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

UWEX-Logo-BLK-small

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 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.

To-Do List: Late-Season Scouting

A critical component of herbicide resistance management and integrated pest management is scouting for pests. In this case, it is time to start thinking about scouting for late-season weed escapes. Late-season weed scouting will help to focus your weed management plans for next year- what worked, did not work, and which weed species may be a problem during the next growing season. If you suspect that you have found an herbicide-resistant weed, please contact your local county Extension agent or Vince Davis at vmdavis@wisc.edu

To learn more about late-season weed scouting in soybean, please watch Vince’s videos:


For more videos, please visit our video page, http://wcws.cals.wisc.edu/videos/

Late-season weed escape survey in Wisconsin identifies a second county with a glyphosate-resistant horseweed population

Ross Recker (Graduate Research Assistant), John Buol (Undergraduate Research Assistant), and Vince Davis (Assistant Professor) Department of Agronomy, UW-Madison

Glyphosate-resistant weeds continue to be a major threat to corn and soybean production across the Nation. A pro-active survey of late-season weed escapes in corn and soybean fields was conducted throughout Wisconsin in 2012 and 2013. One objective of the survey was to identify areas where populations of glyphosate-resistant weeds may exist.

In 2012, a population of horseweed (Conyza Canadensis) collected from Jefferson County, and glyphosate screening in the greenhouse confirmed it was glyphosate-resistant with a 6-fold tolerance compared to a susceptible population. This announcement and more information about horseweed biology and management were previously reported and can be read here.

During the 2013 late-season survey, another putative glyphosate-resistant horseweed population was collected from Columbia County. Again whole-plant dose response experiments were recently conducted in the greenhouse and results confirm this population is also nearly six-fold glyphosate-resistant (Figure 1).


Figure 1. Shoot dry biomass of Columbia County horseweed and susceptible horseweed following treatment with glyphosate at doses up to 3.48 kg ae ha-1 as estimated by a four-parameter log-logistic regression function.

Figure 1. Shoot dry biomass of Columbia County horseweed and susceptible horseweed following treatment with glyphosate at doses up to 3.48 kg ae ha-1 as estimated by a four-parameter log-logistic regression function.

The horseweed seeds were collected from mature plants in a soybean field with a history of no-till, corn/soybean rotation, and glyphosate use. These plants displayed symptomology at the time of collection including shortened internodes (Figure 2) and were scattered randomly at low densities in about a 2 to 3 acre patch. If you have horseweed or other weeds that survive postemergence applications, you should have concern about herbicide resistance. Contact your local county Ag Extension Agent who can help you further evaluate the situation and plan a pro-active resistant management program so you can take action against herbicide resistance.


Figure 2. Horseweed plant late in the 2013 growing season that was not controlled with a previous postemergence glyphosate application in a no-till soybean field in Columbia County, Wisconsin.

Figure 2. Horseweed plant late in the 2013 growing season that was not controlled with a previous postemergence glyphosate application in a no-till soybean field in Columbia County, Wisconsin.

In the toolbox: Invasive Plant Control Database

This slideshow requires JavaScript.

The Midwest Invasive Plant Network has published an online search tool to find information on invasive species control- Invasive Plant Control Database. From this website, go to Resources and then to Tools. The MIPN database works best if you already know the invasive plant species that you have. To help with identification, the Herbarium at the University of Wisconsin-Green Bay has a very nice image catalog of invasive species in Wisconsin, including the counties where they have been found and species descriptions. Another great resource that categorizes the invasive species by habitat is hosted by Wisconsin’s Department of Natural Resources.

Still unsure of the plant species, try University of Wisconsin’s weed identification tool or go to the PlantDOC website to find your local Extension agent and submit a digital image for identification. The Extension Specialist for Invasive Plants at UW-Madison is Mark Renz and his contact information is here.

Resources for Wisconsin Farmers, Part IV

post_plantDOC

Insect, disease, or weed issues? Burning questions? The PlantDOC website can connect you directly to experts in Cooperative Extension.

Before submitting a new question, you can check out the archive. Chances are that someone else has had the same question. Under the “Archives” tab, there will be categories along with a list of the most recent questions.

To submit a question, go to the “Find an Agent” tab, click on your county. This will re-direct you to your county’s Extension website. The staff directory will be in the upper righthand corner. The next step is to contact the agent or educator that seems most appropriate for your question.

If your agent cannot answer the question immediately, the agent can post the question on PlantDOC and a state specialist will answer your question.

Resources for Wisconsin Farmers, Part III

Wisconsin’s Department of Agriculture, Trade, and Consumer Protection produces The Wisconsin Pest Bulletin at weekly or semi-monthly intervals. For email notifications, go here. The bulletins are archived and available as printable pdf documents.

What information can you find in the bulletin?

  • Degree day accumulations for towns located throughout Wisconsin.
  • A pest forecast with scouting guidelines for different pests.
  • Scouting reports for several cropping systems arranged by the following categories: forages and grains, corn, soybeans, fruit, nursery and forest, and vegetables.