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Pest Patrol: Herbicide-resistant wild oats in spring cereals

#PestPatrol with Mike Cowbrough, OMAFRA

A field sample of wild oat screened with three chemical families within the Group 1 mode of action by the Tardif lab. Note there is resistance observed when Puma (a “fop”) was applied, moderate resistance when Achieve (a “dim”) was applied and moderate susceptibility when Axial (a “den”) was applied.

Q. Are there any solutions to control herbicide resistant wild oats in spring cereals?

A. Wild oats have been an undesirable weed in spring cereal crops for many years, and its control has been limited to a handful of herbicides within the same mode of action (specifically, WSSA herbicide Group 1).

Until recently, there have been no other modes of action which can control wild oats in cereals, so the Group 1 herbicides have been used exclusively. Predictably, several populations in Ontario have been identified as resistant to these herbicides.

A project is underway to search for solutions, led by Dr. François Tardif at the University of Guelph, and a preliminary screening trial was conducted in Grey County in 2019 based on queries for how to manage Group 1-resistant wild oats, particularly in barley. There are few options since many of the Group 2 cereal herbicides that control Group 1-resistant wild oats are not labelled for use on barley, due to unacceptable crop injury.

One objective of this preliminary study was to evaluate herbicides that control or suppress wild oats in other crops where barley’s tolerance to those herbicides is unknown. The other objective was to see if these Group 1-resistant wild oat populations could be managed with other Group 1 herbicides.

Confused? Let me explain.

There are three different chemical family groups within the Group 1 mode of action that can be used to control wild oats in barley and wheat: they include the “fops” (e.g. Puma, active ingredient: fenoxaprop), the “dims” (e.g. Achieve, active ingredient: tralkoxydim) and the “dens” (e.g. Axial, active ingredient: pinoxaden). Since each chemical family group is structurally different, it is possible that instead of a weed population being resistant to all three chemical family groups, they may only be resistant to one or two. Work done by the Tardif lab in 2018 demonstrated (Figure 1 at top of page) that some populations in Ontario, although resistant to Puma (a fop), were moderately resistant to Achieve (a dim) and susceptible to Axial (a den). These laboratory observations needed to be evaluated on a field scale, since only a small number of plants within a field are tested, so they may or may not be representative of the entire field.

What did we learn in 2019?

On the positive side, there was one experimental herbicide that seemed to provide partial control of wild oats with very little injury to barley (Figure 2 below). The Western Canada herbicide Assert (Group 2) provided excellent control of wild oats and exhibited excellent crop safety (Figure 3 below). The manufacturer has been contacted regarding the possibility of this product being used in Ontario.

Figure 2: Wild oat control with an experimental soil-applied herbicide in the foreground compared to no herbicide applied in the background. Figure 3: Wild oat control observed in July following an application of Assert (right) compared to an un-sprayed control (left). photo: Supplied

Wild oat response in the field to Group 1 herbicides (Table 1 above) did mirror what was observed in the laboratory study in that wild oats were resistant to Puma, mostly resistant to Achieve and mostly susceptible to Axial. However, switching to Axial would be a short-term management strategy because even though we were able to control close to 80 per cent of the population with Axial (Figures 4 and 5 below), we did observe several plants that survived the application (Figure 6 at bottom) and so it would only be a matter of time until the field population shifted to wild oats that were also resistant to Axial.

Figure 4: Control of wild oats two weeks after an application of Axial. Although many plants were controlled, a significant amount survived. Figure 5: Close-up of a wild oat plant from Figure 4 that was controlled successfully with an application of Axial. photo: Supplied

Dr. Tardif’s research project is on schedule to begin during the 2020 growing season, so if you have had issues with herbicide-resistant wild oats and would like to be involved, drop me a line.

Acknowledgements: I’d like to thank both Kelsey Banks and Deb Campbell for sharing their insight and experience. I would also like to thank Teije of Talking Heads Farms for providing a field site and sharing his observations of what herbicide treatments he liked and didn’t like.

Close-up of a wild oat plant from Figure 4 that was not controlled by the application of Axial. photo: Supplied

Have a question you want answered? Hashtag #PestPatrol on Twitter to @cowbrough or email Mike at [email protected].

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