Wheat Density Alters but Does Not Repress the Expression of a Fluroxypyr-Resistant Kochia (Bassia scoparia) Phenotype

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.

Increasing frequency of multiple herbicide-resistant kochia (Bassia scoparia) in Alberta

“…The short-lived (1-2 yr) kochia seedbank (Beckie et al 2018) represents an opportune target for weed management strategies that mitigate kochia seed production and return to the soil seedbank (Geddes and Davis 2021). With increasing resistance to multiple herbicide MOAs, it is clear that non-chemical approaches, like growing competitive crops, increasing crop seeding rates (Geddes and Kimmins 2021), and judicious -yet strategic -tillage (Obour et al 2021), will have a prominent role in preserving the efficacy of the remaining herbicides available for kochia control. showing the overall frequencies of resistance to these herbicides among the 314 sites evaluated.…”