Recent confirmation of dicamba-resistant kochia [<i>Bassia scoparia</i> (L.) A.J. Scott] in Alberta warrants investigation of resistance to other commonly used synthetic auxin herbicides like fluroxypyr. A randomized-stratified survey of 305 sites in Alberta was conducted in 2017 to determine the status of fluroxypyr-resistant kochia. Overall, 13% of the kochia populations were fluroxypyr-resistant. Only 4% of the populations were both fluroxypyr- and dicamba-resistant, indicating that different mechanisms may confer resistance to these herbicides. When combined with estimates of dicamba resistance, about 28% of kochia populations sampled in Alberta in 2017 were resistant to at least one synthetic auxin herbicide.
In response to concerns about acetolactate synthase (ALS) inhibitor-resistant weeds in wheat production systems, we explored the efficacy of managing Bromus spp., downy and Japanese bromes, in a winter wheat system using alternative herbicide treatments applied in either fall or spring. Trials were established at Lethbridge and Kipp, Alberta, and Scott, Saskatchewan, Canada over three growing seasons (2012)(2013)(2014) to compare the efficacy of pyroxasulfone (a soil-applied very-long-chain fatty acid elongase inhibitor; WSSA Group 15) and flumioxazin (a protoporphyrinogen oxidase inhibitor; WSSA Group 14) against industry-standard ALSinhibiting herbicides for downy and Japanese brome control. Winter wheat injury from herbicide application was minor, with the exception of flucarbazone application at Scott. Bromus spp. control was greatest with pyroxsulam and all herbicide treatments containing pyroxasulfone. Downy and Japanese bromes were controlled least by thiencarbazone and flumioxazin, respectively, whereas Bromus spp. had intermediate responses to the other herbicides tested. Herbicides applied in fall resulted in reduced winter wheat yield relative to the spring applications. Overall, pyroxasulfone or pyroxsulam provided the most efficacious Bromus spp. control compared with the other herbicides and consistently maintained optimal winter wheat yields. Therefore, pyroxasulfone could facilitate management of Bromus spp. resistant to ALS inhibitors in winter wheat in the southern growing regions of western Canada. Improved weed control and delayed herbicide resistance may be achieved when pyroxasulfone is applied in combination with flumioxazin.
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Kochia [<i>Bassia scoparia</i> (L.) A.J. Scott] is a summer-annual tumbleweed that is tolerant of heat, drought and salinity, and capable of causing large yield losses in spring wheat (<i>Triticum aestivum</i> L). Increased incidence of glyphosate and acetolactate synthase (ALS) inhibitor-resistant kochia in Western Canada warrants investigation of alternative herbicides to manage these biotypes. Herbicides applied pre- or post-emergence in spring wheat were evaluated based on crop tolerance and control of ALS inhibitor-resistant kochia accessions with and without the glyphosate resistance trait in five environments near Lethbridge and Coalhurst, Alberta, from 2013-2015. The most effective and consistent treatments for kochia management included sulfentrazone applied pre-emergence, and fluroxypyr/bromoxynil/2,4-D or pyrasulfotole/bromoxynil applied post-emergence. All of these treatments resulted in ≥ 90% visible control in all environments and ≥ 90% kochia biomass reduction compared with the untreated control in Lethbridge 2014 and 2015. MCPA/dichlorprop-p/mecoprop-p, dicamba/2,4-D/mecoprop-p, and dicamba/fluroxypyr resulted in acceptable control among environments (≥ 80% visible control in all environments and ≥ 80% kochia biomass reduction in Lethbridge 2014 and 2015); however the latter two options caused unacceptable (> 10%) wheat visible injury in Coalhurst 2014. Recent confirmations of auxinic herbicide-resistant kochia in Western Canada – due, in part, to use of synthetic auxins to manage glyphosate-resistant kochia in small-grain cereals – will limit kochia management options. When implemented with non-chemical tools as part of an integrated weed management program, alternative herbicide modes of action like protoporphyrinogen oxidase inhibitors before and photosystem II or 4-hydroxyphenylpyruvate dioxygenase inhibitor(s) within spring wheat could mitigate selection for multiple herbicide-resistant kochia.
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