Giant Ragweed with Suspected Glyphosate Resistance

Chris Boerboom, Extension Weed Scientist

Biotypes of nine weed species have been documented as glyphosate resistance in the U.S.  However, glyphosate-resistant weeds have not been OFFICIALLY confirmed in Wisconsin. This article reports on a giant ragweed population that is being investigated for potential glyphosate-resistance. Note that glyphosate-resistance is not new in giant ragweed as six other states in the Midwest have already reported giant ragweed with glyphosate resistance. What is important in this case is 1) this serves as a reminder that the threat of glyphosate resistance is real in Wisconsin (it’s not always going to be someone else’s problem); 2) diverse weed management programs are warranted to reduce risk of resistance; and 3) when poor weed control occurs, resistance should be considered along with other causes of performance problems.

This field was reported in 2008 when a small number of giant ragweed plants survived two applications of glyphosate in soybean. The first application contained 1.1 lb ae/a glyphosate and the second contained 1.15 lb ae/a. (1.1 lb/a = 1.5 qt/a of generic or 33 oz/a of PowerMax).  Several plants that survived were between the soybean rows suggesting that poor spray interception was not a problem. Some plants regrew from lower nodes on the stem after being sprayed whereas others were stunted and then the main shoot continued to grow. These symptoms showed the plants were sprayed and did not emerge after the application. Seeds were collected from several of the plants for greenhouse testing. 

The seeds from each plant were kept separate, were cold treated to break their dormancy, and were planted in the greenhouse. Many seedlings grew and were sprayed with 1.5 lb ae/a glyphosate. All of the plants were damaged by the glyphosate, but some survived, especially seedlings from a couple parent plants. Unfortunately, the giant ragweed seed collected from Arlington that was to be used as a sensitive check did not grow. Therefore, I did not have sensitive plants to determine how well the glyphosate worked in the greenhouse. It seemed that the seedlings from some parent plants were more tolerant to the glyphosate than others. This test did not confirm resistance.

Because this test was not conclusive, a field test was conducted this spring in the field immediately adjacent to the 2008 field. Glyphosate was sprayed at 0.75, 1.5, 2.25, and 3.0 lb ae/a (equivalent to 1 qt/a to 1 gal/a of a generic) in 10 by 30 ft plots with four replications. Most giant ragweed were 6 to 8 inches tall when sprayed. Two weeks after spraying, all plants were dead except for some plants in a 50 by 30 ft area. The standard 0.75 lb/a glyphosate rate killed all the ragweed except for plants in this area (Figure 1). However, many plants survived 0.75 lb/a in this area (Figure 2) and a few plants survived rates up to 3.0 lb/a (Figure 3) at 2 weeks after spraying. We are monitoring the fate of these plants, but several appear to be growing.

Figure 1. Giant ragweed was controlled at 2 weeks after being sprayed with 0.75 lb ae/a glyphosate in most areas of the trial.

At this time, I can only conclude that these giant ragweed are suspected of being glyphosate resistant because I did not officially compare them to a sensitive giant ragweed population. I know that they were sprayed (no spray skips and they didn’t emerge after spraying); weather conditions were favorable, weed size was not excessive, and the glyphosate was effective on a majority of the ragweed plants. 

One point that these photos illustrate with glyphosate resistant plants (if confirmed) is that glyphosate resistant weeds have a lower level of resistance than most other cases of resistance. For instance, triazine or ALS resistant weeds are almost immune to injury from the herbicide because of the high level of resistance, but glyphosate resistant weeds are often damaged and then regrow. This creates the problem that glyphosate resistance is harder to distinguish from other causes of poor glyphosate performance. Causes of poor control or escapes include spray skips, low rates, too large of weeds, weeds under the canopy, weeds emerged after spray application, rainfall too soon after application, dusty leaves, spraying early or late in the day, etc. You can review factors that affect glyphosate performance in at the Glyphosate, Weeds and Crops website http://www.glyphosateweedscrops.org/ Look for the “Understanding Glyphosate to Increase Performance” bulletin (click Available to open the bulletin list). If resistance is suspected, you will also want to search for evidence of resistance. Some standard features of herbicide resistant weeds are: 

• A single weed species escaped controlled, but this weed should have been controlled by the herbicide. A few cases exist where more than one herbicide resistant weed species exists in a field, but this is unlikely when resistance is first discovered. Rule out weeds that have natural tolerance to glyphosate such as yellow nutsedge or field horsetail.

• Dead weeds mixed with live weeds which indicate the herbicide was applied to the escaping weeds.

• The field has a history of repeated use of the same herbicide mode of action (check spray records for herbicide history).

• Poor performance was noted in the same area in previous years or resistance exists in the local area.

• No known application errors, weather conditions, or large weed size to explain the lack  of control. 

If you strongly suspect glyphosate resistance and are seeking management options OR if you are seeking management options to diversify your program, please see the bulletins on giant ragweed, waterhemp, horseweed, and common lambsquarters on the Glyphosate, Weeds and Crops website http://www.glyphosateweedscrops.org/. The management recommendations are at the end of each bulletin.   Click Available to open the bulletin list