The development of glufosinate-resistant soybean cultivars has created opportunities for use of glufosinate applied postemergence for weed control. Four field experiments were conducted in 2021 and 2022 to ascertain the effect of glufosinate rate and the addition of ammonium sulfate on annual weed control in glyphosate/glufosinate/2,4-D–resistant soybean. An increased glufosinate rate of 500 from 300 g ai ha−1 improved control of common ragweed, common lambsquarters, redroot pigweed, and foxtail species and resulted in decreased density and dry biomass of common lambsquarters and foxtail species. The addition of ammonium sulfate to glufosinate increased control of common lambsquarters, 2 and 8 wk after application (WAA), and of foxtail species, 2, 4, and 8 WAA, but did not improve control of common ragweed and redroot pigweed. Increasing the dose of glufosinate from 300 to 500 g ai ha−1 improves control of common ragweed, redroot pigweed, common lambsquarters, and foxtail species; however, the benefit of the addition of ammonium sulfate to glufosinate is weed species-specific.

Region- and system-specific research is needed to understand the viability of delayed cover-crop termination (i.e., planting green) as an integrated weed management (IWM) tactic in no-till soybean. In a 3-yr field experiment, we evaluated the potential for planting green to facilitate elimination of soil-applied, preemergence residual herbicides within a soybean phase of a 6-yr grain–forage cropping systems experiment in Pennsylvania. This IWM tactic was contrasted with a Standard treatment, which included 14 to 21 d pre-plant termination of cereal rye and a two-pass herbicide program with preemergence herbicides. A 63% increase in cereal rye biomass production was observed within the IWM treatment in 2019, but only a 22% and 33% increase in 2020 and 2021, respectively. In 2020, significantly lower volumetric water content (%VWC) was observed within the IWM treatment in dates closest to planting and greater %VWC at multiple dates in June and July compared to the Standard treatment. No differences occurred in soybean populations, but soybean biomass at the V4 growth stage was reduced in the Standard treatment compared to the IWM treatment, which we attribute to injury from preemergence applications. The Standard treatment resulted in greater soybean yield (2,590 kg ha–1) than the IWM treatment (1,870 kg ha–1) in 2020, but yields were similar in other years. The IWM treatment resulted in 58% fewer herbicide inputs, as measured by the number of active ingredients applied, compared to the Standard over the 3-yr study. Yet, peak weed biomass did not differ between treatments. However, the IWM treatment resulted in greater total horseweed density and the number of horseweed plants that exceeded recommended size-based height thresholds (10 cm) compared to the Standard treatment just prior to postemergence applications (35–42 d after planting) in 2020 and 2021, underscoring the importance of integrating surface mulch residues with effective herbicide sites of action.

Waterhemp control in Ontario has increased in complexity due to the evolution of biotypes that are resistant to five herbicide modes of action (Groups 2, 5, 9, 14, and 27 as categorized by the Weed Science Society of America). Four field trials were carried out over a 2-yr period in 2021 and 2022 to assess the control of multiple-herbicide-resistant (MHR) waterhemp biotypes in glyphosate/glufosinate/2,4-D-resistant (GG2R) soybean using one- and two-pass herbicide programs. S-metolachlor/metribuzin, pyroxasulfone/sulfentrazone, pyroxasulfone/flumioxazin, and pyroxasulfone + metribuzin applied preemergence (PRE) controlled MHR waterhemp similarly by 46%, 63%, 60%, and 69%, respectively, at 8 wk after postemergence (POST) application (WAA-B). A one-pass application of 2,4-D choline/glyphosate DMA POST provided greater control of MHR waterhemp than glufosinate. Two-pass herbicide programs of a PRE herbicide followed by (fb) a POST-applied herbicide resulted in greater MHR waterhemp control compared to a single PRE or POST herbicide application. PRE herbicides fb glufosinate or 2,4-D choline/glyphosate DMA POST controlled MHR waterhemp by 74% to 91% and by 84% to 96%, respectively, at 8 WAA-B. Two-pass herbicide applications of an effective PRE residual herbicide fb 2,4-D choline/glyphosate DMA POST in GG2R soybean can effectively manage waterhemp that is resistant to herbicides in Groups 2, 5, 9, 14, and 27.

Narrow-windrow burning (NWB) is a form of harvest weed seed control in which crop residues and weed seeds collected by the combine are concentrated into windrows and subsequently burned. The objectives of this study were to determine how NWB will 1) affect seed survival of Italian ryegrass in wheat and Palmer amaranth in soybean and 2) determine whether a relationship exists between NWB heat index (HI; the sum of temperatures above ambient) or effective burn time (EBT; the cumulative number of seconds temperatures exceed 200 C) and the post-NWB seed survival of both species. Average soybean and wheat windrow HI totaled 140,725 ± 14,370 and 66,196 ± 6224 C, and 259 ± 27 and 116 ± 12 s of EBT, respectively. Pre-NWB versus post-NWB germinability testing revealed an estimated seed kill rate of 79.7% for Italian ryegrass, and 86.3% for Palmer amaranth. Non-linear two-parameter exponential regressions between seed kill and HI or EBT indicated NWB at an HI of 146,000 C and 277 s of EBT potentially kills 99% of Palmer amaranth seed. Seventy-six percent of soybean windrow burning events resulted in estimated Palmer amaranth seed kill rates greater than 85%. Predicted Italian ryegrass seed kill was greater than 97% in all but two wheat NWB events; therefore, relationships were not calculated. These results validate the effectiveness of the ability of NWB to reduce seed survival, thereby improving weed management and combating herbicide resistance.

Limited information exists on the global economic impact of glyphosate-resistant (GR) weeds. The objective of this manuscript was to estimate the potential yield and economic loss from uncontrolled GR weeds in the major field crops grown in Ontario, Canada. The impact of GR weed interference on field crop yield was determined using an extensive database of field trials completed on commercial farms in southwestern Ontario between 2010 and 2021. Crop yield loss was estimated by expert opinion (weed scientists and Ontario government crop specialists) when research data were unavailable. This manuscript assumes that crop producers adjust their weed management programs to control GR weeds, which increases weed management costs but reduces crop yield loss from GR weed interference by 95%. GR volunteer corn, horseweed, waterhemp, giant ragweed, and common ragweed would cause an annual monetary loss of (in millions of Can$) $172, $104, $11, $3, and $0.3, respectively, for a total annual loss of $290 million if Ontario farmers did not adjust their weed management programs to control GR biotypes. The increased herbicide cost to control GR volunteer corn, horseweed, waterhemp, giant ragweed, and common ragweed in the major field crops in Ontario is estimated to be (in millions of Can$) $17, $9, $2, $0.1, and $0.02, respectively, for a total increase in herbicide expenditures of $28 million annually. Reduced GR weed interference with the adjusted weed management programs would reduce farm-gate monetary crop loss by 95% from $290 million to $15 million. This study estimates that GR weeds would reduce the farm-gate value of the major field crops produced in Ontario by Can$290 million annually if Ontario farmers did not adjust their weed management programs, but with increased herbicide costs of Can$28 million and reduced crop yield loss of 95% the actual annual monetary loss in Ontario is estimated to be Can$43 million annually.

The use of different herbicide-resistant soybean technologies in proximity could lead to injury and yield loss due to an application error. Research was initiated to evaluate the effect of 6.25%, 12.5%, and 100% doses of isoxaflutole and mesotrione field use doses applied postemergence to 4-hyroxyphenylpyruvate (HPPD) inhibitor-susceptible soybean, representing physical drift or misapplication doses. Visible injury manifested as chlorosis with slight necrosis, progressing to necrosis and height reduction, with visible height reduction as the only symptom. Isoxaflutole at 6.25% and 12.5% injured V2 soybean more than V1 or V4 soybean at all evaluation dates. This is supported by soybean height data at 14, 28, and 42 d after treatment (DAT). Following the 100% dose, maximum injury occurred at 28 DAT with V1, V2, and V4 soybean injured 90%, 85%, and 72%, but injury declined over time. All isoxaflutole treatments reduced yield at least 310 kg ha−1, with no differences among application timings or between the two lowest doses for a revenue loss of US$147 ha−1 to US$623 ha−1. Visible injury following mesotrione manifested as chlorosis and necrosis advancing to visible height reduction. Following mesotrione at 6.25% and 12.5%, injury ranged from 20% to 36% among application timings and did not differ at 3 to 21 DAT. Soybean heights at 28 and 42 DAT do not support injury observations; therefore, the greater injury was due to chlorosis and necrosis. Following mesotrione at 100%, sensitivity decreased at 3 to 14 DAT when applied to later growth stages, with no differences at 42 DAT. Yields did not differ among application timings, but yield losses were at least 240 kg ha−1, with revenue losses of US$60 ha−1 and US$373 ha−1. Producers are cautioned to prevent off-target movement or errant application of isoxaflutole or mesotrione to HPPD inhibitor-susceptible soybean.

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.

A 3-yr field study was conducted in Keiser, AR, to investigate the response of the naturally occurring weed flora, dominated by Palmer amaranth, under various combinations of 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicide-based programs and crop rotation sequences. In the first year, corn plots were established with three corn HPPD-based herbicide programs designed to represent a range of efficacies and selection pressures for resistance. In the following two years, corn as monoculture or with soybean and/or cotton crops was included in the rotation sequence for selected herbicide programs. Weed emergence, weed biomass, and soil seedbank were assessed through the entire experimental period. The results show that crop rotation, especially a rotation sequence with corn followed by (fb) soybean fb cotton, and the lowest-risk herbicide program involving seven sites of action over the course of the entire crop rotation was effective in reducing the emergence of naturally occurring weeds, including Palmer amaranth, prickly sida, morningglory species, and grass weeds (broadleaf signalgrass, large crabgrass, barnyardgrass, and johnsongrass) by 88.3%, 57.5%, 28.7%, and 76.3%, respectively. Treatments without crop rotation (corn as monoculture for 3 consecutive years) and poor herbicide programs, with one site of action, increased weed emergence, notably of Palmer amaranth and prickly sida, by 73.5% and 74.1%, respectively. The soil seedbank showed a similar trend to weed emergence. This study highlights the fact that reducing the weed seedbank cannot rely on one management practice but requires a multitactic approach with various control methods. HPPD-inhibiting herbicide programs seem to be effective on Palmer amaranth when coupled with crop rotation and should be used with other best management practices.

Limited information exists on the critical time of weed removal (CTWR) with the currently used soybean cultivars in Ontario. A study consisting of eight field experiments was conducted from 2017 to 2019 in Ontario, Canada, to determine the impact of delayed postemergence (POST) herbicide application on soybean yield based on average weed height at application, days after crop emergence (DAE), accumulated crop heat units (CHU) from the date of planting, and soybean growth stage. The regression model estimated the weed size at herbicide application that led to 1%, 2.5%, and 5% yield loss in soybean was 9, 14, and 20 cm under low weed density (averaging 73 to 134 plants m−2) and 3, 4, and 6 cm under high weed density (143 to 153 plants m−2) conditions, respectively. The estimated DAE at herbicide application time that led to 2.5%, 5%, 10%, and 25% yield loss in soybean was 24, 30, 37, and 53 DAE under low weed density and 8, 10, 14, and 23 DAE under high weed density, respectively. The predicted crop stage at herbicide application that resulted in 2.5%, 5%, 10%, and 25% yield loss in soybean was V4, V5, R2, and R5 under low weed density and VE, VC, V1, and V4 under high weed density, respectively. This study concludes that soybean yield loss is influenced by the weed density (low vs/high) and the time of the first POST herbicide application. When the first POST herbicide application was delayed until soybean was at the V2 stage the monetary loss was Can$20.46 and Can$221.20 ha−1 in low and high weed-density environments, respectively.

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.

Dicamba is a synthetic auxin herbicide that may be applied over the top of transgenic dicamba-tolerant crops. The increasing prevalence of herbicide-resistant weeds has resulted in increased reliance on dicamba-based herbicides in soybean production systems. Because of the high volatility of dicamba it is prone to off-target movement, and therefore concern exists regarding its drift onto nearby specialty crops. The present study evaluates 12 mid-Atlantic vegetable crops species for sensitivity to sublethal rates of dicamba. Soybean, snap bean, lima bean, tomato, eggplant, bell pepper, cucumber, summer squash, watermelon, pumpkin, sweet basil, lettuce, and kale were grown in a greenhouse and exposed to dicamba at 0, 0.056, 0.11, 0.28, 0.56, 1.12, 2.24 g ae ha−1, which is, respectively, 0, 1/10,000, 1/5,000, 1/2,000, 1/1,000, 1/500, and 1/250 of the maximum recommended label rate for soybean application (560 g ae ha−1). Vegetable crop injury was evaluated 4 wk after treatment using visual rating methods and leaf deformation index measurements. Overall, snap bean was the most sensitive crop, with dicamba rates as low as 0.11 g ae ha−1 resulting in significantly higher leaf deformation levels compared with the nontreated control. Other Fabaceae and Solanaceae species also demonstrated high sensitivity to sublethal rates of dicamba with rates ranging 0.28 to 0.56 g ae ha−1 causing higher leaf deformation compared with the nontreated control. While cucumber, pumpkin, and summer squash were no or moderately sensitive to dicamba, watermelon showed greater sensitivity with unique symptoms at rates as low as 0.056 g ae ha−1 based on visual evaluation. Within the range of tested dicamba rates, sweet basil, lettuce, and kale demonstrated tolerance to dicamba with no injury observed at the maximum rate of 2.24 g ae ha−1.

Palmer amaranth has developed resistance to at least seven herbicide sites of action in the Cotton Belt of the United States, leaving producers with fewer options to manage this weed. Previous research with corn and newly commercially released soybean systems have found the use of 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides such as isoxaflutole (IFT) to be effective at managing Palmer amaranth. Consequently, a new transgenic cultivar of cotton is being developed with tolerance to IFT, allowing for in-crop applications of the herbicide. Two separate studies were conducted near Marianna, AR, in 2019 and replicated in 2020, to investigate the crop safety and utility of IFT when added to cotton herbicide programs. Herbicide programs featured IFT as a preemergence or early-postemergence option, residual herbicides in subsequent postemergence applications, and the presence or absence of a layby application. The use of IFT did not significantly impact cotton injury or yield, whereas the use of layered residual herbicides, including IFT, increased Palmer amaranth control compared to those without. Regardless of earlier use of IFT, layby applications were needed for season-long control of Palmer amaranth, entireleaf morningglory, broadleaf signalgrass, and johnsongrass, as evidenced by greater than a 20 percentage point improvement in control of all weeds when a layby application was made. Overall, findings from these studies indicate IFT to be a suitable tool for managing Palmer amaranth and will provide an additional site of action for cotton herbicide programs. Sequential herbicide applications and overlaying residuals were found to be paramount for managing Palmer amaranth throughout the season.

The objectives of this study were to determine if the level and consistency of glyphosate-resistant (GR) horseweed control prior to soybean planting can be improved by (i) adding halauxifen-methyl, 2,4-D ester, saflufenacil, metribuzin, or dicamba to glufosinate, (ii) increasing the rate of glufosinate from 500 to 1,000 g ai ha–1, and (iii) adding 28% urea ammonium nitrate (UAN) as the carrier solution. During a 2-yr period (2020–2021), four field trials were conducted on commercial farms located in southwestern Ontario, Canada, with confirmed GR horseweed. Glufosinate controlled GR horseweed 65%, 66%, and 63% at 2, 4, and 8 wk after application (WAA), respectively, and reduced density and biomass 46% and 33% at 8 WAA, respectively. There was no improvement in GR horseweed control from the addition of halauxifen-methyl, 2,4-D ester or saflufenacil to glufosinate and no decrease in density and biomass, with the exception that the addition of saflufenacil to glufosinate reduced density 30% compared to glufosinate alone. The addition of metribuzin to glufosinate improved GR horseweed control by 22%, 22%, and 28% at 2, 4, and 8 WAA, respectively, and further reduced density and biomass 50% and 47%, respectively, at 8 WAA, respectively. The addition of dicamba to glufosinate improved GR horseweed control by 19%, 26%, and 30% at 2, 4, and 8 WAA, respectively, and further reduced density and biomass 54% and 60%, respectively, at 8 WAA. There was no improvement in GR horseweed control by increasing the rate of glufosinate from 500 to 1,000 g ai ha–1 or when using 28% UAN as the carrier solution. The addition of all herbicides to glufosinate, increasing the rate of glufosinate, or using 28% UAN as the carrier solution improved the consistency of GR horseweed control.

Glyphosate-resistant (GR) horseweed interference in soybean can reduce soybean yield up to 93%. Glyphosate plus dicamba, 2,4-D ester, halauxifen-methyl or pyraflufen-ethyl/2,4-D applied preplant (PP) provide variable GR horseweed control in soybean. The objective of this study was to determine if the addition of saflufenacil or metribuzin to glyphosate plus dicamba, 2,4-D ester, halauxifen-methyl, or pyraflufen-ethyl/2,4-D will improve the level and consistency of GR horseweed control. Four trials were conducted over the 2020 and 2021 field seasons in fields with GR horseweed populations. Glyphosate plus dicamba, 2,4-D ester, halauxifen-methyl, or pyraflufen-ethyl/2,4-D controlled GR horseweed 96%, 77%, 71%, and 52%, respectively, at 8 wk after application (WAA). When saflufenacil or metribuzin was added to glyphosate plus dicamba or 2,4-D ester, GR horseweed control was not improved at 8 WAA. When saflufenacil or metribuzin was added to glyphosate plus halauxifen-methyl, GR horseweed control improved by 27% and 25%, respectively, at 8 WAA. When saflufenacil or metribuzin was added to glyphosate plus pyraflufen-ethyl/2,4-D, GR horseweed control was improved by 47% and 37%, respectively, at 8 WAA. The consistency of GR horseweed control was improved when saflufenacil or metribuzin was added to glyphosate plus dicamba, 2,4-D ester, halauxifen-methyl, or pyraflufen-ethyl/2,4-D compared to each herbicide applied alone. Synergism was observed when metribuzin was added to glyphosate plus halauxifen-methyl and when saflufenacil or metribuzin was added to glyphosate plus pyraflufen-ethyl/2,4-D at 8 WAA. Though GR horseweed control was improved with the addition of saflufenacil or metribuzin to glyphosate plus halauxifen-methyl or pyraflufen-ethyl/2,4-D, all treatments including saflufenacil resulted in the highest level and most consistent control.

Commercialization of 2,4-D-resistant soybean varieties allows for postemergence (POST) applications of 2,4-D in soybean. With the increase in POST applications of 2,4-D in soybean, shifts in weed populations may occur. A long-term field trial was conducted over 7 yr in a corn-soybean rotation. Weed populations were subjected to four herbicide strategies with variable levels of 2,4-D reliance. The strategies used included 1) diversified glyphosate strategy with six herbicide sites of action (SOAs); 2) 2,4-D reliant strategy with three SOAs; 3) diversified 2,4-D reliant strategy with seven SOAs; and 4) fully diversified strategy with eight SOAs. Soil residual herbicides were used for both corn and soybean years, except for the 2,4-D-reliant strategy, which used only a residual herbicide during the corn years. A 52% or greater reduction in weed densities for all herbicide strategies, except the 2,4-D-reliant strategy, was observed by the end of the study. However, the density of weeds tolerant to 2,4-D, such as monocots, increased after 3 yr of selection pressure, and more than doubled after 5 yr of selection pressure in the 2,4-D-reliant strategy. Additionally, in the 2,4-D-reliant strategy with three SOAs, species richness was 30% higher in the soil seedbank compared to herbicides strategies with six or more SOAs. In order to delay weed shifts, diversified herbicide strategies with more than three SOAs that include residual herbicides should be used in corn:soybean rotational systems that use 2,4-D-resistant soybean.

BASF Corp. has developed p-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor–resistant cotton and soybean that will allow growers to use isoxaflutole in future weed management programs. In 2019 and 2020, a multi-state non-crop research project was conducted to examine weed control following isoxaflutole applied preemergence alone and with several tank-mix partners at high and low labeled rates. At 28 d after treatment (DAT), Palmer amaranth was controlled ≥95% at six of seven locations with isoxaflutole plus the high rate of diuron or fluridone. These same combinations provided the greatest control 42 DAT at four of seven locations. Where large crabgrass was present, isoxaflutole plus the high rate of diuron, fluridone, pendimethalin, or S-metolachlor or isoxaflutole plus the low rate of fluometuron controlled large crabgrass ≥95% in two of three locations 28 DAT. In two of three locations, isoxaflutole plus the high rate of pendimethalin or S-metolachlor improved large crabgrass control 42 DAT when compared to isoxaflutole alone. At 21 DAT, morningglory was controlled ≥95% at all locations with isoxaflutole plus the high rate of diuron and at three of four locations with isoxaflutole plus the high rate of fluometuron. At 42 DAT at all locations, isoxaflutole plus diuron or fluridone and isoxaflutole plus the high rate of fluometuron improved morningglory control compared to isoxaflutole alone. These results suggest that isoxaflutole applied preemergence alone or in tank mixture is efficacious on a number of cross-spectrum annual weeds in cotton, and extended weed control may be achieved when isoxaflutole is tank-mixed with several soil-residual herbicides.

Non-dicamba-resistant soybean yield loss resulting from dicamba off-target injury has become an increasing concern for soybean growers in recent years. After off-target dicamba movement occurs onto sensitive soybean, little information is available on tactics that could be used to mitigate the cosmetic or yield losses that may occur. Therefore, a field experiment was conducted in 2017, 2018, and 2019 to determine whether certain recovery treatments of fungicide, plant growth hormone, macro- and micronutrient fertilizer combinations, or weekly irrigation could reduce dicamba injury and/or result in similar yield to soybean that was not injured with dicamba. Simulated drift events of dicamba (5.6 g ae ha−1) were applied to non-dicamba-resistant soybean once they reached the V3 or R2 stages of growth. Recovery treatments were applied approximately 14 d after the simulated drift event. Weekly irrigation was the only recovery treatment that provided appreciable levels of injury reduction or increases in soybean height or yield compared to the dicamba-injured plants. Weekly irrigation following the R2 dicamba injury event resulted in an 1% to 14% increase in soybean yield compared with the dicamba-injured control. All other recovery treatments resulted in soybean yields that were similar to the dicamba-injured control, and similar to or lower than the nontreated control. Results from this study indicate that if soybean have become injured with dicamba, weekly irrigation will help soybean recover some of the yield loss and reduce injury symptoms that resulted from off-target dicamba movement, especially in a year with below average precipitation. However, yield loss will likely not be restored to that of noninjured soybean.

Glyphosate-resistant (GR) Palmer amaranth is a troublesome weed that can emerge throughout the soybean growing season in Nebraska and several other regions of the United States. Late-emerging Palmer amaranth plants can produce seeds, thus replenishing the soil seedbank. The objectives of this study were to evaluate single or sequential applications of labeled POST herbicides such as acifluorfen, dicamba, a fomesafen and fluthiacet-methyl premix, glyphosate, and lactofen on GR Palmer amaranth control, density, biomass, seed production, and seed viability, as well as grain yield of dicamba- and glyphosate-resistant (DGR) soybean. Field experiments were conducted in a grower’s field infested with GR Palmer amaranth near Carleton, NE, in 2018 and 2019, with no PRE herbicide applied. Acifluorfen, dicamba, a premix of fomesafen and fluthiacet-methyl, glyphosate, or lactofen were applied POST in single or sequential applications between the V4 and R6 soybean growth stages, with timings based on product labels. Dicamba applied at V4 or in sequential applications at V4 followed by R1 or R3 controlled GR Palmer amaranth 91% to 100% at soybean harvest, reduced Palmer amaranth density to as low as 2 or fewer plants m−2, reduced seed production to 557 to 2,911 seeds per female plant, and resulted in the highest soybean yield during both years of the study. Sequential applications of acifluorfen, fomesafen and fluthiacet premix, or lactofen were not as effective as dicamba for GR Palmer amaranth control; however, they reduced seed production similar to dicamba. On the basis of the results of this study, we conclude that dicamba was effective for controlling GR Palmer amaranth and reduced density, biomass, and seed production without DGR soybean injury. Herbicides evaluated in this study had no effect on Palmer amaranth seed viability.

Glyphosate-resistant horseweed is difficult to manage in no-tillage crop production fields and new strategies are needed. Cover crops may provide an additional management tool but narrow establishment windows and colder growing conditions in northern climates may limit the cover crop biomass required to suppress horseweed. Field experiments were conducted in 3 site-years in Michigan to investigate the effects of two fall-planted cover crops, cereal rye and winter wheat, seeded at 67 or 135 kg ha−1, to suppress horseweed when integrated with three preplant herbicide strategies in no-tillage soybean. The preplant strategies were control (glyphosate only), preplant herbicide without residuals (glyphosate + 2,4-D), and preplant herbicide with residuals (glyphosate + 2,4-D + flumioxazin + metribuzin). Cereal rye produced 79% more biomass and provided 12% more ground cover than winter wheat in 2 site-years. Increasing seeding rate provided 41% more cover biomass in 1 site-year. Cover crops reduced horseweed density 47% to 96% and horseweed biomass by 59% to 70% compared with no cover at the time of cover crop termination. Cover crops provided no additional horseweed suppression 5 wk after soybean planting if a preplant herbicide with or without residuals was applied, but reduced horseweed biomass greater than 33% in the absence of preplant herbicides. Cover crops did not affect horseweed suppression at the time of soybean harvest or influence soybean yield. Preplant herbicide with residuals and without residuals provided at least 52% and 20% greater soybean yield compared with the control at 2 site-years, respectively. Cereal rye and winter wheat provided early-season horseweed suppression at biomass levels below 1,500 kg ha−1, lower than previously reported. This could give growers in northern climates an effective strategy for suppressing horseweed through the time of POST herbicide application while reducing selection pressure for horseweed that is resistant to more herbicide sites of action.

Horseweed and giant ragweed are competitive, annual weeds that can negatively impact crop yield. Biotypes of glyphosate-resistant (GR) giant ragweed and horseweed were first reported in 2008 and 2010 in Ontario, respectively. GR horseweed has spread throughout the southern portion of the province. The presence of GR biotypes poses new challenges for soybean producers in Canada and the United States. Halauxifen-methyl is a recently registered selective herbicide against broadleaf weeds for preplant use in corn and soybean. There is limited literature on the efficacy of halauxifen-methyl on GR horseweed and giant ragweed when combined with currently registered products in Canada. The purpose of this study was to determine the effectiveness of halauxifen-methyl applied alone and tank-mixed to control GR giant ragweed and GR horseweed in glyphosate and dicamba-resistant (GDR) soybean in southwestern Ontario. Six field experiments were conducted separately for each weed species over 2018 and 2019. Halauxifen-methyl applied alone offered 72% control of GR horseweed at 8 wk after application (WAA). Control was improved to >91% when halauxifen-methyl applied in combination with metribuzin, saflufenacil, chlorimuron-ethyl + metribuzin, and saflufenacil + metribuzin. At 8 WAA, halauxifen-methyl provided 11% control of GR giant ragweed, and 76% to 88% control when glyphosate/2,4-D choline, glyphosate/dicamba, glyphosate/2,4-D choline + halauxifen-methyl, and glyphosate/dicamba + halauxifen-methyl were used. We conclude that halauxifen-methyl applied preplant in a tank-mixture can provide effective control of GR giant ragweed and horseweed in GDR soybean.