Abstract
Dose-response experiments for confirmation of herbicide-resistant weeds are almost always conducted using weed monocultures, thereby ignoring the interaction of interspecific plant interference with herbicide efficacy. Controlled-environment dose-response bioassays were conducted using three kochia [Bassia scoparia (L.) A.J. Scott] populations with four spring wheat (Triticum aestivum L.) densities (0, 200, 400, and 600 plants m−2) to determine how increasing intensity of interspecific plant interference altered the fluroxypyr dose-response relationship of resistant and susceptible kochia. The resistant population exhibited 10.8-, 15.0-, 7.0-, and 8.1-fold resistance to fluroxypyr in the absence of crop interference based on plant survival, biomass fresh weight, and visible control at two and four weeks after application, respectively. Increased wheat densities suppressed fluroxypyr-resistant kochia the greatest, resulting in a linear reduction in the fluroxypyr rate causing 50% plant mortality (LD50) and visible control (ED50) for the resistant but not the susceptible populations. This reduced the expression of fluroxypyr resistance based on kochia plant survival (from 10.8- to 4.3-fold resistance) and visible control (from 8.1- to 4.6-fold resistance) as wheat density increased from 0 to 600 plants m−2. Therefore, enhanced interspecific plant interference caused by increased wheat densities altered but did not repress the expression of fluroxypyr resistance in kochia.
Highlights
Further research identified that 13% of these populations were fluroxypyr-resistant, suggesting that, overall, 28% of the kochia populations tested in Alberta in 2017 were synthetic auxin-resistant, and 16% were triple herbicide-resistant to acetolactate synthase (ALS) inhibitors, glyphosate, and at least one synthetic auxin [10]
RockyView18 exhibited 10.8, 15.0, 7.0, and 8.1-fold resistance in the absence of crop interference based on plant survival, biomass fresh weight (FW), and visible control at two and four weeks after herbicide application (WAA), respectively (Tables 1 and 2; Figure S1)
Vulcan17 resulted in resistance factor (R/S) < 4.0 for plant survival and visible control absent of crop interference, but was 12.5-fold more resistant than the susceptible control based on biomass FW; like RockyView18, the biomass FW R/S was not significantly different from 1 (Tables 1 and 2; Figure S1)
Summary
A 2017 survey of Alberta documented resistance to ALS inhibitors, glyphosate, and dicamba (a synthetic auxin herbicide; WSSA Group 4) in 100%, 50%, and 18% of the 305 kochia populations tested, respectively [9]. Increased crop seeding rates interact with herbicide performance resulting in improved weed management [17,18,19,30]. The interaction of interspecific plant interference with herbicide efficacy could reduce the level of resistance exhibited by herbicide-resistant weeds and potentially shift their phenotypic expression of resistance toward that of their susceptible counterparts. The current study was designed to determine whether interspecific plant interference caused by increasing wheat plant densities interacts with the fluroxypyr dose-response relationship of resistant and susceptible kochia, and to determine whether enhanced crop competitiveness could repress the phenotypic expression of fluroxypyr resistance
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