Widespread occurrence of herbicide-resistant weeds and more variable weather conditions across the United States has made weed control in many crops more challenging. Preemergence (PRE) herbicides with soil residual activity have resurged as the foundation for early season weed control in many crops. Field experiments were conducted in Janesville and Lancaster, Wisconsin, in 2021 and 2022 (4 site-years) to evaluate the weed control efficacy of solo (single site of action [SOA]) and premix (two or more SOAs) PRE herbicides in conventional tillage corn. Treatments consisted of 18 PRE herbicides plus a nontreated check. At the Janesville-2021 site, S-metolachlor + bicyclopyrone + mesotrione, atrazine + S-metolachlor + bicyclopyrone + mesotrione, and clopyralid + acetochlor + mesotrione provided >72% giant ragweed control. At the Janesville-2022 site, none of the PRE herbicides evaluated provided >70% giant ragweed control due to the high giant ragweed density and the lack of timely rainfall. At the Lancaster-2021 site, atrazine, dicamba, and flumetsulam + clopyralid provided <45% waterhemp control, but the remaining treatments provided >90% control. At the Lancaster-2022 site, the efficacy of some PRE herbicides was reduced due to the high waterhemp density; however, most herbicides provided >75% control. At the Lancaster-2021 and Lancaster-2022 sites, only dicamba and S-metolachlor did not provide >75% common lambsquarters control. Group 15 PRE herbicides provided >75% control of giant foxtail. Across weed species, PRE herbicides with two (78%) and three (81%) SOAs provided greater weed control than PRE herbicides with a single SOA (68%), indicating that at least two SOA herbicides applied PRE result in better early season weed control. The efficacy of the PRE herbicide treatments evaluated herein varied according to the soil seedbank weed community composition and environmental conditions (i.e., rainfall following application), but the premixes were a more reliable option to improve early season weed control in conventional tillage corn.

Cereal rye cover crop (cereal rye) and preemergence (PRE) herbicides are becoming common practices for managing herbicide-resistant weeds in soybean production. Adopting these two practices in combination raises concerns regarding herbicide fate in soil, given that the cereal rye biomass can intercept the herbicide spray solution, preventing it from reaching the soil. Delaying cereal rye termination until soybean planting (planting green) optimizes biomass accumulation but might also increase PRE interception. To better understand the dynamics between cereal rye and PRE herbicides, a field experiment was conducted to evaluate two soil management practices (tillage and no-till) and two cereal rye termination practices in the planting-green system (glyphosate [1,260 g ae ha−1] and roller-crimper) on the spray deposition and fate of PRE herbicides and soybean yield. The spray deposition was assessed by placing water-sensitive paper cards on the soil surface before spraying the PRE herbicides (sulfentrazone [153 g ai ha−1] + S-metolachlor [1,379 g ai ha−1]). Herbicide concentration in soil (0 to 7.6 cm) was quantified 25 d after treatment (DAT). The presence of no-till stubble and cereal rye biomass reduced the spray coverage compared to tillage at PRE application, which reflected in a reduction in the concentration of both herbicides in soil 25 DAT. Soybean yield was reduced in all three years when the cereal rye was terminated with a roller-crimper but only reduced in one year when terminated with glyphosate. Our findings indicate that mainly cereal rye biomass reduced the concentration of PRE herbicides in the soil due to the interception of the spray solution during application. Although higher cereal rye biomass accumulation can provide better weed suppression according to the literature, farmers should be aware that the biomass can lower the concentration of PRE herbicides reaching the soil, thus intensifying field scouting to ensure that weed control is not being negatively affected.

A comprehensive, Wisconsin state-wide assessment of waterhemp response to a diverse group of herbicide sites of action has not been conducted. Our objective was to characterize the response of a state-wide collection of waterhemp accessions to postemergence (POST) and preemergence (PRE) herbicides commonly used in corn and soybean in Wisconsin. Greenhouse experiments were conducted with more than 80 accessions from 27 counties. POST treatments included 2,4-D, atrazine, dicamba, fomesafen, glufosinate, glyphosate, imazethapyr, and mesotrione at 1× and 3× label rates. PRE treatments included atrazine, fomesafen, mesotrione, metribuzin, and S-metolachlor at 0.5×, 1×, and 3× label rates. Ninety-eight percent and 88% of the accessions exhibited ≥50% plant survival after exposure to imazethapyr and glyphosate POST 3× rate, respectively. Seventeen percent, 16%, and 3% of the accessions exhibited ≥50% plant survival after exposure to 2,4-D, atrazine, and dicamba, respectively, applied POST at the 1× rate. Survival of all accessions was ≤25% after exposure to 2,4-D or dicamba applied POST at the 3× rate, or glufosinate, fomesafen, and mesotrione applied POST at either rate evaluated. No plant of any accession survived exposure to glufosinate at either rate. Forty-five percent and 3% of the accessions exhibited <90% plant density reduction after exposure to atrazine applied PRE at the 3× rate and fomesafen PRE at the 1× rate, respectively. Plant density reduction of all accessions was ≥96% after exposure to fomesafen applied PRE at the 3× rate, or metribuzin, S-metolachlor, and mesotrione applied PRE at the 1× rate. Our results suggest that waterhemp resistance to imazethapyr and glyphosate applied POST is widespread in Wisconsin, whereas resistance to 2,4-D, atrazine, and dicamba applied POST is present to a lower extent. One accession (A75, Fond du Lac County) exhibited multiple resistance to imazethapyr, atrazine, glyphosate, and 2,4-D when applied POST. Overall, atrazine applied PRE was ineffective for waterhemp control in Wisconsin. Proactive resistance management and the use of effective PRE and POST herbicides are fundamental for waterhemp management in Wisconsin.

Horseweed is a North American indigenous plant species commonly found in Nebraska cropping systems. Horseweed management is challenging because of horseweed’s prolific seed production, long-distance seed dispersal via wind, competitiveness, and rapid evolution of herbicide resistance. Understanding the horseweed emergence pattern across Nebraska can contribute to implementing effective and more sustainable tactics to minimize its impact on cropping systems. Field studies were conducted during fall and spring from 2016 to 2018 in Lincoln (corn and soybean), North Platte (wheat stubble and soybean), and Scottsbluff (corn and fallow) to investigate the emergence pattern of horseweed accessions from Lincoln, North Platte, and Scottsbluff, NE. Results show that most horseweed seedling emergence occurred in fall (99%) and only a few seedlings emerged in spring across locations, except in the wheat stubble experiment at North Platte, where higher spring emergence was detected (3% to 22%). In four out of six experiments, the density of total emerged seedlings of each accession was greatest when established in their site of origin. Our results suggest that late fall and/or early spring is likely the best timing for horseweed management across Nebraska.

The continued dispersal of Palmer amaranth can impose detrimental impacts on cropping systems in Wisconsin. Our objective was to characterize the response of a recently introduced Palmer amaranth accession in southern Wisconsin to postemergence (POST) and preemergence (PRE) herbicides commonly used in corn and soybean. Greenhouse experiments were conducted with the Wisconsin putative herbicide-resistant accession (BRO) and two additional control accessions from Nebraska, a glyphosate-resistant (KEI2) and a glyphosate-susceptible (KEI3) accession. POST treatments were 2,4-D, atrazine, dicamba, glufosinate, glyphosate, imazethapyr, lactofen, and mesotrione at 1X and 3X label rates. PRE treatments were atrazine, mesotrione, metribuzin, S-metolachlor, and sulfentrazone at 0.5X, 1X, and 3X label rates. Plant survival of each accession was ≥63% after exposure to imazethapyr POST 3X rate. Survival of BRO and KEI2 was 44% (±13) and 50% (±13), respectively, after exposure to atrazine POST 3X rate. Survival of BRO was 69% (±12) after exposure to glyphosate POST 1X rate, whereas survival of KEI2 was 44% (±13) after exposure to glyphosate POST 3X rate. After exposure to 2,4-D POST 1X rate, KEI2 and KEI3 survival was 38% (±13) and 50% (±13), respectively. Survival of all accessions was ≤31% after exposure to 2,4-D POST 3X rate or dicamba, glufosinate, lactofen, and mesotrione POST at either rate. Plant density reduction of KEI2 was 77% (±13) after exposure to atrazine PRE 1X rate, whereas density reduction of BRO was 56% (±13) after exposure to atrazine PRE 3X rate. Plant density reduction of all accessions was ≥94% after exposure to mesotrione PRE 1X and 3X rates or metribuzin, S-metolachlor, and sulfentrazone PRE at either rate. Our results suggest that each accession is resistant (≥50% survival) to imazethapyr POST, that BRO and KEI2 are resistant to atrazine and glyphosate POST, and that KEI2 and KEI3 are resistant to 2,4-D POST. The recently introduced BRO accession exhibited multiple resistance to imazethapyr, atrazine, and glyphosate POST. In addition, atrazine PRE was ineffective for BRO control, suggesting that diversified resistance management strategies will be critical for its effective management.

Recent legalization of industrial hemp in the United States has led to increased interest among stakeholders to produce hemp for grain and fiber. However, owing to the lack of herbicides registered for use in hemp, producers are left with limited weed management strategies. Moreover, much of the agricultural land that could be used to cultivate industrial hemp may be prone to carryover of previously applied residual herbicides or physical drift from herbicides sprayed nearby. Industrial hemp sensitivity to herbicides is not well documented. Dose–response studies were conducted under controlled conditions in Madison, WI, screening two industrial hemp grain cultivars for tolerance to 44 preemergence and postemergence herbicides commonly used in corn and soybean. Treatments consisted of herbicides applied at 0×, 0.125×, 0.25×, 0.50×, 0.75×, 1×, 2×, and 4× the recommended maximum labeled rates based on soil type. Preemergence applications were delivered immediately after planting, whereas postemergence applications took place when hemp plants reached 5 to 10 cm in height. Nontreated plants served as the control and were used to estimate percent biomass reduction; dose–response curves were generated. Biomass reduction was >50% for rates under the suggested label rate for 23 preemergence and 21 postemergence herbicides tested. All herbicides tested resulted in >25% biomass reduction at the 0.125× rate, except for clopyralid applied preemergence and postemergence and saflufenacil applied preemergence. This is concerning, as the label rates are determined for effective weed control and the mitigation of herbicide resistance. Overall, these results indicate that industrial hemp is very sensitive to most herbicides tested. Growers should consider herbicide use history and surrounding crops when determining industrial hemp field selection to prevent significant plant injury due to herbicide carryover and drift. Further research into alternative methods of weed control will be vital to establishing hemp as a dominant crop once again.

Owing to the lack of effective POST herbicide options, producers typically rely on nicosulfuron as the main POST grass herbicide in sweet corn production systems. In 2019, a Wisconsin sweet corn producer reported fall panicum control escapes after spraying nicosulfuron. Seeds from mature plants were collected to (1) measure fall panicum response to acetolactate synthase (ALS)-inhibiting herbicides, (2) elucidate the resistance mechanism, and (3) evaluate its response to alternative POST herbicides. Greenhouse and laboratory investigations were conducted to assess fall panicum response to ALS-inhibiting herbicides and elucidate the resistance mechanism. Dose–response results showed that fall panicum was highly resistant to nicosulfuron with a resistance ratio of >12.9-fold (survived rates >254 g ai ha−1, or 8× the field label rate). Molecular and genetic studies indicated that there are multiple ALS gene copies in fall panicum and that resistance was due to a mutation in one copy, resulting in an Asp-376-Glu amino acid substitution. Additional greenhouse experiments indicate that clethodim (105 g ai ha−1), quizalofop-p-ethyl (70 g ae ha−1), glyphosate (864 g ae ha−1), and glufosinate (650 g ai ha−1) are effective POST options to manage the ALS-resistant fall panicum (>90.0% control and 96.8% biomass reduction) in rotational years. The 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides isoxaflutole (105 g ai ha−1), mesotrione (105 g ai ha−1), tembotrione (92 g ai ha−1), and tolpyralate (39 g ai ha−1) did not provide effective POST fall panicum control. Because these herbicides are commonly used for POST weed control in sweet corn, more investigations are required to evaluate combinations of HPPD-inhibiting herbicides with herbicides from other sites of action for POST fall panicum control. Herein we confirm the first case of herbicide resistance in fall panicum in the United States.

Herbicides with soil-residual activity have the potential for carryover into subsequent crops, resulting in injury to sensitive crops and limiting productivity if severe. The increased use of soil-residual herbicides in the United States for management of troublesome weeds in corn- and soybean-cropping systems has potential to result in more cases of carryover. Soil management practices have different effects on the soil environment, potentially influencing herbicide degradation and likelihood of carryover. Field experiments were conducted at three sites in 2019 and 2020 to determine the effects of corn (clopyralid and mesotrione) and soybean (fomesafen and imazethapyr) herbicides applied in the fall at reduced rates (25% and 50% of labeled rates) and three soil management practices (tillage, no-tillage, and a fall-established cereal rye cover crop) on subsequent growth and productivity of the cereal rye cover crop and the soybean and corn crops, respectively. Most response variables (cereal rye biomass and crop canopy cover at cover crop termination in the spring, early-season crop stand and herbicide injury ratings, and crop yield) were not affected by herbicide carryover. Corn yield was lower when soil was managed with a cereal rye cover crop compared with tillage at all three sites, while yield was lower for no-till compared with tillage at two sites. Soybean yield was lower when managed with a cereal rye cover crop compared with tillage and no-till at one site. Findings from this research indicate a low carryover risk for these herbicides across site-years when label rotational restrictions are followed and environmental conditions favorable for herbicide degradation exist, regardless of soil management practice on silt loam or silty clay loam soil types in the U.S. Midwest region.

The role of weed suppression by the cultivated crop is often overlooked in annual row cropping systems. Agronomic practices such as planting time, row spacing, tillage and herbicide selection may influence the time of crop canopy closure. The objective of this research was to evaluate the influence of the aforementioned agronomic practices and their interaction with the adoption of an effective preemergence (PRE) soil residual herbicide program on soybean canopy closure and yield. A field experiment was conducted in 2019 and 2020 in Arlington, WI, as a 2×2×2×2 factorial in a randomized complete block design, including early (late April) and standard (late May) planting time, narrow (38 cm) and wide (76 cm) row spacing, conventional tillage and no-till, and soil-applied PRE herbicide (yes and no; flumioxazin 150 g ai ha−1 + metribuzin 449 g ai ha−1 + pyroxasulfone 190 g ai ha−1). All plots were maintained weed-free throughout the growing season. In both years, early planted soybeans reached 90% green canopy cover (T90) before (7 to 9 d difference) and yielded more (188 to 902 kg ha−1 difference) than the standard planted soybeans. Narrow-row soybeans reached T90 earlier than wide-row soybeans (4 to 7 d difference), but yield was similar between row spacing treatments. Conventional tillage resulted in a higher yield compared to a no-till system (377 kg ha−1 difference). The PRE herbicide slightly delayed T90 (4 d or less) but had no impact on yield. All practices investigated herein influenced the time of soybean canopy closure but only planting time and tillage impacted yield. Planting soybeans earlier and reducing their row spacing expedites the time to canopy closure. The potential delay in canopy development and yield loss if soybeans are allowed to compete with weeds early in the season would likely outweigh the slight delay in canopy development by an effective PRE herbicide.

Enlist E3TM soybean cultivars permit over-the-top application of labeled glyphosate, glufosinate, and 2,4-D choline products. Increased Enlist E3TM trait adoption and use of 2,4-D choline postemergence across U.S. soybean production systems raise concerns regarding potential for 2,4-D off-target movement (OTM). A large-scale drift experiment was established near Sun Prairie, WI, and Arlington, WI, in 2019 and 2020, respectively. A 2,4-D-resistant soybean cultivar was planted in the center of the field (∼3 ha), while the surrounding area was planted with a 2,4-D-susceptible cultivar. An application of 785 ae ha−1 2,4-D choline plus 834 g ae ha−1 glyphosate was completed within the center block at R2 and V6 growth stages on August 1, 2019, and July 3, 2020, respectively. Filter papers were placed in-swath and outside of the treated area in one upwind transect and three downwind transects to estimate particle deposition. Low-volume air samplers ran for the 0.5-h to 48-h period following application to estimate 2,4-D air concentration. Injury to 2,4-D-susceptible soybean was assessed 21 d after treatment (0% to 100% injury). The 2,4-D deposition in-swath was 9,966 and 5,727 ng cm−2 in 2019 and 2020, respectively. Three-parameter log-logistic models estimated the distance to 90% reduction in 2,4-D deposition (D90) to be 0.63 m and 0.90 m in 2019 and 2020, respectively. In 2020, the 2,4-D air concentration detected was lower for the upwind (0.395 ng m−3) than the downwind direction (1.34 ng m−3), although both were lower than the amount detected in-swath (4.01 ng m−3). No soybean injury was observed in the downwind or upwind directions. Our results suggest that 2,4-D choline applications following label recommendations pose little risk to 2,4-D-susceptible soybean cultivars; however, further work is needed to understand 2,4-D choline OTM under different environmental conditions and the presence of other susceptible crops.

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.

The use of photosystem II (PSII)-inhibitor and/or protoporphyrinogen oxidase (PPO)-inhibitor PRE herbicides in soybean may, under adverse environmental conditions, result in early season crop injury. A field study was conducted near Brule and North Platte, Nebraska, during the 2016 and 2017 growing seasons with the objective to evaluate the impact of PRE herbicides metribuzin (PSII-inhibitor) and sulfentrazone (PPO-inhibitor) on early season soybean development, final plant stand, and yield using 22 soybean varieties adapted to southwestern Nebraska. Herbicide treatments consisted of metribuzin (560 g ai ha−1) and sulfentrazone (280 g ai ha−1) applied within 3 d after planting and a nontreated control (NTC). Sulfentrazone reduced green canopy vegetation at the V2 growth stage by 22% and final plant stand at physiological maturity by 10% compared with the NTC. The number of pods per plant was 16% higher for sulfentrazone and the number of seeds per plant was 15% and 4% higher for sulfentrazone and metribuzin compared with the NTC, respectively. Sulfentrazone and metribuzin resulted in a slightly higher yield (3%) compared with the NTC, thus no yield reduction from PRE herbicides was observed in this study. These results support other findings that sulfentrazone and metribuzin have potential to cause early-season crop injury; however, when applied according to their label recommendations and following regional agronomic management practices, this impact may not translate into soybean yield reduction while such herbicides provide effective soil residual weed control.

A stakeholder survey was conducted from April through June of 2018 to understand stakeholders’ perceptions and challenges about cropping systems and weed management in Brazil. The dominant crops managed by survey respondents were soybean (73%) and corn (66%). Approximately 75% of survey respondents have grown or managed annual cropping systems with two to three crops per year cultivated in succession. Eighteen percent of respondents manage only irrigated cropping systems, and over 60% of respondents adopt no-till as a standard practice. According to respondents, the top five troublesome weed species in Brazilian cropping systems are horseweed (asthmaweed, Canadian horseweed, and tall fleabane), sourgrass, morningglory, goosegrass, and dayflower (Asiatic dayflower and Benghal dayflower). Among the nine species documented to have evolved resistance to glyphosate in Brazil, horseweed and sourgrass were reported as the most concerning weeds. Other than glyphosate, 31% and 78% of respondents, respectively, manage weeds resistant to acetyl-CoA carboxylase (ACCase) inhibitors and/or acetolactate synthase (ALS) inhibitors. Besides herbicides, 45% of respondents use mechanical, and 75% use cultural (e.g., no-till, crop rotation/succession) weed control strategies. Sixty-one percent of survey respondents adopt cover crops to some extent to suppress weeds and improve soil chemical and physical properties. Nearly 60% of survey respondents intend to adopt the crops that are resistant to dicamba or 2,4-D when available. Results may help practitioners, academics, industry, and policy makers to better understand the bad and the good of current cropping systems and weed management practices adopted in Brazil, and to adjust research, education, technologies priorities, and needs moving forward.

Use of synthetic auxin herbicides has increased across the midwestern United States after adoption of synthetic auxin-resistant soybean traits, in addition to extensive use of these herbicides in corn. Off-target movement of synthetic auxin herbicides such as dicamba can lead to severe injury to sensitive plants nearby. Previous research has documented effects of glyphosate on spray-solution pH and volatility of several dicamba formulations, but our understanding of the relationships between glyphosate and dicamba formulations commonly used in corn and for 2,4-D remains limited. The objectives of this research were to (1) investigate the roles of synthetic auxin herbicide formulation, glyphosate, and spray additives on spray solution pH; (2) assess the impact of synthetic auxin herbicide rate on solution pH; and (3) assess the influence of glyphosate and application time of year on dicamba and 2,4-D volatility using soybean as bioindicators in low-tunnel field volatility experiments. Addition of glyphosate to a synthetic auxin herbicide decreased solution pH below 5.0 for four of the seven herbicides tested (range of initial pH of water source, 7.45–7.70). Solution pH of most treatments was lower at a higher application rate (4× the labeled POST rate) than the 1× rate. Among all treatment factors, inclusion of glyphosate was the most important affecting spray solution pH; however, the addition of glyphosate did not influence area under the injury over distance stairs (P = 0.366) in low-tunnel field volatility experiments. Greater soybean injury in field experiments was associated with high air temperatures (maximum, >29 C) and low wind speeds (mean, 0.3–1.5 m s−1) during the 48 h after treatment application. The two dicamba formulations (diglycolamine with VaporGrip® and sodium salts) resulted in similar levels of soybean injury for applications that occurred later in the growing season. Greater soybean injury was observed after dicamba than after 2,4-D treatments.

Failure to control Palmer amaranth with glyphosate and protoporphyrinogen IX oxidase (PPO)-inhibitor herbicides was reported across southwestern Nebraska in 2017. The objectives of this study were to 1) confirm and 2) validate glyphosate and PPO-inhibitor (fomesafen and lactofen) resistance in 51 Palmer amaranth accessions from southwestern Nebraska using genotypic and whole-plant phenotypic assay correlations and cluster analysis, and 3) determine which agronomic practices might be influencing glyphosate resistance in Palmer amaranth accessions in that location. Based on genotypic assay, 88% of 51 accessions contained at least one individual with amplification (>2 copies) of the 5-enolypyruvyl-shikimate-3-phosphate synthase (EPSPS) gene, which confers glyphosate resistance; and/or a mutation in the PPX2 gene, either ΔG210 or R128G, which endows PPO-inhibitor resistance in Palmer amaranth. Cluster analysis and high correlation (0.83) between genotypic and phenotypic assays demonstrated that EPSPS gene amplification is the main glyphosate resistance mechanism in Palmer amaranth accessions from southwestern Nebraska. In contrast, there was poor association between genotypic and phenotypic responses for PPO-inhibitor resistance, which was attributed to segregation for PPO-inhibitor resistance within these accessions and/or the methodology that was adopted herein. Genotypic assays can expedite the process of confirming known glyphosate and PPO-inhibitor resistance mechanisms in Palmer amaranth from southwestern Nebraska and other locations. Phenotypic assays are also a robust method for confirming glyphosate resistance but not necessarily PPO-inhibitor resistance in Palmer amaranth. Moreover, random forest analysis of glyphosate resistance in Palmer amaranth indicated that EPSPS gene amplification, county, and current and previous crops are the main factors influencing glyphosate resistance within that geographic area. Most glyphosate-susceptible Palmer amaranth accessions were found in a few counties in areas with high crop diversity. Results presented here confirm the spread of glyphosate resistance and PPO-inhibitor resistance in Palmer amaranth accessions from southwestern Nebraska and demonstrate that less diverse cropping systems are an important driver of herbicide resistance evolution in Palmer amaranth.

Six experiments were conducted in 2018 on field sites located in Arkansas, Indiana, Michigan, Nebraska, Ontario, and Wisconsin to evaluate the off-target movement (OTM) of dicamba under field-scale conditions. The highest estimated percentages of dicamba injury in non–dicamba-resistant (DR) soybean were 55%, 44%, 39%, 67%, 15%, and 44% injury for noncovered areas and 55%, 5%, 13%, 42%, 0%, and 41% injury for covered areas during dicamba application in Arkansas, Indiana, Michigan, Nebraska, Ontario, and Wisconsin, respectively. The level of injury generally decreased as the downwind distance increased under covered and noncovered areas at all sites. There was an estimated 10% injury in non-DR soybean at 113, 8, 11, 8, and 8 m; and estimated 1% injury at 293, 28, 71, 15, and 19 m from the edge of treated fields downwind when plants were not covered during dicamba application in Arkansas, Indiana, Michigan, Ontario, and Wisconsin, respectively. Assessment of filter-paper collectors placed from 4 to 137 m downwind from the edge of the sprayed area suggested the dicamba deposition reduced exponentially with distance. The greatest injury to non-DR soybean from dicamba OTM occurred at Nebraska and Arkansas (as far as 250 m). Non-DR soybean injury was greatest adjacent to the dicamba sprayed area, but injury decreased with no injury beyond 20 m downwind or in any other direction from the dicamba sprayed area in Indiana, Michigan, Ontario, and Wisconsin. The presence of soybean injury under covered and noncovered areas during the spray period for primary drift suggests that secondary movement of dicamba was evident at five sites. Additional research is needed to determine the exact forms of secondary movement of dicamba under different environmental conditions.

In 2017, dicamba-resistant (DR) soybean was commercially available to farmers in the United States. In August and September of 2017, a survey of 312 farmers from 60 Nebraska soybean-producing counties was conducted during extension field days or online. The objective of this survey was to understand farmers’ adoption and perceptions regarding DR soybean technology in Nebraska. The survey contained 16 questions and was divided in three parts: (1) demographics, (2) dicamba application in DR soybean, and (3) dicamba off-target injury to sensitive soybean cultivars. According to the results, 20% of soybean hectares represented by the survey were planted to DR soybean in 2017, and this number would probably double in 2018. Sixty-five percent of survey respondents own a sprayer and apply their own herbicide programs. More than 90% of respondents who adopted DR soybean technology reported significant improvement in weed control. Nearly 60% of respondents used dicamba alone or glyphosate plus dicamba for POST weed control in DR soybean; the remaining 40% added an additional herbicide with an alternative site of action (SOA) to the POST application. All survey respondents used one of the approved dicamba formulations for application in DR soybean. Survey results indicated that late POST dicamba applications (after late June) were more likely to result in injury to non-DR soybean compared to early POST applications (e.g., May and early June) in 2017. According to respondents, off-target dicamba movement resulted both from applications in DR soybean and dicamba-based herbicides applied in corn. Although 51% of respondents noted dicamba injury on non-DR soybean, 7% of those who noted injury filed an official complaint with the Nebraska Department of Agriculture. Although DR soybean technology allowed farmers to achieve better weed control during 2017 than previous growing seasons, it is apparent that off-target movement and resistance management must be addressed to maintain the viability and effectiveness of the technology in the future.

Waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] is a troublesome weed occurring in cropping systems throughout the U.S. Midwest with an ability to rapidly evolve herbicide resistance that could be associated with competitive disadvantages. Little research has investigated the competitiveness of different A. tuberculatus populations under similar environmental conditions. The objectives of this study were to evaluate: (1) the interspecific competitiveness of three herbicide-resistant A. tuberculatus populations (2,4-D and atrazine resistant [2A-R], glyphosate and protoporphyrinogen oxidase [PPO]-inhibitor resistant [GP-R], and 2,4-D, atrazine, glyphosate, and PPO-inhibitor susceptible [2AGP-S]) with soybean [Glycine max (L.) Merr.]; and (2) the density-dependent response of each A. tuberculatus population within a constant soybean population in a greenhouse environment. Amaranthus tuberculatus competitiveness with soybean was evaluated across five target weed densities of 0, 2, 4, 8, and 16 plants pot−1 (equivalent to 0, 20, 40, 80, and 160 plants m−2) with 3 soybean plants pot−1 (equivalent to 300,000 plants ha−1). At the R1 soybean harvest time, no difference in soybean biomass was observed across A. tuberculatus populations. At A. tuberculatus densities <8 plants pot−1, the 2AGP-S population had the greatest biomass and stem diameter per plant. At the R7 harvest time, the 2AGP-S population caused the greatest loss in soybean biomass and number of pods compared with the other populations at densities of <16 plants pot−1. The 2AGP-S population had greater early-season biomass accumulation and stem diameter compared with the other A. tuberculatus populations, which resulted in greater late-season reduction in soybean biomass and number of pods. This research indicates there may be evidence of interspecific competitive fitness cost associated with the evolution of 2,4-D, atrazine, glyphosate, and PPO-inhibitor resistance in A. tuberculatus. Focus should be placed on effectively using cultural weed management practices to enhance crop competitiveness, especially early in the season, to increase suppression of herbicide-resistant A. tuberculatus.

Resistance to atrazine (a photosystem II [PSII] inhibitor) is prevalent in waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] across the U.S. Midwest. Previous research suggests that target-site mutation or rapid metabolism of atrazine mediated by glutathione S-transferase (GST) conjugation confers resistance in A. tuberculatus from Illinois. The distribution and mechanism of resistance to atrazine in A. tuberculatus populations from Nebraska (NE) are unknown. In this research we (1) evaluated the response and frequency of resistance in NE A. tuberculatus to soil-applied PSII (metribuzin and atrazine) and protoporphyrinogen oxidase (sulfentrazone) inhibitors, as well as POST-applied atrazine; and (2) determined the mechanism of atrazine resistance in NE A. tuberculatus. The chloroplastic psbA gene, coding for a D1 protein (the target site of atrazine) was sequenced in 85 plants representing 27 populations of A. tuberculatus. Furthermore, 24 plants selected randomly from four atrazine-resistant (AR) populations were used to determine the metabolism of atrazine via GST conjugation. Results from the soil-applied herbicide evaluation suggest that metribuzin (0.56 kg ai ha−1) and sulfentrazone (0.28 kg ai ha−1) were effective on A. tuberculatus management. PRE and POST screenings against atrazine in the greenhouse indicate that atrazine (1.345 kg ai ha−1) was not effective on 39% and 73% of the A. tuberculatus populations evaluated (total of 109 and 85 populations, respectively), suggesting the prevalence of atrazine resistance in A. tuberculatus in NE. Sequence analysis of the psbA gene found no known point mutations conferring atrazine resistance. However, the AR plants conjugated atrazine via GST activity faster than the known atrazine-susceptible A. tuberculatus. Overall, the outcome of this study demonstrates the predominance of metabolism-based resistance to atrazine in A. tuberculatus from NE, which may predispose this species to evolve resistance to other herbicides. The use of integrated management strategies for A. tuberculatus is crucial for the control of this troublesome species.