It is the use of multiple, diverse modes of action that controls or prevents herbicide-resistant weeds—not the volume of herbicide or herbicides with similar modes of action. The majority of single modes of action herbicides need to be tank-mixed with other herbicides to ensure activity and prevent resistance.

As herbicide-resistant weeds continue to spread, especially into the upper Midwest, it is clear that amaranthus species (pigweed family) are really tough to control and are causing more and more problems.

"The major problem throughout the country is mainly going to be amaranthus species like common waterhemp, tall waterhemp and Palmer amaranth—those small-seeded broadleaves,” said Carroll Moseley, Ph.D., Syngenta herbicide brand manager.

“The take away for me is the more active ingredients and the more different modes of action that we can throw at weeds to decrease the selection pressure as compared to one straight active ingredient, the better off we will be as an industry,” he said.

ISU Scientist Calls it Redundancy

Mike Owen, Ph.D., Iowa State University Extension weed scientist, talks about redundancy in using two different herbicides against specific weeds such as waterhemp. He says the ultimate or theoretical way to keep resistance from occurring is through the use of two herbicides with different modes of action that have the same level of weed control—efficacy and length of control.

From a hypothetical perspective, if two herbicides have different levels of control against a weed like waterhemp, then resistance has a good chance to occur as one of the herbicides exerts more selection pressure than the other.

“If you have two herbicides that have activity on waterhemp and one of them has extremely good activity, let’s say 98 percent, and the other herbicide has pretty good activity on waterhemp, let’s say 90 percent, that means there is a selective differential. The herbicide with 98 percent control is going to place more selection pressure on the waterhemp than the herbicide with 90 percent control. In theory, if you keep applying them over and over, the herbicide that has 98 percent control is going to select for weeds that don’t respond to that herbicide,” Owen said.

Atrazine is a Natural Answer

So, a third herbicide and mode of action may be a natural answer for real-world situations. For field-corn production, that third mode of action has consistently been atrazine.

"The reality is the more diversity that you can include in a weed management program, the better off you are. And then in regard to broadleaf weed management in corn, the inclusion of atrazine is an excellent way to provide diversity of mechanisms of action, that in itself will help mitigate the evolution of resistance to other herbicides,” Owen explained.

As for resistance developing to atrazine because it is used with nearly every other corn herbicide on the market, Owen suggests it hasn’t been a major concern other than in situations of continuous corn and continuous atrazine use. The main reason that every other year use of atrazine in a corn/soybean rotation hasn’t spurred weed resistance to atrazine is because the compound is not used as the sole weed control product, like glyphosate has been used for more than a dozen years.

With resistance management in mind, Mosely said, Syngenta prepackages combinations of active ingredients with atrazine including S-metolachlor and mesotrione. This provides three different actives with three different modes of action on amaranthus species.  

“In general, resistance to the triazine herbicides results in a very significant fitness penalty in those resistant weeds, meaning that they are not strong or robust. They are not as competitive with the corn as the same type of weed that isn’t resistant. So, if you take the atrazine out of the system, there are a lot of natural ecological forces that will drive those resistant populations down,” Owen noted in explaining the common situation with atrazine-resistant weeds. He said there are some weed exceptions to this situation.

Using waterhemp as the example again, Owen said, “If you stop using glyphosate, the waterhemp that has evolved resistance to the glyphosate are still going to remain in the system because of dormancy and so forth. That resistance trait is dominant, and it does not cause the glyphosate-resistant waterhemp to be less fit than the non-resistant waterhemp. So, it stays at pretty high numbers in the population once established whether or not glyphosate is used. On the other hand, if you remove atrazine from the system, in most instances the atrazine-resistant waterhemp will be less fit than the atrazine-sensitive waterhemp. The atrazine-sensitive waterhemp will be more competitive and cause the atrazine-resistant waterhemp populations to decline when atrazine isn’t around.”  

Low Use Rate Not a Concern

“Although there are several low-use-rate products in the market, these products generally include an ALS inhibitor and/or an HPPD inhibitor. Atrazine and, in many cases, an acetamide herbicide should be part of the picture if a grower is dealing with amaranthus species and the potential for resistance,” Moseley contends. “If atrazine is part of the mixture, a low-use-rate combination is no longer being used.”

Owen said, “The perception that a low-use-rate product is better from an environmental perspective than a high-use-rate product is kind of silly from the perspective of regardless of whatever herbicide you are using, it takes x number of molecules to get the job done. So, from a biological perspective, high-use-rate and low-use-rate herbicides are doing the same thing.

“We need to look at things from a biological perspective so that we are applying the appropriate amount of herbicide to get the job done, and regardless of high or low use rate, accuracy and precision are incredibly important things to mind as you mix, handle and apply herbicides.”

Moseley noted a Syngenta perspective on weed resistance in corn as he suggested all companies need consistent messages about product use to accomplish weed resistance management. Promoting combination products that contain two active ingredients, when waterhemp or any other weed has shown resistance to both of them, is not always the best recommendation, he said.

“There are millions of acres of waterhemp that are tolerant to ALS-inhibitor herbicides, and we now have some HPPD mode of action resistance identified, too. So, exposing thousands of acres of corn to just those two active ingredients wouldn’t make sense,” Moseley said.

“We have already diminished the value of glyphosate by the mere fact that we have allowed resistance and non-performance to take place in many pigweed species,” Moseley said. “It is critical that we do what we can to protect the other active ingredients because we don’t see many new modes of action coming into corn or soybeans for weed control in the next few years.”