Herbicide Resistance Management for Common Lambsquarters and Horseweed

In the June 13th issue of the Wisconsin Crop Manager, I discussed herbicide resistance management for giant and common ragweed. This week’s featured herbicide resistance threats are common lambsquarters, Chenopodium album, and horseweed, Conyza canadensis. Weed scientists across the Midwest and Midsouth have identified eleven species of weeds that are of most concern for herbicide resistance because of their ability to compete with crops and to develop resistance to different herbicide sites of action. In 1979, University of Wisconsin weed scientists identified a population of common lambsquarters resistant to atrazine, a photosystem II inhibitor (www.weedscience.org). Lambsquarters populations in Michigan and Ohio have been found resistant to ALS inhibitors. In 2013, University of Wisconsin researchers identified horseweed plants resistant to glyphosate. Ohio and Delaware have horseweed populations resistant to both ALS inhibitors and glyphosate. Resistance to a single site of action has occurred in over twenty states.

Now is the time to start thinking about fall horseweed management. Emergence typically occurs in the early spring and again in the fall. Long-term no-till systems tend to harbor significant horseweed populations. Scouting in mid to late summer to locate any escapes from spring herbicide applications is important for herbicide resistance management and to decide whether to switch to a horseweed management program that includes both spring and fall control measures. Fall herbicide applications can help to reduce horseweed populations in problem fields. Also, if dandelion is an issue, then there are two reasons to consider fall herbicide applications. University of Wisconsin researchers found that fall dandelion control is best prior to next year’s corn crop. Their results are available as a slide presentation. Kevin Bradley at the University of Missouri has a video, less than five minutes, discussing the importance of application timing for horseweed control.

Ohio State and Purdue University have a great fact sheet on horseweed management. Also, the TakeAction website has a fact sheet with spring and fall herbicide recommendations.

Common lambsquarters can be difficult to manage because of an early and sustained emergence period, long seed persistence, and competitive ability. A 50 percent reduction of seed in the soil seedbank requires about 12 years and 78 years for a 99 percent reduction. Management goals should include: starting with a clean field, using a pre-emergence residual herbicide, scouting, and applying a post-emergence herbicide if necessary. For specific management recommendations, please consult the TakeAction fact sheet.

Crop Diagnostic Training Center Summer Workshops


University of Wisconsin’s Crop Diagnostic Training Center will be hosting two hands-on workshops this summer: Diagnostic Troubleshooting July 30 and Crop and Pest Management on August 13. For more information, please download the workshop flyer. Registration is available online.

Herbicide injury during and after emergence in soybean

Pre-emergence and post-emergence herbicides can injure soybean plants. This year, soybean injury symptoms following pre-emergence applications have been relatively slight for May 13 and May 19 planting dates. In the herbicide evaluation program, soybean injury symptoms that we typically observe are stunting, drawstring (puckering), chlorosis (yellowing), and necrosis. However, it is important to keep in mind that injury symptoms may be due to weather, soil conditions, or disease.

To diagnose an injury problem at or during emergence, first check the root system of the plant. Seedling root growth inhibitors, ALS inhibitors, and growth regulators change the root architecture in different ways. Next step, if the roots appear normal, take a look at the leaves for yellowing (chlorosis), bleaching, or drawstring (puckering). To help with diagnosis, University of Wisconsin Extension has a two-page guide, available for download and an online diagnostic tool.

After emergence, PPO inhibitor injury tends to appear on the leaves that receive the application but younger leaves will not show any injury symptoms. Typical damage includes yellowing (chlorosis) and browning of the leaf surface in spots (necrosis) (Fig. 1A). Researchers at Purdue University have a five minute video discussing PPO inhibitor and fluopyram (ILeVO) seed treatment injury.

It is important to remember that despite the damage, in most cases yield is not affected. Also, PPO inhibitors add another site of action to your resistance management plan and effectively control a variety of broadleaf weed species. Growth regulators cause leaf cupping or epinasty (downward growth habit) (Fig. 1B). The leaves of soybean plants with ALS inhibitor injury show chlorosis and distinctive reddish leaf veins (Fig. 1C).


Injury symptoms typically appear seven to fourteen days after application and will gradually decrease through the season when the plants resume normal growth. Phytotoxicity data are available in the WCWS research reports. Each trial contains a summary section that will mention if any phytotoxicity symptons were observed. If the injury differed by treatment and exceeded five percent then a bar graph is included. Crop injury resulting from an application made according to the label instructions usually does not cause a reduction in yield. “Pest Management in Wisconsin Field Crops” shows the relative risk of soybean injury from different herbicides on pages 143 and 144, available at Cooperative Extension’s Learning Store.

Herbicide resistance management for giant and common ragweed

Weed scientists across the Midwest and Midsouth have identified eleven species of weeds that are of most concern for herbicide resistance because of their ability to compete with crops and to develop resistance to different herbicide sites of action (SOA). They are common waterhemp, Palmer amaranth, horseweed (marestail), giant ragweed, common ragweed, common lambsquarters, kochia, Italian ryegrass, barnyard grass, johnsongrass, and giant foxtail. All of the species have shown resistance to between two and nine herbicide sites of action.

Herbicides are typically classified in two ways by (1) mode of action or (2) site of action. The mode of action is generally how the herbicide inhibits plant growth and development. The site of action is specifically which biochemical pathway in the plant that the herbicide disrupts. Each site of action is associated with a number and the number is located on the first page of most herbicide labels. The TakeAction herbicide classification chart clearly displays the herbicides and premixes by sites of action. A successful resistance management plan will include multiple effective sites of action applied at the full rate to weeds less than four inches in height.

In 2011 and 2013, University of Wisconsin weed scientists identified two separate giant ragweed, Ambrosia trifida, populations resistant to either glyphosate (SOA 9) or ALS inhibitors (SOA 2). In 2013, a population of common ragweed, Ambrosia artemisiifolia, was confirmed resistant to ALS inhibitors. Not sure what a glyphosate resistant giant ragweed plant looks like, check out a time lapse video from Purdue University comparing susceptible and resistant biotypes of giant ragweed, two minutes fifteen seconds in length.

At this point in the season, the most important task for herbicide resistance management is to scout fields several times before and after an herbicide application. Scouting after pre-emergence applications, prior to a post-emergence timing, can ensure that the post application is on target for weed species and size. It is important to pay attention to any weed species that is becoming more abundant across the field and then to double-check that your planned herbicide program is effective at controlling that species. Also, factoring in current weed size with weather forecasts is important for scheduling that next application. After applying herbicides, the next task is to follow-up with scouting at 14 days after the post application to verify adequate control. For more information about effective scouting, please visit the TakeAction website.

Both ragweed species reduce crop yields because early season germination and growth interferes with crop establishment prior to canopy closure. Giant ragweed can be difficult to control with a one-pass herbicide program because seedlings can emerge from deeper soil depths where a pre-emergence herbicide may not reach and some populations tend to emerge over an extended period. For herbicide resistance management recommendations, please consult the TakeAction fact sheet. Giant ragweed tends to be more of a problem in conventional tillage systems whereas common ragweed infestations are common in reduced and no-till systems. Management recommendations for common ragweed infestations in corn and soybean are available on the WCWS website.

Post-emergence herbicides for corn and soybean

At this time of the year, integrated weed management programs focus on scouting and diversifying management practices including non-chemical methods and herbicide sites-of-action. For more information, please visit the United Soybean Board’s TakeAction website for field management guidelines and to learn more about herbicide sites of action. Michigan State University’s Weed Science website has detailed web pages on common weeds in annual crops with biological information and management recommendations. After each field season, the Herbicide Evaluation Program here at the university publishes efficacy data in a research report. Summary ratings for many weed species are located in “Pest Management in Wisconsin Field Crops” available as a free pdf or in print at Cooperative Extension’s Learning Store.

Pigweed Identification

Weed identification at the seedling and immature stages can be difficult but is often necessary because scouting should occur before weeds reach 4 inches in height. At emergence before a full set of true leaves appear, pigweeds can be confused with other weed species such as wild buckwheat, eastern black nightshade, and ladysthumb. In addition, the pigweeds: Palmer amaranth, waterhemp, redroot pigweed, Powell amaranth, and smooth pigweed, are not easily separated by species at the immature stages. The first step is to look at the leaves and stems because Palmer amaranth and waterhemp do not have any hairs compared to Powell, redroot, and smooth pigweed, which do have hairs but they may not be obvious at the immature stage. If the plant looks like it may be Palmer amaranth or waterhemp, then the next step is to look at the leaf shape and petiole. Palmer amaranth has a more rounded leaf shape and a petiole that is longer than the leaf itself.

For a list of resources available by species, guides for the most common weed seedlings, and links to the WeedID smartphone apps, take a look at the Weed Info page. There are several Extension resources available to help with pigweed identification including:

  • “Palmer amaranth seedling identification,” Purdue University, 7.5 minute video

The videos provide a nice introduction to identification, particularly, if using the printed guides. Pigweeds present a tough set of management challenges, for instance, very high growth rates, extended emergence over most of the growing season, and high seed production. The United Soybean Board’s TakeAction website has publications on the management of Palmer amaranth and waterhemp. Populations of Palmer amaranth and waterhemp have been confirmed resistant to glyphosate in Wisconsin. To read more about glyphosate-resistant pigweeds in Wisconsin, please consult these fact sheets: Palmer amaranth resistance and waterhemp resistance.

Herbicide Injury Diagnosis for Corn Seedlings at Emergence

Depending upon the herbicide, injury can occur after a pre-emergence application when corn is germinating in cool, wet soils. This year, if corn was planted in mid-April then you may observe some injury. However, it is important to remember that other environmental factors can mimic herbicide injury symptoms such as corn emerging in crusted or compacted soil. For this spring, WCWS has a re-designed online diagnostic tool, available at http://wcws.cals.wisc.edu/herbicide-injury-diagnostic-tool or from the main page, go to ‘Resources’ and then to ‘Tools’. The diagnostic tool asks three basic questions 1) When do injury symptoms appear? 2) Are both broadleaves and grasses affected or just one group? and 3) What are the symptoms and where do they occur? The original web-based tool was developed by Tim Trower and Chris Boerboom to accompany a handy two-page guide. The following changes were made to the new version:

  1. Each page shows your previous answers.
  2. A ‘Start over’ button is located at the bottom of each page.
  3. For each herbicide mode-of-action, an herbicide chart from the TakeAction Herbicide Classification poster or on the WCWS website under ‘Resources’, ‘Documents’(Fig. 1).
  4. Simplified guides to symptoms that mimic herbicide injury during and after emergence are included on each mode-of-action page (Fig. 2).
  5. Photo galleries for both corn and soybean injury symptoms are located on the same page (Fig. 3).

Figure 1. Herbicide site-of-action groups, chemical  families, active ingredients, and product examples for the seedling shoot growth inhibitor mode-of-action.  Specific sections of the larger TakeAction chart are on each mode-of-action page.

Figure 1. Herbicide site-of-action groups, chemical families, active ingredients, and product examples for the seedling shoot growth inhibitor mode-of-action. Specific sections of the larger TakeAction chart are on each mode-of-action page.

Figure 2. Mimics of herbicide injury to corn during or at emergence.

Figure 2. Mimics of herbicide injury to corn during or at emergence.

Figure 3. Example of a photo gallery for corn and soybean herbicide injury symptoms.

Figure 3. Example of a photo gallery for corn and soybean herbicide injury symptoms.

For pre-emergence applications in corn, the seedling shoot growth inhibitors, particularly the chloroacetamides, may injure seedlings when soils are cool and wet. Injury will not always be apparent aboveground. For example, corn plants with seedling root growth inhibitor damage will display clubbed root tips and grasses will be more affected than broadleaves. To get an idea of injury risk, check out the herbicide tables in “Pest Management in Wisconsin Field Crops” available in pdf and print formats at Cooperative Extension’s Learning Store.

When Does a Weed Become “Super”?

The term “superweed” has come to mean any weed that has become difficult to manage even if there has only been one or two management techniques employed. However, according to the Weed Science Society of America, this isn’t what a true superweed is. Just because a weed has developed resistance to one management technique (such as how dandelions begin to produce seed differently in a regularly mowed lawn), does not make it “super”. A true super weed has the ability to develop resistance to all kinds of management techniques, including multiple herbicides and mechanical, biological, or cultural management techniques. This new definition of superweed is hoped to clear up any confusion and encourage proper weed management techniques. More information about what a superweed really is can be found here.