ISSUE 13   July 27, 2006

HERBICIDE RESISTANT WEED REVIEW

Although Agri-professionals and growers get tired of information on herbicide resistant weeds, I thought two recent events justify this article. Weed scientists in WI tested and confirmed an ALS resistant common ragweed biotype for a grower in northwest Wisconsin. This common ragweed was specifically tested for resistance to FirstRate. This is hardly remarkable since ALS-resistant common and giant ragweed was first reported in 1998 in several states. The scientists remark that this well-informed grower was not aware that ALS resistance in common or giant ragweed has happened and is increasing in may states. So, just as an update, Im reminding people of the section in the weed guide that summarizes weed resistance in ND - pages 106-107.

If you suspect herbicide-resistant weeds, use this checklist to determine if resistance is a potential.

There were no known application errors and the correct herbicide rate was added to the spray tank.

The environment was favorable for herbicide performance (unstressed plants).

Only one weed species escaped control (weed escapes may be localized in patches).

The suspected resistant weed is mixed in with other weeds that were controlled.

The herbicide typically controls the weed species that escaped control.

The herbicide (or herbicides with the same mode of action) has been used frequently in the past or other cases of resistance have been confirmed in the area.

The second event was an agri-professional who was expressing concern about a grower who has been growing continuous Roundup Ready soybeans for about 7 years. The concern was that this type of management may lead to glyphosate resistant weeds.

I agree with this concern. I also noticed a recent farm magazine article about a Nebraska grower who has been growing continuous soybeans for 19 years. In recent years, these soybeans have been Roundup Ready and typically receive two glyphosate applications per year. The comment in the article from the grower is that to date, "he has seen no weed resistance to Roundup."

I can only make two comments on this type of management in regards to herbicide resistance.

1) If you wait until resistance develops, its too late.

In regards to risk management, its like saying "Air bags are expensive. If I have an accident, I might think about getting air bags in my next car." Its a little late at that point.

2) The glyphosate-resistant common ragweed and waterhemp that developed on Missouri farms were in continuous Roundup Ready soybeans. We cannot predict the exact risk of developing glyphosate-resistant weeds in any field, but we sure know that growing continuous Roundup Ready crops and relying heavily on glyphosate for weed control is the worst case scenario.

Actually, a glyphosate-resistant weed is not the biggest concern. Weeds which are both glyphosate resistant and resistant to one or more other herbicides (this is called multiple resistance) are the biggest concern. Take waterhemp as an example and answer this question (courtesy of Bob Hartzler, ISU).

To which soybean herbicide has waterhemp not developed resistance?

1. Blazer
2. Pursuit
3. Glyphosate
4. Reflex
5. Sencor

Ok, this was a trick question because waterhemp has developed resistance to all of these herbicides at different places in the Midwest. Some fields already have waterhemp that are resistant to two or three herbicide modes of action. If a farm gets waterhemp that is resistant to two or more of these herbicides, does it affect how easily waterhemp is controlled or how much it will cost? That is the question that we need to consider.

To consider which weeds might have the greatest risk for herbicide resistance, we can look back and see which weeds have already shown the ability to become resistant. In the table below, I have summarized the number of modes of action that each weed has become resistant for 12 Midwestern states.

For example, Illinois has reported waterhemp populations with resistance up to three different modes of action (ALS, PPO, and photosynthetic inhibitors). In addition to these 13 weeds, there are seven other herbicide resistant weeds that are only reported in a single state. Some states have not reported all of the resistance that they actually have so this list under-reports the number of resistance species.

This summary might serve as a herbicide resistant weed watch list. The seven broadleaf weeds at the top of the list have demonstrated the ability to become resistant to herbicides. This includes the potential for glyphosate resistance. Of these seven weeds, 3 have confirmed glyphosate resistance (horseweed, common ragweed, and waterhemp) and research is underway to determine the potential for glyphosate resistance to two others (lambsquarters and giant ragweed). Whether its glyphosate resistance or resistance to another herbicide mode of action, these are definitely weeds to watch and plan appropriate weed management programs.

Herbicide Resistant Weed Summary (number of herbicide modes of action within each state)

ND

SD

NE

MN

IA

MO

WI

IL

MI

IN

OH

PE

Total states

Lambsquarters

1

1

1

1

2

1

2

1

8

Pigweed species

1

   

1

   

1

1

2

1

1

2

8

Kochia

3

1

 

1

1

 

2

2

 

2

   

7

Waterhemp

   

1

 

2

2

1

3

1

 

1

 

7

Common ragweed

     

1

 

1

1

1

2

1

1

 

7

Giant ragweed

1

1

1

1

1

5

Horseweed

         

1

   

2

1

2

1

5

Foxtail species

1

   

2

2

 

1

       

1

5

Cocklebur

     

1

1

1

       

1

 

4

Sunflower

 

1

   

1

1

           

3

E.B. nightshade

1

         

1

1

       

3

Wild oat

2

   

1

               

2

Smartweed

       

1

       

1

   

2

 

WEED RESISTANCE TO GLYPHOSATE

University of Missouri weed scientists have confirmed waterhemp resistant to glyphosate herbicide. Herbicide tests in a western Missouri soybean field have confirmed that tall waterhemp is the sixth glyphosate-resistant weed in the U.S. and the ninth such weed in the world.

Up to eight times the labeled rate of glyphosate was used. The waterhemp was injured initially but recovered. Plants were light green to yellow in the center, a symptom of glyphosate treatment. But the bulk of each plant's leaves are green and healthy. Their confirmation this week placed tall waterhemp on the international herbicide-resistant weeds Web site, www.weedscience.org. The site, dedicated to information on weed resistance, lists 183 weed species that have been proven to be resistant to one herbicide or another.

The scientists are also working on another field, in central Missouri, containing glyphosate-resistant common ragweed for several seasons. That field has been in a soybean-soybean-wheat rotation with almost sole use of glyphosate herbicide for weed control.

Waterhemp plants are either male or female, which means females rely on pollen shed from surrounding male plants.

If the resistant trait is carried in the pollen, which we are fairly confident it is, then you have pollen traveling to fields all around the resistant plants. Each female waterhemp plant produces hundreds of thousands of seeds, ensuring a ready supply of plants for the following season.

They found good news in their field plots. The glyphosate-resistant waterhemp is killed by a number of popular pre-emergence soybean and corn herbicides.

The eight other confirmed glyphosate-resistant weeds throughout the world include buckhorn plantain, common ragweed, goosegrass, hairy fleabane, horseweed or marestail, Italian ryegrass, palmer amaranth and rigid ryegrass.

Richard Zollinger
NDSU Extension Weed Specialist
r.zollinger@ndsu.edu


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