

Social Media: We have a lot of ways to keep you up to date on what is happening at the Plant Clinic and about other plant and pest issues. Follow the U of I Plant Clinic on Facebook, YouTube or, Blogger.


Terminating cover crops

The number of acres in Illinois seeded with various cover crops has increased during the past few seasons.  Those cover crops that do not winter kill are usually controlled the following spring with herbicides.  Dr. Bob Hartzler, extension weed scientist at Iowa State University, recently published an excellent article in which he describes several factors that can impact the performance of spring-applied herbicides used to terminate cover crops.  We sincerely appreciate Dr. Hartzler’s willingness to allow us to reproduce his article which follows.

There is an increased interest in the use of cover crops due to the many agronomic and environmental benefits they offer.  An important consideration when incorporating cover crops into the system is their termination.  Failure to completely control cover crops at planting results in them acting as a weed and competing with the crop.  Several factors influence the effectiveness of burndown treatments, including the cover crop species and growth stage, the herbicides and rates used, application parameters and environment.

Research in Missouri has shown that cereal rye and hairy vetch are usually consistently controlled with appropriate treatments, whereas winter wheat, annual ryegrass and red clover can be more difficult to kill.

As with controlling weeds, cover crops are easier to kill early in the spring while they are small.  They become progressively more difficult to kill as they approach reproductive stages.  The Roundup PowerMax label states better performance is achieved when applied before the boot stage of cereal rye.  Increasing the herbicide rate and the spray volume to improve coverage will improve the consistency of control when dealing with large, mature cover crops.

Temperatures tend to fluctuate widely during the cover crop termination period and can lead to variable results.   Spraying when temperatures favor active plant growth minimizes problems; however, abnormally cold nights (<40° F) may reduce activity even when favorable temperatures (>60° F) occur during the day.

Glyphosate is the standard herbicide used for terminating cover crops.  Glyphosate is recommended at 0.63 lb a.e./A (18 oz Roundup PowerMax) for cereal rye 16 inches or less in height, and at higher rates for larger rye.  Tank-mixing other herbicides with glyphosate may reduce the activity of glyphosate.   Control of cereal rye with glyphosate was reduced up to 50% when tank-mixed with atrazine or Canopy, whereas 2,4-D, dicamba or Sharpen had little or no effect on rye control (K. Bradley, Univ. Missouri).  The antagonism observed with tank-mixes was greater with late applications than applications made to small rye.

Unfortunately, we have a limited ability to access the susceptibility of cover crops to burndown treatments.  The most consistent control will be achieved with applications made in early spring while cover crops are small and actively growing. Avoid spraying in the early morning or evening during periods with less than optimum temperatures.  The glyphosate label provides great flexibility in application rate.   While the Roundup PowerMax label states an 18 oz rate for cereal rye, this rate should only be used under ideal conditions.   Increase the rate when tank-mixing with other products, with larger rye, or when applications are made during cool periods.


Management of Palmer amaranth in Illinois

Palmer amaranth is a weed species that must be thoughtfully and carefully managed; simply attempting to control Palmer amaranth often leads to ineffective herbicide applications, substantial crop yield loss, and increasing weed infestations.  Ignored or otherwise not effectively managed, Palmer amaranth can reduce corn and soybean yield to near zero.  The threat of Palmer amaranth during the 2014 growing season is very real across a large portion of Illinois.

In January 2014, the weed science program at the University of Illinois developed recommendations for management of Palmer amaranth in agronomic crops.  These recommendations were developed in accordance with the somewhat unique growth characteristics of this weed species.  The goals of the recommendations are twofold: 1) to reduce the potential for Palmer amaranth to negatively impact crop yield, and 2) to reduce Palmer amaranth seed production that ultimately augments the soil seed bank and perpetuates the species.

Before delineating the specific management recommendations, we present three general principles of Palmer amaranth management:

1)      Prevention is preferable to eradication.  Prevention refers to utilizing tactics that prevent weed seed introduction and weed seed production.  Palmer amaranth is not native to Illinois, so any population discovered in the state originated from seed that somehow was moved into the state.  The myriad of ways in which Palmer amaranth seeds can be transported, however, makes preventing seed introduction extremely challenging.  Once Palmer amaranth populations become established, utilizing any and all tactics to prevent seed production becomes of paramount importance.

2)      It is not uncommon for annual herbicide costs to at least double once Palmer amaranth becomes established.  There are simply no soil- or foliar-applied herbicides that will provide sufficient control of Palmer amaranth throughout the entire growing season.  At least three to five herbicide applications per growing season are common in areas where Palmer amaranth is well established.

3)      Control of Palmer amaranth should not be less than 100 percent; in other words, the threshold for this invasive and extremely competitive species is zero.  Female Palmer amaranth plants produce tremendous amounts of seed and in less than five years a few surviving plants can produce enough seed to completely shift the weed spectrum in any particular field.

Species Biology: Germination and emergence of Palmer amaranth

Palmer amaranth seed germination and seedling emergence are best described as continuous.  Similar to waterhemp, multiple Palmer amaranth emergence events are possible throughout much of the growing season.  However, previous research has demonstrated that Palmer amaranth seed has a higher germination rate than most other Amaranthus species (including waterhemp), and demonstrates a germination percentage higher than waterhemp at both low and high temperatures.  These germination and emergence characteristics help explain why Palmer amaranth can seemingly “displace” waterhemp from a field within only a few years after Palmer’s introduction.  Palmer amaranth that emerges before waterhemp in the spring and later in the growing season after waterhemp emergence has stopped, gives the species a competitive advantage over waterhemp and most other weed species.

Recommendations based on Palmer amaranth germination and emergence characteristics:

1)      Be certain to control all emerged Palmer amaranth plants before planting corn or soybean.  Burndown herbicides or thorough tillage are effective tactics to control emerged Palmer amaranth plants before planting.  Keep in mind, however, that glyphosate will not control glyphosate-resistant Palmer amaranth and growth regulator herbicides (such as 2,4-D or dicamba) are most effective on Palmer amaranth plants less than 4 inches tall.  If preplant scouting (which is especially important prior to planting soybean) reveals Palmer amaranth plants taller than 4 inches, consider using tillage instead of herbicides to control the plants.

2)      Apply a full rate (based on label recommendations for soil texture and organic matter content) of an effective soil-residual herbicide not sooner than seven days prior to planting nor more than three days after planting.  Many soil-residual herbicides that are effective for controlling waterhemp are also effective for controlling Palmer amaranth.  Soil-applied herbicide families that demonstrate control or suppression of Palmer amaranth include the triazines (atrazine, simazine, metribuzin), dinitroanilines (trifluralin, pendimethalin), chloroacetamides (metolachlor, acetochlor, dimethenamid, etc.), and protox inhibitors (flumioxazin, sulfentrazone, saflufenacil).  Do not apply less than the rate recommended by the product label.  In soybeans, products containing sulfentrazone (Authority) or flumioxazin (Valor) have provided effective control of Palmer amaranth.  Application rates of products containing these active ingredients should provide a minimum of 0.25 lb ai/acre sulfentrazone or 0.063–0.095 lb ai/acre flumioxazin.

Species Biology: Palmer amaranth growth rate

The growth rate and competitive ability of Palmer amaranth exceed those of other Amaranthus species.  Research has demonstrated that Palmer amaranth plants have the highest values for several growth parameters, including plant volume, dry weight and leaf area among the Amaranthus species common to agronomic cropping systems.  Perhaps most important with respect to application timing of foliar-applied herbicides, Palmer amaranth demonstrates the fastest rate of height increase.  Whereas waterhemp can add one inch of new growth per day under good growing conditions, Palmer amaranth can add two to three inches per day.  The effectiveness of most foliar-applied herbicides dramatically decreases when Palmer amaranth plants are taller than four inches.

Do not rely solely on glyphosate to control Palmer amaranth.  Molecular assays have indicated resistance to glyphosate appears to be relatively common among recently identified Palmer amaranth populations in Illinois.

Recommendations based on Palmer amaranth growth rate:

1)      Begin scouting fields within 14–21 days after crop emergence.  We recommend this interval even for fields previously treated with a soil-residual herbicide applied close to planting.

2)      Foliar-applied herbicides must be applied before Palmer amaranth plants exceed four inches in height.  Reiterating, the effectiveness of most foliar-applied herbicides dramatically decreases when Palmer amaranth plants are taller than four inches.  Postemergence herbicides that demonstrate control or suppression of Palmer amaranth include synthetic auxin herbicides (dicamba, 2,4-D), diphenylethers (acifluorfen, lactofen, fomesafen), glufosinate, glyphosate, and HPPD inhibitors (mesotrione, tembotrione, topramezone).  Palmer amaranth can germinate and emerge over an extended period of time, so there is often a wide range of plant sizes by the time postemergence herbicides are applied.  This can present problems with spray interception by smaller plants under the protective canopy of larger plants.  Adjustments in spray volume and pressure can help to overcome some of the challenges with coverage.

3)      Consider including a soil-residual herbicide during the application of the foliar-applied herbicide.  A soil-residual herbicide applied with the foliar-applied herbicide can help control additional Palmer amaranth emergence and allow the crop to gain a competitive advantage over later-emerging weeds.

4)      Fields should be scouted 7–14 days after application of the foliar-applied herbicide to determine:

  1. herbicide effectiveness
  2. if the soil-residual herbicide included with the POST application is providing effective control
  3. if additional Palmer amaranth plants have emerged.

If scouting reveals additional Palmer amaranth plants have emerged, make a second application of a foliar-applied herbicide before Palmer amaranth plants are four inches tall.

Species Biology: Palmer amaranth seed production

Palmer amaranth (like waterhemp) is a dioecious species, meaning plants are either male or female.  Male plants produce pollen and female plants produce seed, which makes Palmer amaranth an obligate outcrossing species.  Outcrossing species tend to have more genetic diversity then self-pollenated species and this can hasten the evolution of herbicide resistance.  Transfer of herbicide-resistance traits via pollen can quickly spread these traits across the landscape.  Research has demonstrated that female Palmer amaranth plants are capable of producing numbers of seed comparable to that of waterhemp (several hundred thousand to over one million).

Recommendations based on Palmer amaranth seed production:

1)      Physically remove any remaining Palmer amaranth plants before the plants reach the reproductive growth stage.  Plants should be severed at or below the soil surface and carried out of the field.  Severed plants can root at the stem if left on the soil surface, and plants can regenerate from stems severed above the soil surface.

The pdf  accompanying this article contains additional information about the management of Palmer amaranth.  This publication was made possible through funding by the United Soybean Board and collaboration among several university and private industry partners.

PalmerAmaranth (2)


2014 Illinois Crop Management Conferences Registration Now Open

The latest research information on crop production and management issues will be discussed at four University of Illinois Crop Management Conferences this winter. These two-day conferences are designed to address a wide array of topics pertinent to crop production, pest management, and natural resource issues and provide a forum for discussion and interaction between participants and university researchers.

Certified Crop Advisers can earn up to 13 hours of CEU credit. Advance registration, no later than one week before each conference, is $130 per person. Late and on-site registration is $150. Dates and location for the four regional conferences are listed below. Links to the complete agendas and registration information for each conference are located on the Crop Sciences Research and Education Center web page here.

 

January 22-23: Mt. Vernon – Krieger/Holiday Inn Convention Center. For more information, contact Robert Bellm, (618-427-3349); rcbellm@illinois.edu . Register online at http://extension.illinois.edu/go/icmcmtvernon

January 29-30: Springfield – Northfield Inn Conference Center. For more information, contact Robert Bellm, (618-427-3349); rcbellm@illinois.edu . Register online at http://extension.illinois.edu/go/icmcspringfield

February 6:  Champaign – i-Hotel and Conference Center. For more information, contact Dennis Bowman, 217-244-0851); ndbowman@illinois.edu . Register online at http://extension.illinois.edu/go/icmcchampaign

February 12-13: Malta – Kishwaukee College Conference Center. For more information, contact Russ Higgins (815-274-1343); rahiggin@illinois.edu . Register online at http://extension.illinois.edu/go/icmcmalta


Fall Herbicides to Control Marestail (Horseweed)

Widespread and often very dense populations of marestail in soybean fields last spring caught the attention of farmers and other weed management practitioners.  Many came to the difficult realization that marestail is not a problem weed species only in the more southern portions of Illinois.  It’s difficult to say with complete accuracy how far north these infestations occurred, but mature marestail was easily observed during recent travels through Kankakee and Will counties.  As we mentioned earlier this year, many reported poor marestail control from herbicides applied prior to planting (primarily no-till soybean), especially when burndown applications contained only glyphosate or glyphosate plus 2,4-D.  The increasing frequency of glyphosate-resistant marestail populations, the rush to plant whenever field conditions were conducive, and the less-than-ideal environmental conditions when many burndown applications were made, contributed to a challenging situation for which a good solution was not always readily available.

Marestail is native to North America and like many other plant species completes its life cycle in one year.  Unlike many other annual species, however, marestail can exist as a winter or summer annual.  Populations of winter annual marestail typically emerge during the fall months, within a few days or weeks after seed is dispersed from the parent plant.  Summer annual populations can emerge in early or late spring, perhaps as late as early summer in some instances.  In northern areas of Illinois, most marestail demonstrates a winter annual life cycle, whereas a substantially higher proportion of spring emergence occurs in areas south of (approximately) Interstate 70.  Both winter and summer annual life cycles can be found across central Illinois.

Fall-emerging plants form a basal rosette that represents the plant’s overwintering stage.  In the spring, plants bolt by rapidly elongating the main stem.  Mature horseweed plants may reach heights in excess of 6 feet, but plants ranging from 3 to 5 feet are perhaps most common.  Flowers are produced in a panicle-type inflorescence at the top of the plant.  The seeds are known as achenes, and are produced with an attached “parachute” (known as a pappus) to aid in wind-borne dispersal.  Research has demonstrated that mature marestail plants can produce in excess of 200,000 seeds, with fall-emerging plants frequently producing more seeds than spring-emerging plants.  Marestail seed can travel long distances with its dispersal mechanism, which becomes especially important when considering the spread of herbicide-resistant biotypes.  Mature seeds do not demonstrate much dormancy, but rather germinate soon after contact with the soil surface.  Seeds do not remain viable in the soil seedbank for very long.

We have received many questions about applying herbicides following harvest to control emerged marestail plants.  Fall-applied herbicides often provide more effective and consistent control of emerged marestail as compared with spring-applied (i.e., burndown) herbicides.  We suggest applying 2,4-D (1.0 lb acid equivalent per acre) anytime between mid-October and late November to control emerged marestail.  This treatment should not be expected to provide much soil-residual activity, so marestail plants that emerge after application will most likely not be controlled.  Do not rely solely on glyphosate (either in the fall or spring) to control emerged marestail.  Other herbicides (including glyphosate) can be tankmixed with 2,4-D to broaden the spectrum of winter annual species controlled.

Do not simply assume that fields treated with fall-applied herbicides will be free of marestail next spring.  Be sure to scout fall-treated fields before spring planting and take appropriate measures (i.e., supplemental herbicides, tillage, etc.) to control any existing marestail plants.  Do not plant soybean into an existing marestail population.  Residual herbicides should be applied close to soybean planting to control summer annual species, including spring-emerging marestail.

We do not recommend fall herbicide applications as an avenue to provide residual control of summer annual weed species.  Control of summer annual species, such as waterhemp, is often improved when soil-residual herbicides are applied closer to planting compared with several weeks (or months) prior to planting.  If a soil-residual herbicide will be part of a fall herbicide application, we suggest selecting an application rate that will provide control of winter annuals throughout the remainder of 2013, and recommend against increasing the application rate in hopes of obtaining control of summer annual species next spring.