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.