Update on Palmer amaranth Distribution in Illinois

Palmer amaranth (Amaranthus palmeri) has garnered much attention recently in both academic discussions and popular press releases, and with good reason.  Among the weedy species of Amaranthus, Palmer amaranth has the fastest growth rate and is the most competitive with the crops common to Midwest agronomic cropping systems.  Soybean yield losses approaching 80% and corn yield losses exceeding 90% have been reported in the peer-reviewed scientific literature.  Palmer amaranth can be effectively managed in Illinois agronomic crops, but the greatest likelihood for successful management is with systems that employ multiple effective management tactics.  Palmer amaranth is perhaps the personification of a weed species that requires an integrated management approach.

Unlike waterhemp, Palmer amaranth is not indigenous to Illinois.  Palmer amaranth evolved as a desert-dwelling species in the southwestern United States, including areas of the Sonoran Desert.  However, genotypic and phenotypic adaptability have allowed Palmer amaranth to expand its distribution and colonize the vastly different agricultural landscapes across much of the eastern half of the United States.  The presence of Palmer amaranth in agricultural landscapes of the eastern United States is perceived by some to be only a recent phenomenon, but Dr. Jonathan Sauer, noted expert of Amaranthus taxonomy, noted in 1957 that, “It looks as if there has been recent and substantial northeastward expansion of A. palmeri resulting in its present wide area of cohabitation with interior species,” (Sauer 1957).

Will Palmer amaranth populations, likely introduced by seeds moved into Illinois from areas where Palmer amaranth has become the dominant pigweed species, be able to adapt to the landscape and growing conditions of Illinois?  Perhaps a more important question is to define the damage niche of Palmer amaranth populations in Illinois agronomic cropping systems.

Recent research conducted in Illinois by Dr. Adam Davis, USDA-ARS plant ecologist at the University of Illinois, has examined these important questions.  Results from these experiments have demonstrated that there are few landscape-level barriers to the establishment of Palmer amaranth populations in Illinois, and that these populations, once established, are competitive with crop species.  In other words, these results indicate that it’s not a question of if Palmer amaranth will become established in Illinois, but rather when and where it will become established.

For the past two seasons, weed scientists at the University of Illinois and Southern Illinois University have conducted surveys (field and laboratory) to determine where in Illinois Palmer amaranth now occurs.  Figure 1 illustrates the current known distribution of Palmer amaranth in Illinois based on 2012–2013 surveys by university weed scientists.  Palmer amaranth populations have been confirmed in counties colored orange or blue; orange designates glyphosate-resistant populations are present in these counties.  It should be neither construed nor assumed that Palmer amaranth populations occur only in the counties colored on this map. It is altogether likely that Palmer amaranth populations are present in many other counties.

Similar to female waterhemp plants, female Palmer amaranth plants produce an abundance of seeds.  These small black seeds are easily moved within and between fields in myriad ways, including harvesting and tillage equipment.  If you identify Palmer amaranth in one or more fields, what can you do this fall to limit movement of the seed?  The following suggestions were developed by weed scientists in Illinois to help curtail the movement of Palmer amaranth seed.

1)      Fields with Palmer amaranth populations should be the last fields harvested this fall and the last fields planted next spring.

2)      Mark or flag areas where Palmer amaranth plants have produced seed.  These areas should be intensively scouted the following season and an aggressive Palmer amaranth management plan implemented to prevent future seed production.

3)      Do not mechanically harvest mature Palmer amaranth plants with crop harvesting equipment.  Physically remove the plants immediately prior to harvest and either leave the plants in the field or place in a sturdy garden bag and remove the plants from the field.  Bury or burn the bags in a burn barrel as soon as possible.

4)      Fields in which Palmer amaranth seeds were produced should NOT be tilled during the fall or following spring.  Leaving the seeds near the soil surface increases the opportunities for seed predation by various granivores.

It’s not too early to begin planning an integrated Palmer amaranth management program.  An integrated herbicide program should include soil-residual herbicides applied at full recommended use rates within two weeks of planting and followed by postemergence herbicides applied before Palmer amaranth plants exceed 3 inches tall.  Most herbicides that control waterhemp also control Palmer amaranth, but successful, long-term management of Palmer amaranth in Illinois will likely require more than herbicides.

 

Literature cited:

Sauer, J.  1957.  Recent migration and evolution of the dioecious Amaranths.  Evolution 11:11–31.

Palmer Amaranth ID Form

Guidelines for the Identification and Management of Palmer amaranth


Palmer amaranth: what should you do if you find it in your fields?

Recently, we have identified populations of Palmer amaranth in several Illinois counties.  The density of many populations is relatively low, and often these plants occur only in small patches.  However, a few scattered plants this year can lead to severe infestations within only a few years.

We continue to accept tissue samples from suspected Palmer amaranth plants and use tools of molecular biology to identify whether the sample is Palmer amaranth or another species of Amaranthus.  Information on how to collect and submit tissue samples from suspected Palmer amaranth plants can be found in the “Palmer Amaranth Identification” form that accompanies this article.

So, what should you do if you discover Palmer amaranth growing in your fields?  We have developed the following suggested guidelines and also have made these available in a format that can be downloaded and printed.  Keep in mind these guidelines assume a scenario where the population of Palmer amaranth is only a few plants per acre:

1)      If you discover a plant that you think may be Palmer amaranth, you can verify its identity by sending a leaf tissue sample to the University of Illinois (please find a sampling protocol in the Palmer amaranth ID form accompanying this article) for identification using molecular biology techniques.

2)      Plants confirmed or suspected of being Palmer amaranth should be physically removed from the field prior to flowering.  Do not rely on herbicides for control.  Physical removal can include hoeing or hand-pulling plants from the soil.  If hoeing is used, be sure to sever the plant stem at or below the soil surface to reduce the potential for regrowth, and remove plants from the field as they will re-root from stem fragments.

3)      If Palmer amaranth plants are not identified until after brown-to-black colored seeds are present on female plants, we suggest leaving the plants undisturbed in order to avoid inadvertently spreading seed.

4)      Mark or flag areas where Palmer amaranth plants produced seed.  These areas should be intensively scouted the following season and an aggressive Palmer amaranth management plan implemented to prevent future seed production.

5)      Do not mechanically harvest mature Palmer amaranth plants.  Physically remove the plants prior to harvest and either leave the plants in the field or place in a sturdy garden bag and remove the plants from the field.  Bury or burn the bags in a burn barrel as soon as possible.

6)      Fields in which Palmer amaranth seeds were produced should NOT be tilled during the fall or following spring.  Leaving the seeds near the soil surface increases the opportunities for seed predation by various granivores.

7)      Herbicides that control waterhemp also control Palmer amaranth.  An integrated herbicide program should include soil-residual herbicides applied at full recommended use rates of within two weeks of planting and followed by postemergence herbicides applied before Palmer amaranth plants exceed 3 inches tall.

And remember, fields containing Palmer amaranth should be the last fields harvested in 2013 and the last fields planted in 2014.

Palmer Amaranth ID Form

Guidelines for the Identification and Management of Palmer amaranth


Brownstown Agronomy Research Center Field Day – July 25

The 2013 Brownstown Agronomy Research Center Field Day, presented by the University Of Illinois Department Of Crop Sciences, will be held on Thursday, July 25. Extension researchers and specialists will address issues pertinent to the current growing season. Tours will start at 8 a.m., with the second and third groups leaving the headquarters around 8:20 a.m. and 8:40 a.m. The tours will last about two and a half hours and will be followed by lunch provided by U of I Extension.

Shaded tour wagons will take participants to each stop. These topics will be addressed:

  • Nitrogen Sensors & Variable-rate N Applications – Dennis Bowman
  • Wheat Disease I.D. & Management – Dr. Carl Bradley
  • Emerging Developments in Weed Management – Doug Maxwell
  • Crop Rotation:  Another Risk Management Tool – Dr. Emerson Nafziger
  • Agronomic and Environmental Assessment of Cover Crops – Dr. Angie Peltier

 The 208-acre Brownstown Agronomy Research Center has been conducting crop research on the claypan soils of southern Illinois since 1937. More than 30 research and demonstration projects are conducted at the Center every year. Visitors are always welcome.

The research center is located south of Brownstown on IL Route 185, approximately 4 miles east of the IL Route 40 / 185 junction.

For more information, contact Robert Bellm (618-427-3349); rcbellm@illinois.edu
Visit us on the web at http://web.extension.illinois.edu/barc/


Controlling Large Horseweed and Waterhemp in Soybean

The volume of inquiries about how to control large (taller than 12 inches) horseweed (a.k.a. marestail) and waterhemp in soybean has remained consistent over the past 10 days.  The answer can be summarized as follows: there are NO postemergence herbicides that will consistently control these very large weeds in soybean, especially if these weeds are resistant to glyphosate.

Glyphosate-resistant waterhemp can be controlled by foliar-applied PPO inhibitors (such as lactofen (Cobra), fomesafen (Flexstar) or acifluorfen (Ultra Blazer)) in conventional or glyphosate-resistant soybean varieties, or by glufosinate (Liberty) in glufosinate-resistant (Liberty Link) soybean varieties.  However, it is very important to remember that these herbicides do not extensively translocate within the weed following their absorption through the leaf surface, and control of large weeds is often not as consistent as control of small (5 inches or less) weeds.  Reducing application rates of these herbicides often results in reduced waterhemp control.  In our research, we have observed less control of glyphosate-resistant waterhemp with tankmixes of glyphosate and fluthiacet (Cadet) or 2,4-DB compared with tankmixes of other PPO-inhibiting herbicides.  Keep in mind that PPO-resistant waterhemp biotypes are very common, and these biotypes are sometimes also resistant to glyphosate.  The only effective methods to control waterhemp plants resistant to PPO inhibitors and glyphosate include physically (i.e., weed hook or hoe) or mechanically (i.e. cultivate) removing the plants from the field.

The following text, written by Dr. Mark Loux, extension weed scientist at the Ohio State University, provides excellent information about managing marestail in double-crop soybean.

A weed free start is the most critical aspect of a weed management program for double-crop soybeans.  This can be challenging to achieve where glyphosate-resistant marestail are present after wheat harvest.  Problems with marestail include the following:  1) most populations are now glyphosate-resistant and many of these are also ALS-resistant (we suspect ALS-resistant marestail exists in Illinois); 2) it’s usually not possible to use 2,4-D ester and wait 7 days until double-crop soybean planting; and 3) marestail that were tall enough to be cut off by harvesting equipment will be even more difficult to control.  Our research indicates that there are no herbicide treatments that consistently control glyphosate-resistant marestail populations that have regrown following mechanical disturbance or prior herbicide treatment.  Certainly one of the best options is to plant LibertyLink soybeans, which allows for a POST application of Liberty to help control plants that survive a preplant burndown.  The following are the most effective burndown options for control of marestail prior to double-crop soybean emergence:

Liberty (32 to 36 oz) + Sharpen (1 oz) + MSO + AMS (can also add metribuzin)

Liberty (32 to 36 oz) + metribuzin (4 to 8 oz of 75DF) + AMS

Glyphosate (1.5 lb ae/A) + Sharpen (1 oz) + MSO + AMS

We suggest using a spray volume of 20 gpa for any of these treatments, and avoiding nozzles that produce large droplets.  Results with a combination of glyphosate and 2,4-D may be more variable then the treatments listed.

With regard to the control of weeds that can emerge after double-crop soybean planting, and the entire weed control system, the following approaches can be considered.

1.  Plant any type of soybean, and include a residual herbicide with the burndown treatment so that POST herbicides are not needed.  A good strategy in Roundup Ready or nonGMO soybeans even where POST treatment is needed, since POST marestail control might be impossible in these systems.  Residual herbicides used at this time of the year should be restricted to those that have little or no carryover risk – such as metribuzin, Valor, or low rates of chlorimuron or cloransulam products.

2.  Plant a LibertyLink soybean, and apply Liberty POST as needed.  Probably the best option for control of later-emerging marestail or plants that regrow after the burndown, assuming that there is any Liberty available.

3.  Plant a Roundup Ready soybean and apply glyphosate POST.  Should work for most weeds, but not a good choice if the POST application needs to control marestail.

4.  Plant a nonGMO soybean and apply conventional POST herbicides (Flexstar, Fusion, Select, etc) as needed.  This system has the most potential for soybean injury, but seed may be cheaper than the other systems.  Not a good choice if the POST application needs to control marestail.


Reminder…University of Illinois Weed Science Field Research Tour

We would like to take this opportunity to once again extend the invitation to attend the 2013 University of Illinois Weed Science Field Day, to be held Wednesday, June 26th 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 barbeque 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.  Advanced registration is not required.

We look forward to visiting with you at the Urbana weed science field day on June 26th.  Please contact us at 217-333-4424 if you have any questions.


Screening Waterhemp for Herbicide Resistance

We are pleased to announce that, with continued financial support from the Illinois Soybean Association, we are offering free screening for herbicide resistance in waterhemp during the 2013 growing season.  To date, over 1000 plant samples have been screened through this collaborative effort between the weed science program at the University of Illinois and the Illinois Soybean Association.  This valuable service helps farmers and researchers better track and understand the evolution of herbicide resistance in waterhemp across the state.

Details for collecting plant samples are as follows:

1) Following application of glyphosate, select five waterhemp survivors in the field.

2) Remove the top inch or two (containing young, newly emerged, healthy leaves) from each plant and seal inside in a sandwich-sized Ziploc bag. Use a separate bag for each plant.

3) Place the bags in an envelope and send by overnight delivery to Dr. Chance Riggins, 320 ERML, 1201 W. Gregory Dr., Urbana, IL 61801. Do NOT submit samples to the University of Illinois Plant Clinic.  Ideally, samples should be sent the same day they are collected. If necessary, however, they can be stored for a day or two in a refrigerator (but do not freeze) until shipped.  Do NOT mail samples on Friday or Saturday.

4) The sample submission form included with this article can be printed and should be completed for each field sampled.

There is no fee for this service, but please understand that we cannot guarantee when results will be available. Also, because of the way in which we conduct our resistance tests, a test result of “sensitive” to glyphosate does not rule out the possibility that the plant actually is resistant, but by a mechanism that is different than for what we are testing. Finally, we want to assure you that we will respect the privacy of those sending samples: we will not make the exact location of any samples, or names associated with samples, available to anyone without your permission. If you have any questions, feel free to contact Pat Tranel (217-333-1531; tranel@illinois.edu).

 

Waterhemp Sample Submission Form 2013


Is it Waterhemp or Palmer Amaranth?

Palmer amaranth (Amaranthus palmeri) is a summer annual broadleaf weed species taxonomically related to other pigweed species (waterhemp, smooth, redroot) common in Illinois agronomic cropping systems.  Palmer amaranth is not indigenous to Illinois, but rather evolved as a desert-dwelling species in the southwestern United States including areas of the Sonoran Desert.  Genotypic and phenotypic adaptability have allowed Palmer amaranth to expand its distribution and colonize the vastly different agricultural landscapes across much of the eastern half of the United States, including Illinois.

Research has demonstrated that Palmer amaranth has a higher growth rate and is more competitive than other pigweed species.  Growth rates approaching 3 inches per day and yield losses of 78% (soybean) and 91% (corn) attributed to Palmer amaranth interference have been reported in the scientific literature.  Seed production capability of female Palmer amaranth plants is similar to that of female waterhemp plants.

Early and accurate identification of Palmer amaranth plants coupled with implementation of an integrated management program are essential to reduce the potential for crop yield loss due to interference of Palmer amaranth.  Proper management of Palmer amaranth populations can help reduce the potential for successful seed production that will augment the soil seedbank and perpetuate the population in future growing seasons.

The cotyledon leaves of Palmer amaranth are relatively long compared with other Amaranthus species (Figure 1).

Figure 1. Cotyledons of Palmer amaranth (top) and waterhemp (bottom).

 

Like all weedy Amaranthus species in Illinois, the true leaves (those produced after the cotyledon leaves) of Palmer amaranth have a small notch in the tip.  Occasionally, a single hair can be found in the leaf notch of Palmer amaranth (Figure 2). This hair may not be present in each leaf notch of a Palmer amaranth plant, and tends to be less common on leaves of waterhemp plants.

Figure 2.

Figure 3. Leaves of Palmer amaranth sometimes have white or purple chevrons.

The stems and leaves have no or few hairs and the stems feel smooth to the touch.  Leaves are alternate on the stem and are generally lance-shaped or egg-shaped with prominent white veins on the underside.  As plants become older, they often assume a poinsettia-like appearance and sometimes have a white or purple chevron on the leaves (Figure 3).  Leaves are attached to the stem by petioles that are usually longer than the leaf blade.

Accurate identification of weedy Amaranthus species during early vegetative stages can be difficult because many exhibit similar morphological characteristics (i.e., they look very much alike).  During the 1990s, waterhemp provided an excellent example of how difficult it can be to differentiate among the various Amaranthus species, especially when plants are small (Figures 4 and 5).

 

 

 

Figure 4. Palmer amaranth seedling.

Figure 5. Waterhemp seedling.

 

 

 

 

 

 

 

 

 

 

 

 

To assist weed management practitioners in accurately identifying Palmer amaranth, you may send us tissue samples from suspected Palmer amaranth plants and we will use tools of molecular biology to identify whether the sample is Palmer amaranth or another species of Amaranthus.  Information on how to collect and submit tissue samples from suspected Palmer amaranth plants can be found in the “Palmer Amaranth Identification” form that accompanies this article.  Please download this form, provide as much information as possible, and submit it along with the tissue samples to the address listed at the top of the form.

 

Palmer Amaranth ID Form


Controlling Horseweed (Marestail) after Crop Emergence

The past several days have experienced a noticeable increase in questions about options to control marestail after crop emergence.  Many have reported poor marestail control from herbicides applied prior to planting (primarily no-till soybean), especially when these burndown applications contained only glyphosate or glyphosate and 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, have contributed to a challenging situation for which a good solution might not be available.  Several herbicides can control emerged marestail plants that are less than six inches tall, but few herbicides will control plants that are 12 inches or taller.

The majority of Illinois corn acres receive one or more tillage operations prior to planting, and these tillage operations generally control emerged marestail.  However, marestail might be a problem in corn acres planted with limited or no preplant tillage.  Growth regulator herbicides, such as dicamba and 2,4-D, can provide control or suppression of emerged marestail.  Some ALS-inhibiting herbicides also have activity against marestail, and control is sometimes improved when (if allowed by label) these herbicides are tankmixed with dicamba or 2,4-D.  Glufosinate, applied alone or with atrazine in glufosinate-resistant corn hybrids, is another option for control.  Keep in mind that if glufosinate was applied as a burndown treatment prior to corn planting, no in-crop applications of glufosinate can be made.  Foliar-applied HPPD inhibitors combined with atrazine are other options that can provide control/suppression of emerged marestail less than about six inches tall.

Marestail that survived a burndown herbicide in no-till soybean can be very difficult to control after soybean have emerged.  Cloransulam or chlorimuron-containing products can be applied postemergence to suppress or control emerged marestail.  These products should be applied after the first soybean trifoliolate has expanded and before marestail height exceeds 6 inches.  These products can be tankmixed with glyphosate for postemergence applications in glyphosate-resistant soybean.  Glyphosate is an option for control of glyphosate-sensistive biotypes and glufosinate is an effective option when used in glufosinate-resistant (Liberty Link) soybean varieties.  Foliar-applied PPO-inhibiting herbicides will not control emerged marestail.

Another option to consider is to physically remove the surviving plants with a hoe, weed hook, or by pulling them from the soil.  Before summarily dismissing this as a viable option, remember that marestail plants can be very competitive with soybean and can produce around 200,000 seeds per plant that are easily dispersed long distances by wind.  A few plants that reach maturity this year can produce a sufficient amount of seed to infest an entire field within only a couple of seasons.  Removing surviving marestail plants before they produce seed could save a great deal of time, money, and consternation in future seasons.


Mark Your Calendars for the 2013 AGMasters Conference

The 2013 AGMasters Conference will be held at the i Hotel and Conference Center in Champaign, IL on December 2 and 3. The conference will begin with a morning general program followed with 1 1/2 days of specialized sessions. Participants will be able to pick and choose the sessions of most interest to them. These sessions are designed to encourage interaction between instructors and students and cover a broad range of topics including crop production challenges, soil fertility, water resource management, entomology, plant pathology, weed science, and introductory statistics. Each session is taught twice and is limited to 40 students (per session). Registration for the most popular topics is very competitive. The overall conference is limited to the first 160 registrants. The conference will be pieced together over the summer and registration information will become available by early fall. For now, please add these dates to your calendar and look for more conference information to follow in this Bulletin. Conference co-chairs include Dennis Bowman, Carl Bradley, Aaron Hager, Sandy Osterbur and me, all members of the Crop Sciences Department. As the growing season unfolds, please contact any of us with your suggestions for the 2013 program. We welcome your input.

Mike Gray


Supplemental Information about Soil-Residual Herbicides Applied to Emerged Corn

The following table provides supplemental information to an article (“Soil-Residual Herbicides Applied to Emerged Corn”) published last week.  Table 1 summarizes information about postemergence applications of more traditional soil-applied corn herbicides.  Please consult the respective product label for additional information.

 

Table 1.  Maximum Corn Size for Postemergence Applications of Soil-Residual Herbicides

Herbicide Maximum Corn Size for Broadcast Application
Prequel, Princep, Verdict Before corn emergence
Balance Flexx, Corvus 2 leaf collars
Bullet, Micro-Tech 5 inches
Bicep Lite II Magnum, Cinch ATZ, Cinch ATZ Lite, Parallel Plus, Stalwart Xtraa 5 inches
Breakfree, Breakfree ATZ, Breakfree ATZ Lite, Degree, Degree Xtra, FulTime, Harness, Harness Xtra, Keystone, Keystone LA, SureStart/TripleFLEX, Surpass, TopNotch 11 inches
Atrazine, Bicep II Magnum, Expert, Guardsman Max, G-Max Lite, Lexar EZ, Lumax EZ, Outlookb 12 inches
Resolve DF 12 inches (V5)
Hornet WDG 20 inches (V6)c
Python WDG 20 inches or V6
Resolve Q 20 inches (through V6)
Callisto, Prowl H2O, Zemax 30 inches (V8)
Cinch, Dual II Magnum, Me-Too-Lachlor II, Stalwart C, Parallel 40 inches
TriCor Prior to tassel emergence

aAll of these products are labeled for directed applications to corn up to 12 inches tall.

bOutlook is labeled for layby applications to corn up to 36 inches tall.

cHornet is labeled for directed application to corn up to 36 inches tall.