One of these days you will admit to yourself, and maybe some farming partners, that you have a weed resistance problem.  You have been “resisting” that admission for some time, but are wondering if the spray you are applying/will soon be applying will really control that patch of waterhemp or giant ragweed marestail. 

It has been eating on your psyche that you can’t control those weeds, but maybe it is time to move beyond the psychological battle and arm yourself with a battle strategy. After all, you are engaged in a war on weeds.

You may have been trying your best for the past few years to control hard to control weeds, but have not made much headway.  Iowa State University weed specialist Bob Hartzler rhetorically asks, “Is your herbicide program fighting or creating resistance problems?” 

Hartzler’s recent factsheet says there have been many resources from both public and private researchers that will provide great assistance, some of which are overlooked by producers who opt for traditional methods or misuse highly promoted new products. He stresses the need to know how herbicides work, in general terms, by attacking different proteins in the weed, but one does not have to become an expert on that. 

All you have to know is an industry code number, and ensure that you are using a variety of different numbered chemicals to spread out the control options.  At that point you have moved from knowing the “site of action” of an herbicide to ensuring that you are using herbicides with different “sites of action.”  He says, “Utilizing herbicide programs that include several sites of action that are effective on troublesome weeds (e.g. waterhemp, giant ragweed) is the key to managing resistance issues with herbicides.”

Hartzler says if you are having a problem with a specific weed, such as waterhemp, then you must use a variety of herbicides with sufficient strength, or you will be adding to your resistance problems.  Using an example from an Iowa cooperative, he said there were herbicides used that represented six different “sites of action” which would ordinarily be a good strategy. However, he was quite concerned by the economical treatment being offered, which may not control weeds, but gives them additional strength.  He said each different herbicide may do a good job on the weeds it was designed to control, but none would fight waterhemp in the strengths that were being used.

Specifically, he said the Group 2 herbicide being used would not be effective, because almost all waterhemp is resistant to Group 2. 

The Group 27 herbicide is effective on waterhemp, but the rate being used will not likely provide full-season waterhemp control.  The Group 5 herbicide is at too low of a rate to be effective. 

The Group 4 herbicide is designed to be a pre-emergent herbicide, but the waterhemp are late sleepers and would not be close to germination at the time of a pre-emergent application. The Group 9 herbicide being used should be effective on susceptible waterhemp.  The Group 4 herbicide was in a pre-mix with a Group 19 herbicide and the latter is only designed to enhance the activity of the former and will not control waterhemp.  So he says a broad-spectrum tank mix being marketed to many Iowa farmers will have little impact on any waterhemp problem they may have.

Hartzler says the potential for any control depends on the rate used, the timing of the application, the presence of resistance weeds, and several other factors, and it is difficult to attribute success to any specific herbicide. While he said the agronomist developing the herbicide prescription may have had the best intentions, one of the two herbicides that could control waterhemp was being applied in a strength that was one-quarter of the recommended rate.

Regarding sites of action needed to control problem weeds, Hartzler says there is no correct answer but two are better than one and three are better than two.  And he adds that the same ones should not be used year after year.

Summary:
Managing weeds that have resistance to many herbicides is not easy, and many farmers are fighting the battle annually.  The key is creating a treatment program that will address a large number of “sites of action” which attach several protein functions within the weed at the same time.  Resistant weeds, such as waterhemp, can be controlled with a variety of herbicides at different times and methods of application, however, they should be at a strength that will be effective, and not used two years in a row.

Source: FarmGate blog