Preharvest Intervals for Postemergence Soybean Herbicides

Almost all postemergence soybean herbicides have a preharvest interval or a soybean developmental stage beyond which applications cannot be made specified on their respective label.  Labels of some products may indicate both a developmental stage (before soybean bloom, for example) and a preharvest interval.  Preharvest intervals indicate the amount of time that must elapse between the herbicide application and crop harvest.  Failure to observe the preharvest interval may result in herbicide residue levels in the crop in excess of established limits.  Table 1 contains information regarding preharvest intervals for a number of postemergence soybean herbicides.

Herbicide Preharvest Interval/Maximum Soybean Growth Stage
Assure II/Targa 80 days
Anthem Maxx/Cadet 60 days
Basagran No interval on label
Classic 60 days
Cobra/Phoenix 45 days/do not apply after R6
Dual Magnum1/Dual II Magnum1 90 days/through third trifoliolate
Enlist Duo Do not apply after R2
Extreme/Tackle 85 days
FirstRate Apply prior to R2
Flexstar/Rhythm 45 days
Flexstar GT 45 days
Fusilade DX 60 days
Fusion Prebloom
Liberty/Interline/Cheetah/Cheetah Max 70 days
Roundup PowerMax2 Broadcast: through R2

Harvest aid: 14 days

Harmony SG 60 days
Marvel 60 days
Outlook1 5th trifoliolate
Poast/Poast Plus 75 days
Prefix/Vise 90 days
Pursuit 85 days
Raptor Prebloom
Resource 60 days
Select or SelectMax 60 days
Storm 50 days
Synchrony XP 60 days
Torment 85 days
Ultra Blazer 50 days
Warrant1/Warrant Ultra Before R2
Zidua1 3rd trifoliolate

 1These products will not control emerged weeds but can be applied postemergence for soil-residual weed control.

2Data, taken from the Roundup PowerMax label, are for broadcast applications in glyphosate-resistant soybean varieties.  Intervals change for applications (spot treatment and preharvest) made to non-glyphosate-resistant soybean varieties.

Postemergence Options to Control Waterhemp in Soybean

Waterhemp continues to be one of the most widespread and troublesome broadleaf weed species with which Illinois farmers must contend.  Factors related to the species’ biology, such as prolonged germination and emergence, obligate outcrossing, and high seed production, contribute to management challenges.  The evolution of herbicide resistance in Illinois waterhemp populations adds another very challenging obstacle for effective management.  Recently, many have sent questions and concerns regarding the inability to control waterhemp with various postemergence soybean herbicides.  A description of herbicide resistance in Illinois waterhemp populations, along with postemergence herbicide options to control waterhemp, follows.

Prior to the evolution of herbicide resistance in waterhemp, ALS-, PPO-, EPSPS- and GS-inhibiting herbicides controlled waterhemp postemergence in soybean.  Resistance to ALS-inhibiting herbicides (such as Raptor and Classic), first confirmed in Illinois during the mid-1990s, has become so widespread that this class of herbicides is largely considered functionally ineffective against waterhemp.  Resistance to PPO-inhibiting herbicides (such as Flexstar, Cobra, and Ultra Blazer) was first identified in Adams County in 2001, and the first instance of resistance to the EPSPS-inhibiting herbicide glyphosate (Roundup, etc.) was confirmed in Fayette County in 2006.  To date, no instance of waterhemp resistance to the GS-inhibiting herbicide glufosinate (Liberty, Interline, Cheetah) has been reported.

Figure 1 shows the range expansion of glyphosate-resistant waterhemp from 2012–2015.  These data are based on samples submitted to the University of Illinois for resistance verification with molecular marker assays.  Figure 2, using the same data source as Figure 1, shows the current range of PPO-resistant waterhemp in Illinois.  It should not be assumed that resistance does not occur in non-shaded counties; a better interpretation is simply that we have yet to test a positive sample from those counties.  In other words, it is altogether likely resistance to glyphosate and PPO inhibitors occurs in all Illinois counties.

Waterhemp resistant to PPO-inhibiting herbicide can be controlled with glyphosate, and glyphosate-resistant waterhemp can be controlled by PPO-inhibiting herbicides.  However, there are no effective herbicide options to control waterhemp resistant to both glyphosate and PPO inhibitors in conventional or glyphosate-resistant soybean varieties.  As mentioned previously, ALS-inhibiting herbicides are ineffective, and 2,4-DB will not improve control.  Inter-row cultivation or hand removal represent two options to control multiple-resistant waterhemp.

It remains very unlikely that a herbicide with a novel site of action will be commercialized in the foreseeable future.  At the same time, the frequency of multiple resistant waterhemp will only increase.  Many eagerly anticipate the ability to apply 2,4-D or dicamba to new herbicide-resistant soybean varieties, but the long-term utility of these herbicides to control multiple-resistant waterhemp will be compromised without thoughtful and implemented stewardship practices.

Range expansion of glyphosate-resistant waterhempPPO-resistant waterhemp in Illinois

Reminder of the 2016 Weed Science Field Research Tour

We would like to take this opportunity to once again extend the invitation to attend the 2016 University of Illinois Weed Science Field Research Tour, to be held Wednesday, June 29th 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.  The tour will provide ample opportunity to look at research plots and interact with weed science faculty, staff, and graduate students. Participants can compare their favorite corn and soybean herbicide programs to other commercial programs.  The tour will conclude around noon with a catered lunch at the Seed House.  Cost for the Urbana weed science field tour is $10, which will help defray the cost of the field tour book, refreshments and lunch.

We look forward to visiting with you at the Urbana weed science field day on June 29th.  Please contact us at 217-333-9646 if you have any questions.


Maximum Corn Growth Stage for Postemergence Herbicides

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


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

Herbicide Maximum corn heights and growth stagesa
2,4-D Broadcast before corn exceeds 8” tall; use drop nozzles when corn is taller than 8”.
Accent Q Broadcast up to 20” tall or through the V6 stage.  Apply with drop nozzles when corn is 20–36” tall or before the V10 stage.
Anthem Maxx Apply from corn emergence through the V4 (visible fourth leaf collar) stage.
Armezon Pro Apply from corn emergence to the 8-leaf stage or 30” tall.
Atrazine Apply before corn exceeds 12” tall.
Basagran No height specified on label.
Basis Blend Apply to corn from spike through 2 collar stage. Do not apply to corn having 3 fully emerged collars or over 6” tall.
Beacon Broadcast when corn is 4–20” tall.  After corn is 20” tall or exhibits more than 6 collars use directed applications up to tassel emergence.
Cadet Apply until corn is 48″ tall or prior to tasseling.
Callisto/Callisto GT May be applied to corn up to 30” tall or up to the 8-leaf stage.
Callisto Xtra Apply before corn exceeds 12” tall
Capreno Broadcast applications must be made to corn from the 1-leaf collar stage through the 5-leaf collar (V5) stage.
Clarity or Banvel Apply between corn emergence and the 5-leaf stage or 8” tall; apply 0.5 pt/A rate when corn is 8 to 36” or if 6th leaf is emerging, or if 15 days prior to tassel emergence.  Do not apply when soybean are growing nearby if: 1) corn is more than 24” tall, 2) soybean are more than 10” tall, 3) soybean have begun to bloom.
DiFlexx/DiFlexx Duo Apply broadcast when corn is at the spike through 6-leaf collar (V6) growth stage, or 36” tall, whichever occurs first.
Glyphosate (glyphosate-resistant corn) Apply broadcast through the V8 stage or until corn reaches 30” tall.  Use drop nozzles for applications to corn 30–48” tall.
Halex GT (glyphosate-resistant corn) Apply to corn up to 30″ tall or the 8-leaf stage.
Harmony SG Apply to 2–6 leaf corn with 1–5 collars or up to 16” tall.
Hornet WDG Apply broadcast until corn reaches 20” tall or V6 stage.  Apply with drop nozzles to corn up to 36” tall.
Impact/Armezon Can be applied up to 45 days before harvest.  Do not apply Armezon past the V8 growth stage.
Laudis Apply up to the V8 growth stage.
Liberty (glufosinate-resistant corn) Broadcast until corn is 24” in height or in the V7 growth stage (7 developed leaf collars).  Use drop nozzles for corn 24–36” tall.
Marksman Apply between corn emergence and the 5-leaf or 8” height stage.
Moxy Apply prior to tassel emergence.
NorthStar Broadcast applications are made when corn is between 4 –20” tall (V2–V6).  Use directed applications when corn is 20–36” tall.
Permit Can be applied from spike through layby.
Realm Q May be broadcast applied to corn up to 20” tall or exhibiting 6 leaf collars.
Require Q Apply to corn 4–20” tall.  Do not apply to corn exhibiting 7 or more leaf collars.
Resolve Q Do not apply to corn taller than 20” or exhibiting 7 or more leaf collars.
Resource Apply to corn from the 2-leaf through 10-leaf stage.
Shotgun Broadcast applications to corn up to 4 leaves or 8” tall; directed applications for 5-leaf or 8–11 ¾” tall corn.
Solstice May be applied broadcast up to the V8 growth stage or 30” tall.
Spirit Broadcast applications to corn 4–20” tall.  Use drop nozzles when field corn is 20–24” tall or exhibits more than 6 collars (V6).
Starane Ultra Apply broadcast to corn with up to 5 fully exposed leaf collars (V5).
Status Do not apply to corn taller than 36” or past the V10 stage.
Steadfast Q Apply to corn up to 20” tall or exhibiting 6 leaf collars.
Stinger Apply to corn from emergence through 24” tall.
Yukon Apply broadcast or with drop nozzles to corn from spike to 36” tall.  Drop nozzles are recommended when corn exceeds 20”.
Zemax May be applied after corn emergence until plants reach 30” tall or up to the V8 stage.

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




Herbicide Resistance in Waterhemp: PPO inhibitor and Glyphosate Testing University of Illinois Plant Clinic 2016

What is your waterhemp doing ? Waterhemp glyphosate and PPO inhibitor resistance testing was offered by the Plant Clinic for the first time in 2015. Due to the strong demand for this testing, we are offering it again this year. We adapted protocols developed by Dr. Tranel’s lab in the Dept. of Crop Sciences to transition the molecular protocols for glyphosate and for PPO Inhibitor resistance from a research laboratory to a service lab offering. In 2015, we screened plants from 338 fields (approx. 1350 plants) in 5 states, including 240 fields from Illinois. To the best of our knowledge, we are the only Plant Clinic in the country offering this service.

The data  generated showed that the majority of the fields sampled contained waterhemp resistant to at least one of these two classes of herbicides. The Plant Clinic was sponsored by the Regional Crop Management Extension Conferences to purchase some of the necessary equipment for this endeavor.

2015 Counties with waterhemp samples testing positive for PPO inhibitor or glyphosate resistance, University of Illinois Plant Clinic.

2015 Counties with waterhemp samples testing positive for PPO inhibitor or glyphosate resistance, University of Illinois Plant Clinic.

To submit waterhemp samples for testing: After applying herbicide, select up to 5 waterhemp survivors. Remove the top inch or two from each plant (containing young, newly-emerged, healthy leaves. Older leaves and leaves that are damaged by herbicide will be more difficult for us to work with), and seal it in a sandwich-sized tip-top plastic bag. Use a separate bag for each plant. Place the bags in an envelope and send via overnight delivery to the University of Illinois Plant Clinic, along with a Waterhemp Testing sample submission form (see below). Ideally samples should be sent the same day they’re collected, but if necessary they can be stored for a day or two in a refrigerator. Do not freeze the samples. Do not send samples on Friday or Saturday.

Select top few inches from the waterhemp  for testing

Select top few inches from the waterhemp for testing

Sample submission forms are available for download on our website: Fill out a new form for each field. There is a $50 fee for the testing, which includes both the glyphosate and PPO-inhibitor resistance tests. Authors: Suzanne Bissonnette and Diane Plewa

University of Illinois Plant Clinic: Celebrating 40 Years of Service to Illinois

Anniversary Overview of Plant Clinic: Welcome to another year of service at the Plant Clinic! Since 1976, the University of Illinois Extension Plant Clinic has served as a clearinghouse for plant problems. Housed first in the Department of Plant Pathology and now Crop Sciences, the Plant Clinic was originally developed to help County Cooperative Extension staff and campus-based Extension specialists with requests for diagnoses on a wide variety of plants. By acting as a centralized diagnostic laboratory, the Plant Clinic  serves as a source of information about plant problems in Illinois.  While our primary mission is to provide diagnostic service to Illinois,  the Clinic maintains permits to receive plant, pest, and soil samples from the continental US and territories.

For most of its existence, the Plant Clinic was open from May through October. In 2010, we began year round operation.  During the off-season diagnostic staff write grants, compile reports, write fact sheets, and present at conferences and meetings around the state to support the outreach mission.  Our Nematology diagnostic clinic staff process samples and bioassays year round. The Plant Clinic has taken a lead role in the Illinois First Detector Invasive Species Workshops which started in 2013, as a part of our NIFA CPPM-EIP grant that supports IPM and diagnostics outreach. The workshops are held every year in various locations across Illinois and educate green professionals, city and municipal employees, and concerned public about invasive plants, insects, and diseases that threaten Illinois horticulture and agriculture.

U of I Plant Clinic Diagnostic Lab

U of I Plant Clinic Diagnostic Lab

For the past several years, the Plant Clinic has processed over 4,000 plant and soil samples annually. The vast majority of the plant samples are analyzed for disease and insect problems, though plant and insect identification is also performed. The soil samples are analyzed for nematode populations, including Soybean Cyst Nematode and vermiform pathogenic nematodes. Last year a new service testing for herbicide resistance in waterhemp was offered. Protocols for molecular testing for glyphosate and PPO-inhibitor resistance were adapted from ones developed in Dr. Tranel’s laboratory at the University of Illinois, 338 fields (representing 1350 plants) were analyzed. Plants were submitted from Illinois and 4 other Midwestern states.

The Plant Clinic works with the National Plant Diagnostic Network, Illinois Department of Agriculture and the National Sentinel Plant Network to stay aware of new threats in Illinois. Last year we found several new pests in Illinois, including jumping worms (an invasive earthworm) in northern Illinois, and tar spot of corn in north/central Illinois (this disease was found in Illinois and Indiana in 2015 and was a first find in the country). The Plant Clinic also works with the Illinois Department of Agriculture and Illinois Crop Improvement Association to certify diseases present crops for export, and has a partnership with the Illinois Department of Natural Resources to monitor the health of natural areas in Illinois.

The Plant Clinic employs undergraduate and graduate students, providing them with hands-on experience working in a plant diagnostic laboratory and expanding their outreach skills. Staff write articles for various online newsletters, including the Home, Yard, and Garden Pest Newsletter ( and The Bulletin ( The Plant Clinic participated in the ACES Family Academies in 2015, where youth ages 6-13 got a chance to use microscopes, inoculate plants, and wash soil to collect nematode eggs. Departmental service includes opening the laboratory for tours and hands-on activities for students, and outreach at events such as Agronomy Day held every August.

Sample Information for the 2016  Season: Plant Clinic services include plant and insect identification, diagnosis of disease, insect, weed, and chemical injury problems (chemical injury on field crops only), nematode assays, herbicide resistance testing of waterhemp to PPO and glypohsate, and help with nutrient related problems, as well as recommendations involving these diagnoses. Microscopic examinations, laboratory culturing, virus assays, qPCR, ELISA and nematode assays are some of the techniques used at the Plant Clinic. Many samples can be diagnosed within a few days. Should culturing be necessary, isolates may not be ready to make a final reading for 10-14 days. Standard nematode processing also requires 1-2 weeks depending on the procedure. Some nematode bio-assays can take up to 4 months. We send your final diagnosis and invoice to you through both the US mail and email.

Please refer to our website at for additional details on samples, sample forms, fees, and services offered. If you have questions about what, where, when, or how to sample call us at 217-333-0519. When submitting a sample, please 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. Pictures of the affected plants or areas can also be sent with the sample to give us a better idea of what is occurring in the environment.

Our fees vary depending on the procedure necessary. General diagnosis including culturing is $15, ELISA and other serological testing is $25, nematode analysis for SCN or PWN is $20, specialty nematode testing (such as corn) is $40. Call ahead for other specialty nematode testing or bio-assays. Checks should be made payable to the University of Illinois Plant Clinic. Please contact us if you are uncertain of which test is needed.

For more information about the Plant Clinic, including how to contact us and submit a sample, please see our website at: We are celebrating 40 years of service to the state of Illinois all season long on our Facebook page ( and are looking forward to another 40 years of helping people with their plant problems!  authors Suzanne Bissonnette and Diane Plewa

What is That Yellow-Flowered Plant?

The springtime color scheme provided by winter annual weed species in many no-till fields has shifted from the hearty purple of flowering henbit and purple deadnettle to the bright yellow flowers of two species.  Yellow rocket and cressleaf groundsel (a.k.a. butterweed) both produce bright yellow flowers and are common across much of the southern half of Illinois.  Although flower color is similar, the plants are distinct species.  Most of the yellow-flowered plants currently in fields is butterweed.

Native to the United States, butterweed (Packera glabella) can be found from Texas east to Florida, northward along the Atlantic coast to Virginia, and west to Nebraska.  Herbarium specimens from the Illinois Natural History Survey indicate that butterweed specimens were collected in Illinois as early as 1932.  The earliest herbarium specimens of butterweed generally originated from counties in southern Illinois.  During the 1980s and 90s, the Illinois Natural History Survey augmented their collection with specimens from more northern counties, such as Champaign and Vermillion.  While scouting fields in Kankakee County recently, butterweed was easy to find.  Typically favoring moist to saturated soils, butterweed thrives in areas such as wastelands, pastures, fence-rows, and roadsides.  With the increased adoption of no-till and reduced tillage conservation practices, butterweed has become more prevalent in areas devoted to agronomic crop production.

Butterweed completes its life cycle within one year (an annual growth habit).  Field research was conducted from fall 2004 through spring 2006 at the University of Illinois to determine the emergence timing and growth characteristics of butterweed in no-till fields.  Butterweed emergence was predominately during the fall months and was essentially completed by November, although some emergence occurred in the spring.  From these results, we generally consider butterweed to be a winter annual species.

Following emergence, the formation of rosettes occurs prior to overwintering.  The rosette leaves have petioles that connect the leaves to the stem.  Often the under side of the rosette leaves are deep purple.  Bolting (stem elongation), flowering and seed production occurs the following spring, often during late April to early May.  The stem of butterweed is glaborous and hollow.  After bolting, petioles are absent from leaves on the upper part of the plant.  The leaves are pubescent, generally irregular in shape, and are deeply cut to the midrib.  The elongated stem often has a purplish tint. A member of the Asteraceae family, butterweed produces two types of composite flowers.  The outside portion of the flower contains ray florets while the center part contains disk florets.  The flowers are bright yellow and grouped in clusters that are located on several flowering stalks of the plant.  Seeds are easily disseminated via wind due to the white hairs (pappus) on the apex of the achene.

Yellow rocket (Barbarea vulgaris) is a winter annual species in the mustard (Brassicaceae) plant family.  The plant can produce numerous stems that grow from a basal crown surrounded by a rosette of deeply green leaves.   These basal leaves range in length from approximately 2–8 inches and have a large terminal lobe that is somewhat heart-shaped at its base.  Stem leaves are arranged alternately and become progressively shorter toward the top of the plant.  Flowers are produced on spike-like racemes and consist of four petals that form a cross.  Seed pods (siliques) are about 1 inch long and nearly square in cross section.

Yellow rocket and butterweed side-by-side

Yellow rocket and butterweed flowers

University of Illinois Weed Science Field Research Tour

We invite you to attend the 2016 University of Illinois Weed Science Field Day on Wednesday, June 29th 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 look forward to visiting with you at the Urbana weed science field day on June 29th. Please contact us at 217-265-0344 or 217-333-9646 if you have any questions.

Marestail Control Prior to Planting

Marestail can be one of the most challenging weeds to control prior to planting no-till soybean.  Already this season some have reported poor marestail control following applications of glyphosate plus 2,4-D.  Poor control can be caused by several factors, including large plant size and resistance to glyphosate.  If a marestail population is resistant to glyphosate, a pint of 2,4-D in the spring is generally inconsistent/ineffective when it’s the only product in a tankmix active on the resistant population.

Adding Sharpen or metribuzin to glyphosate plus 2,4-D can improve marestail control.  Include MSO with Sharpen and be sure to adhere to planting intervals in treated fields where another soil-applied PPO inhibitor will be used.  Glufosinate (Liberty, Interline, etc.) or Gramoxone SL are other options to control marestail before planting.  Control is often improved when these products are tankmixed with metribuzin and 2,4-D.  Both glufosinate and Gramoxone are contact herbicides, so be sure to adjust application equipment (nozzles, spray volume, etc.) to ensure thorough spray coverage.

Tillage is another option to control emerged marestail.  Delay tillage until field conditions are suitable and be sure to till deep enough to completely uproot all existing vegetation.

Cover Crop Termination

The following information about cover crop termination is taken from the 2016 Weed Control Guide for Ohio, Indiana and Illinois.  The information was originally written by extension weed scientists at Purdue University.

Cover crops are unique in that most are planted primarily to reduce soil erosion and otherwise enhance soil quality, and are not harvested for their seed, fruit, or forage (although some are grazed or used as forage). Instead, cover crops are terminated before planting of summer annual grain crops such as soybeans and corn. When not effectively terminated, cover crops have the potential to become weeds in the grain crop and can slow soil drying and warming in the spring. Many cover crop species have characteristics that make them both desirable as cover crops, and troublesome as weed species. Weedy cover crop escapes not only affect the current production crop, but also can produce seeds and establish a seed bank that will result in future weed problems.

Cover crops can be terminated by a number of meth­ods, although herbicide application is the most common method. When selecting a herbicide program for termi­nation of a cover crop, consider:

  • the cover crop species
  • the cover crop growth stage
  • other weed species present
  • the production crop to be planted
  • the weather conditions at application

Cover Crop Species. Cover crop systems that contain only grass species or only broadleaf species can be terminated using selective grass or broadleaf herbicides. However, producers will often grow combi­nations of grass, legumes, and non-legume broadleaf species together to receive the maximum benefits that each group presents. Successfully terminating a cover crop that contains grasses and broadleaves will require a nonselective herbicide such as glyphosate, glufos­inate, or paraquat. It is possible to combine a selective grass herbicide (sethoxydim, clethodim, quizalifop, fluazifop) and selective broadleaf herbicide (2,4-D, dicamba) to terminate a mixed crop, but it is not advisable because many of these combina­tions can be antagonistic and poor control will result. Combining glyphosate with either 2,4-D or dicamba can ensure more complete termination of broadleaf spe­cies than spraying glyphosate, 2,4-D, or dicamba alone. Effective herbicide control of grasses and broadleaves varies by species. Consult a weed control guide or herbicide label to ensure the herbicide will be effective on a particular cover crop species. See species-specific recommendations below for herbicide programs for some common cover crops.

Cover Crop Growth Stage. The growth stage and height of the cover crop at the time of termination is critical in determining what herbicide and rate will be most effective. Crops that are bolting, jointing, or pro­ducing reproductive structures can be difficult to control with herbicides and may require other termination meth­ods. Always take cover crop heights into consideration because taller, more mature plants may require higher herbicide rates than smaller, less mature plants.

Other Weed Species Present. Before choosing a herbicide to terminate a cover crop, carefully consider all the plant species that are present — including weeds. Decide on a herbicide plan before planting or seeding the cover crop, and then amend the plan according to any additional weed species that occur.

Cash Crop to Be Planted. When planning a herbi­cide termination program, use only herbicides that are labeled for burndown or preplant applications with the summer annual crop you will plant. Be sure you also observe crop rotational restrictions. For example, there is a 14-day restriction when planting soybean after us­ing high rates of 2,4-D in a cover crop termination. The rotational restrictions for corn after applications of selec­tive grass herbicides (sethoxydim, clethodim, quizalifop, fluazifop) range from 30 to 120 days.

Weather Conditions at Application. Environmental conditions affect herbicide performance, and unfortu­nately these are factors that cannot be controlled or predicted. Typically, cover crop terminations take place in the early spring, so while the exact weather may vary, temperatures tend to be cool with variable cloudiness and high soil moisture. Take these typical weather con­ditions into account when planning an herbicide termi­nation program — cool, cloudy conditions slow the rate at which herbicides kill plants. Wet soil can also keep sprayers out of fields, which delays spray applications and allows cover crops to reach undesirable heights and growth stages.

A wide variety of cover crop species are available and recommended for specific cropping systems, soil types, and regions. The following section provides herbicide termination recommendations for the cover crop species most commonly planted in Indiana, Ohio and Illinois.

Annual ryegrass (Lolium multiflorum), also called Ital­ian ryegrass or common ryegrass, has become a very popular cover crop throughout the Midwest. Do not confuse annual ryegrass with cereal rye (Secal cereal). Annual ryegrass is a good cover crop because of its ability to rapidly germinate in the fall, grow aggressively in the spring, and add substantial root and forage mass to the soil profile. However, this plant’s aggressive and competitive nature can also make it a weed problem in grain crops. The introduction of annual ryegrass as a cover crop in Indiana and the possibility of it escaping as a weed is a concern. Annual ryegrass has established itself as a weed in orchards, vineyards, and grain crops throughout the western and southern United States and is recognized by multiple scientific weed societies as an invasive weed species. Annual ryegrass is also able to quickly adapt to herbicide selection pressure. The International Survey of Herbicide Resistant Weeds (Heap 2015) reports herbicide-resistant annual ryegrass populations in ten states and across six herbicide sites of actions. Follow these guidelines for successful termi­nation of annual ryegrass cover crops:

  • Make applications prior to 8″ plant height
  • Glyphosate rates of at least 1.25 lb ae/A are required, although 2.5 lb is preferred for annual ryegrass termi­nation
  • Ryegrass must be actively growing, and it is recom­mended that applications occur only following three consecutive days when air temperatures have been above 45 F
  • The addition of saflufenacil to glyphosate can improve control of annual ryegrass
  • Combinations of paraquat, metribuzin and 2,4-D or dicamba can control small ryegrass (<6″ in height), but are not recommended for control of larger plants
  • Avoid using PSII herbicides (atrazine & metribuzin) in mixtures with glyphosate, as they can cause antago­nism and poor control of annual ryegrass.

Cereal rye and oats. Glyphosate at a rate of 0.75 lb ae/A will effectively control both species up to 18 inches tall. Mixtures of glyphosate plus 2,4-D, chlorimuron, chloransulam, atrazine, or saflufenacil can also be ap­plied for additional control of other cover crop species (specifically broadleaf species) and residual control of summer annual broadleaf weeds. The nonselective her­bicides paraquat and glufosinate are less effective than glyphosate on these species.

Crimson clover and Austrian winter peas are two popular legume species used as cover crops that typi­cally do not winter kill and require a spring termina­tion. Escapes and failed control of crimson clover and Austrian peas have been documented as rare, so they pose less threat as potential weed species in production crops than annual ryegrass. Information on control of these species with herbicides is limited, but cover crop guides advise that glyphosate and 2,4-D easily control crimson clover and winter peas.