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A Word of Caution About Two New Weed Control Technologies

Posted by Blythe Howell | March 25, 2019

Effective, new herbicides are not as common as they once were. When we are lucky enough to have a new one come to the wheat industry, it is an exciting time for those of us involved in the management of troublesome weeds. It is particularly exciting when these new products can help us manage weeds like Italian ryegrass and downy brome, two weed species that have developed widespread biotypes resistant to many of our commonly used herbicides.

Zidua and Anthem Flex were introduced into the wheat market in 2015. Both products contain pyroxasulfone, which is an inhibitor of long-chain fatty acid synthesis (Group 15), that provides effective preemergence control of Italian ryegrass.  You may want to visit the WSU Weed Control Reports from 2014-2018 to see how these products worked for Italian ryegrass control near Pullman.



No other weed in the Pacific Northwest has more herbicide-resistant biotypes than Italian ryegrass. It is an obligate outcrossing species with a great deal of genetic variation that makes selection for herbicide resistance more likely than in many other weed species in the region. Zidua and Anthem Flex have provided wheat growers in the high rainfall zone, where Italian ryegrass is most common, with an effective means of controlling this troublesome weed. However, research conducted in Australia and published in 2012, before pyroxasulfone was labeled for use in wheat, demonstrated that the use of sublethal doses of pyroxasulfone for three consecutive seasons selected for rigid ryegrass (closely related to Italian ryegrass) plants that were resistant to a 3x rate of pyroxasulfone. This should serve as a warning that frequent use of Zidua or Anthem Flex will likely result in rapid selection of Italian ryegrass biotypes resistant to this herbicide.

This is especially concerning now that herbicides containing pyroxasulfone are being labeled for use in pulse crops in addition to wheat. Authority Supreme, which contains pyroxasulfone plus sulfentrazone (the active ingredient in Spartan 4F), is labeled for use in dry pea and chickpea. Anthem Flex may soon be labeled for use in dry pea, chickpea, and lentil. Growers will need to decide which crop to use these products 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.

Another new weed management technology coming to wheat growers in the PNW is the CoAXium wheat production system. This production system uses a non-GMO herbicide tolerance trait in CoAXium wheat varieties and Aggressor herbicide (quizalifop is the active ingredient in Aggressor herbicide) to control troublesome grassy weeds in wheat. The CoAXium wheat production system offers growers with an effective means of controlling feral rye, downy brome, and jointed goatgrass in wheat.



Quizalafop is an ACCase inhibitor (Group 1) and it is the active ingredient in Assure II herbicide, which has been used in pulse production in the PNW for many years. Consequently, many of the grassy weeds in pulse crops have been exposed to this active ingredient already and some resistant biotypes already exist, for example, in Italian ryegrass. Dr. Dan Ball, retired Oregon State University weed scientist, identified a downy brome biotype resistant to quizalofop in 2007. Herbicide resistance to ACCase inhibiting herbicides is common. All of this suggests that if the CoAXium wheat production system is not carefully managed, herbicide resistant biotypes are likely to be selected very quickly, rendering this promising system ineffective.
Zidua, Anthem Flex, and the CoAXium wheat production system are good weed control technologies, but if they are overused they will not last long and there are unlikely to be any new technologies released soon that can readily replace them for controlling some of our most troublesome grassy weeds in wheat. Use them wisely!

Drew Lyon.
For questions or comments, contact Dr. Drew Lyon by email at or by phone 509-335-2961.

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