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I Can’t Say This Enough!

Posted by Drew Lyon | January 13, 2022

When it comes to delaying the development of herbicide resistance in weeds, herbicide mixing and rotation are critical. In an earlier blog post titled, “Two Are Better Than One”, I spoke about the strong benefits of mixing herbicide mechanisms of action. I noted that one of the 12 Best Management Practices (BMPs) for reducing the risks of herbicide resistance is:

Use multiple, effective MOAs (Mechanisms of Action) against the most troublesome weeds and those prone to herbicide resistance.

I provided examples from Australia and Eastern Washington where herbicide mixtures provided superior control of troublesome weeds compared with the use of a single mechanism of action. Some new information out of Australia reinforces the message and may serve as a warning for people who have come to rely on Zidua, Anthem Flex, or Authority Supreme for the control of Italian ryegrass in wheat or pulse crops (see labels for details on use).

In a large herbicide resistance screening project, Dr. Roberto Busi with the University of Western Australia, collected 579 rigid ryegrass (closely related to our Italian ryegrass) samples from 298 farms across four states of Australia. He screened two million ryegrass seeds in over 15,000 individual resistance tests using 21 herbicides – 12 standalones and 9 two-way mixtures. His take home message is “Herbicide mixtures rock!” The frequency of resistance to stand-alone pre-emergence herbicides ranged from 10 to 34% while the frequency of resistance to herbicide mixtures ranged from 0 to 6%. Despite the increased cost of herbicide mixtures, I think they should and will soon become necessary for successful weed control as herbicide-resistant weeds spread across the wheat production regions of Eastern Washington.

Another worrisome message from this work in Australia was the high level of resistance of rigid ryegrass to pyroxasulfone (sold as Sakura herbicide in Australia and as Zidua, Anthem Flex, and Authority Supreme in the US). They reported resistance rates to Sakura herbicide of 46% in South Australia, 40% in Victoria, and 9% in Western Australia. I cautioned people about the risk posed by overreliance on this new effective active ingredient in a Timely Topic a few years ago titled, “A Word of Caution About Two New Weed Control Technologies”. I stated that, “growers will need to decide which crop to use these products (Zidua, Anthem Flex, and Authority Supreme) in and avoid using them in every phase of their crop rotation or face the very real possibility of losing these products as effective controls for Italian ryegrass.” Over the last year or two, I have received reports that these products are not as effective as they used to be. That is the inevitable result of overuse.

Fortunately, as the Australian study showed, mixing Sakura with trifluralin (Treflan HFP and other trade names) or triallate (Avadex  MicroActiv or Far-Go) reduced resistance rates to under 10% in South Australia and Victoria. We have seen similar results in some of our field trials. If you have not been using herbicide mixtures in your operations, I believe it is time to start before we burn through what we have left for effective herbicides in wheat production systems.

Have you used herbicide mixtures in your operations or those that you service? If so, what did you use and how did it work?

2 thoughts on "I Can’t Say This Enough!"

  1. Steve Riggers says:


    Thanks for your perspective on this topic. It’s helpful to have the Australian data with respect to HR especially in the case of Italian rye grass. Too many farmers on the Prairie are seeing pyroxasulfone as the Lone Ranger riding in to save the day! What really concerns me is that because of last years drought, many of these problem weeds did not appear at levels that got farmer’s attention. I’m guessing that many decided not to apply post plant herbicides to the crop this fall, ignoring the weed seed that did not germinate during the 2021 growing season.

    1. Drew Lyon says:

      Steve, it is not just the farmers on the Prairie that are relying solely on pyroxasulfone for Italian ryegrass control in wheat. For the last few years, pyroxasulfone has been the most effective herbicide for Italian ryegrass control in wheat so it has been the “go to” chemistry. Unfortunately, the use rate in wheat has always been a bit marginal because of a concern for crop injury. Consequently, selection of Italian ryegrass biotypes just somewhat more resistant to pyroxasulfone is all that is needed to reduce the efficacy of pyroxasulfone. I think we are well on our way to selecting those somewhat more tolerant biotypes. That is why I think it is imperative that growers stop using pyroxasulfone as a standalone product for Italian ryegrass control. Once we lose pyroxasulfone as an effective herbicide for Italian ryegrass control, we will have very little left in our arsenal.

      To your second point about many growers not applying post-plant herbicides to their winter wheat last fall because of a lack of weed emergence in 2021, the good news is that even though preemergence treatments work best, Zidua and Anthem Flex can be applied to winter wheat postemergence in the late winter or early spring (see labels for details). I would suggest adding some metribuzin to try to pick up some of the already emerged Italian ryegrass that pyroxasulfone applied postemergence will not control. If your biotype of Italian ryegrass is still susceptible to one of the postemergence grass products, such as PowerFlex HL, it would be nice to include such a treatment in the program as well.

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