Amid widespread occurrence of herbicide-resistant weeds in the United States, the use of PRE herbicides and cover crops have resurged once again as important strategies for weed management in cropping systems. The objective of this experiment was to evaluate the length of soil residual weed control from PRE soybean herbicides and the detrimental impact of these herbicides on cover crop species using field treated soil in greenhouse bioassays. Greenhouse bioassays were conducted using soil from field experiments conducted in 2018 and 2019 in Arlington and Lancaster, WI. PRE herbicides consisted of imazethapyr, chlorimuron-ethyl, and cloransulam-methyl (acetolactate synthase [ALS] inhibitors); metribuzin (photosystem II [PS II] inhibitor); sulfentrazone, flumioxazin, and saflufenacil (protoporphyrinogen oxidase [PPO] inhibitors); acetochlor, S-metolachlor, dimethenamid-P, and pyroxasulfone (very long-chain fatty acid [VLCFA] inhibitors); and a nontreated control. Greenhouse bioassays were conducted using soil (depth, 0 to 10 cm) sampled at 0, 10, 20, 30, 40, and 50 d after treatment (DAT). Palmer amaranth and giant foxtail (weeds), and radish and cereal rye (cover crops) were used as bioindicators of herbicide levels in the soil. Bioassay results showed extended soil residual control of Palmer amaranth with sulfentrazone and pyroxasulfone; extended residual control of giant foxtail was observed with pyroxasulfone and S-metolachlor. Chlorimuron-ethyl and metribuzin were the most injurious herbicides to radish and cereal rye shortly after application, respectively, but minimal injury was observed from soil samples collected 50 DAT, indicating the use of PRE and fall-seeded cover crops in southern Wisconsin can be compatible. These results can support growers and practitioners with selection of effective PRE herbicides for Palmer amaranth and giant foxtail control and reduced impact on fall-seeded radish and cereal rye cover crops, altogether leading to more effective, diverse, and sustainable weed management programs.

Background: The prevalent and repeated use of acetyl‐coenzyme A carboxylase (ACCase)‐inhibiting herbicides for Bromus tectorum L. control in fine fescue (Festuca L. spp) grown for seed has selected ACCase‐resistant B. tectorum populations. The objectives of this study were to (1) evaluate the response of nine B. tectorum populations to the ACCase inhibitors clethodim, sethoxydim, fluazifop‐P‐butyl, and quizalofop‐P‐ethyl and the acetolactate synthase (ALS) inhibitor sulfosulfuron and (2) characterize the resistance mechanisms.Results: Bromus tectorum populations were confirmed to be resistant to the ACCase‐inhibiting herbicides tested. The levels of resistance varied among the populations for clethodim (resistance ratio, RR = 5.1–14.5), sethoxydim (RR = 18.7–44.7), fluazifop‐P‐butyl (RR = 3.1–40.3), and quizalofop‐P‐ethyl (RR = 14.5–36). Molecular investigations revealed that the mutations Ile2041Thr and Gly2096Ala were the molecular basis of resistance to the ACCase‐inhibiting herbicides. The Gly2096Ala mutation resulted in cross‐resistance to the aryloxyphenoxypropionate (APP) herbicides fluazifop‐P‐butyl and quizalofop‐P‐ethyl, and the cyclohexanedione (CHD) herbicides clethodim, and sethoxydim, whereas Ile2041Thr mutation resulted in resistance only to the two APP herbicides. All B. tectorum populations were susceptible to sulfosulfuron (RR = 0.3–1.7).Conclusions: This is the first report of target‐site mutations conferring resistance to ACCase‐inhibiting herbicides in B. tectorum. The results of this study suggest multiple evolutionary origins of resistance and contribute to understanding the patterns of cross‐resistance to ACCase inhibitors associated with different mutations in B. tectorum. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

This review summarizes what is currently known about herbicide resistance in Bromus spp. worldwide. Additional information on the biology and genetics of Bromus spp. is provided to further the understanding of resistance evolution and dispersal of the different species. Cases of herbicide resistance have been confirmed in Bromus catharticus Vahl., Bromus commutatus Schrad. (syn.: Bromus racemosus L.), Bromus diandrus Roth, Bromus japonicus Thunb. (syn.: Bromus arvensis L.), Bromus madritensis L., Bromus rigidus Roth (syn.: Bromus diandrus Roth ssp. diandrus), Bromus rubens L., Bromus secalinus L., Bromus sterilis L., and Bromus tectorum L. in 11 countries. Bromus spp. populations have evolved cross- and multiple resistance to six herbicide sites of action: acetyl-coenzyme A carboxylase, acetolactate synthase, photosystem II, very-long-chain fatty-acid, 5-enolpyruvylshikimate-3-phosphate synthase, and 4-hydroxyphenylpyruvate dioxygenase inhibitors. Resistance mechanisms varied from target-site to non–target site or a combination of both. Bromus spp. are generally highly self-pollinated, but outcrossing can occur at low levels in some species. Bromus spp. have different ploidy levels, ranging from diploid (2n = 2x = 14) to duodecaploid (2n = 12x = 84). Herbicide resistance in Bromus spp. is a global issue, and the spread of herbicide-resistance alleles primarily occurs via seed-mediated gene flow. However, the transfer of herbicide-resistance alleles via pollen-mediated gene flow is possible.

The repeated evolution of herbicide resistance in agriculture provides an unprecedented opportunity to understand how organisms rapidly respond to strong anthropogenic‐driven selection pressure. We recently identified agricultural populations of the grass species Bromus tectorum L. with resistance to multiple herbicides. To understand the evolutionary origins and spread of resistance, we investigated the resistance mechanisms to acetolactate synthase (ALS) inhibitors and photosystem II inhibitors, two widely used herbicide modes of action, in 49 B. tectorum populations. We assessed the genetic diversity, structure and relatedness in a subset of 21 populations. Resistance to ALS inhibitors was associated with multiple nonsynonymous mutations in ALS, the target site gene, despite the relatively small geographic region where populations originated, suggesting ALS inhibitor resistance evolution occurred multiple times in the region. We also found evidence that mechanisms not related to the target site evolved and were common in the populations studied. Resistance to photosystem II inhibitors was confirmed in two populations and was conferred by nonsynonymous mutations in the plastid gene psbA. Population genomics analyses suggested that ALS resistance in most populations, at the nucleotide level, spread via gene flow, except for one population where we found evidence that Pro‐197‐His mutations may have evolved in three separate events. Our results suggest that both gene flow via pollen and/or seed dispersal and multiple local evolutionary events were involved in the spread of herbicide‐resistant B. tectorum. Our results provide an empirical example of the rapid repeated evolution of a trait under strong anthropogenic selection and elucidate the evolutionary origins of herbicide resistance in a plant species of agricultural importance.

Downy brome (Bromus tectorum L.) is a difficult species to control in the dryland wheat (Triticum aestivum L.) production areas of northeastern Oregon. The selection of herbicide-resistant B. tectorum populations has further complicated B. tectorum management. A survey of wheat growers was conducted in 2021 and 2022 to understand B. tectorum management practices. The survey included four questions based on the growing seasons from 2017 to 2022 related to crop rotation, tillage versus no-tillage, irrigation versus dryland, and herbicide programs. To determine the extent of herbicide resistance, seeds were collected from 49 B. tectorum populations in wheat fields in northeastern Oregon and tested for resistance. Herbicides tested were clethodim, glyphosate, imazamox, mesosulfuron, metribuzin, propoxycarbazone, pyroxasulfone, pyroxsulam, quizalofop, and sulfosulfuron. Winter wheat–summer fallow rotation was the most predominant cropping system in the region. Most of the fields were in no-tillage systems, and none were irrigated. Pyroxasulfone applied preemergence and acetolactate synthase (ALS) inhibitors applied postemergence were the most often used herbicides for B. tectorum control in winter wheat. Glyphosate was the most frequently used herbicide for B. tectorum control in summer fallow. Resistance screenings confirmed that 46 of the 49 B. tectorum populations were resistant to ALS inhibitors with different cross-resistance patterns. Two populations were resistant to metribuzin and exhibited multiple resistance to ALS inhibitors. All populations were susceptible to clethodim, glyphosate, pyroxasulfone, and quizalofop. The widespread occurrence of ALS inhibitor–resistant B. tectorum populations limits effective postemergence herbicide options in winter wheat.

Weeds pose a significant threat to Oregon field crops by reducing yields and compromising the quality of the harvested products. In fall 2024, Oregon stakeholders were surveyed to identify the key challenges and needs related to weed management in field crops. The survey consisted of eight questions, developed based on expert input and review of relevant literature, divided into three sections: respondent demographics, resource and support needs, and current weed management challenges and economic impacts. A total of 184 responses were collected statewide, with growers and crop consultants as the primary participants. Most respondents were from western Oregon, which may introduce a geographic bias to the findings. Respondents expressed particular interest in new herbicide technologies and herbicide resistance. Field days were the most preferred method for receiving weed management information. Annual bluegrass, Italian ryegrass, and roughstalk bluegrass were reported as the most problematic grass weeds, while wild carrot was the most problematic broadleaf weed. Most respondents reported an average weed control cost of US$125 to US$250 per hectare, and the majority indicated they were very concerned about herbicide resistance in their fields. The results from this survey will help guide future research and outreach efforts to improve weed management strategies in Oregon’s field crops.