Potato producers in Canada’s Atlantic provinces of Prince Edward Island (PE) and New Brunswick rely on photosystem II (PSII)-inhibiting herbicides to provide season-long weed control. Despite this fact, a high proportion of common lambsquarters populations in the region have been identified as resistant to this class of herbicides. Crop-topping is a late-season weed management practice that exploits the height differential between weeds and a developing crop canopy. Two field experiments were conducted in Harrington, PE, in 2020 and 2021, one each to evaluate the efficacy of a different crop-topping strategy, above-canopy mowing or wick-applied glyphosate, at two potato phenological stages, on common lambsquarters viable seed production and potato yield and quality. Mowing common lambsquarters postflowering decreased viable seed production (72% to 91%) in 2020 but increased seed production (78% to 278%) in 2021. Mowing had minimal impact on potato marketable yield across cultivars in both years. In contrast, treating common lambsquarters with wick-applied glyphosate had variable impacts on seed output in 2020 but dramatically reduced seed production (up to 95%) in 2021 when treatments were applied preflowering. Glyphosate damage to potato tubers was not influenced by timing and resulted in a 14% to 15% increase in culled tubers due to black spotting and rot. Our results highlight the importance of potato and common lambsquarters phenology when selecting a crop-topping strategy and demonstrate that above-canopy mowing and wick-applied glyphosate can be utilized for seedbank management of herbicide-resistant common lambsquarters in potato production systems.

Postemergence herbicides used to control weeds in the space between raised, plastic-covered beds in plasticulture production systems are typically banded, and herbicides are applied to weeds and to where weeds do not occur. To reduce the incidence of off-targeted applications, the University of Florida developed a smart-spray technology for row middles in plasticulture systems. The technology detects weed according to categories and applies herbicides only where the weeds occur. Field experiments were conducted at the Gulf Coast Research and Education Center in Balm, FL, in fall 2021 and spring 2022. The objective was to evaluate the efficacy of postemergence applications of diquat and glyphosate in row middles in jalapeno pepper fields when banded or applied with smart-spray technology. The overall precision of the weed detection model was 0.92 and 0.89 for fall and spring, respectively. The actuation precision achieved was 0.86 and 1 for fall and spring, respectively. No significant differences were observed between banded and targeted applications either with glyphosate or diquat in terms of broadleaf, grass, and nutsedge weed density. No significant pepper damage was observed with either herbicide or application technique. The smart-spray technology reduced herbicide application volume by 26% and 42% in fall and spring, respectively, with no reduction in weed control or pepper yield compared to a banded application. Overall, the smart-spray technology reduced the herbicide volume applied with no reductions in weed control and no significant effects on crop yield.

Ethofumesate is a broad-spectrum, soil-applied herbicide used to control broadleaf and grass weeds in sugarbeet crops. Ethofumesate is commonly applied preemergence at rates ranging from 1.25 to 4.2 kg ai ha−1, or applied postemergence (POST), up to 0.38 kg ai ha−1. The Generic Crop Science company has developed a new Ethofumesate 4SC label that has increased ethofumesate POST rates up to 4.48 kg ha−1 in sugarbeet with more than two true leaves per plant. Field and greenhouse experiments were conducted in 2018 and 2019 to evaluate sugarbeet tolerance and herbicide efficacy. Field tolerance experiments indicated sugarbeet stature from ethofumesate applied POST at 0.28, 0.56, and 1.12 kg ha−1 was the same as that of the nontreated control, but ethofumesate at 2.24 kg ha−1 reduced sugarbeet stature, although that rate did not affect yield components. Ethofumesate applied POST at 4.48 kg ha−1 reduced sugarbeet stature and affected sugarbeet yield components. Ethofumesate applied alone POST provided weed control of up to 85%, 76%, and 84% on common lambsquarters, redroot pigweed, and waterhemp, respectively, in field efficacy experiments. Mixing ethofumesate at 1.12 kg ha−1 with glyphosate does not provide a second effective herbicide for POST control of common lambsquarters or redroot pigweed, but it does provide residual control of these weeds when at least one-half inch of penetrating rainfall occurs, following application. In greenhouse experiments, ethofumesate alone or ethofumesate plus glyphosate applied to common lambsquarters, redroot pigweed, or waterhemp at heights of less than 2.5 cm provided the best combination of burndown and soil residual control compared with weeds that were 2.5 to 5 cm tall. Ethofumesate applied POST at 1.12 kg ha−1 plus glyphosate provided the best combination of tolerance and efficacy, especially on waterhemp.

With the increase in hectares planted to auxin-resistant cotton, the number of preplant, at-plant, and postplant applications of dicamba and 2,4-D choline to aid in the control of troublesome broadleaf weeds, including glyphosate-resistant Palmer amaranth, has increased. More dicamba and 2,4-D choline applications mean an increased risk of off-target movement. Field studies were conducted in 2019 to 2021 at the Texas Tech University New Deal Research Farm to evaluate dicamba-resistant cotton response to various rates of 2,4-D choline when applied at four growth stages (first square [FS] + 2 wk, first bloom [FB], FB + 2 wk, and FB + 4 wk). Applications of 2,4-D choline were applied at 1,060 (1X), 106 (1/10X), 21 (1/50X), 10.6 (1/100X), 2.1 (1/500X), and 1.06 (1/1000X) g ae ha−1 to Deltapine 1822 XF cotton. Relative to the nontreated control, yield losses were observed in all years at FS + 2 wk and FB from rates of 2,4-D choline ≥ 1/100X. At the FB + 4 wk application, only the 1X rate of 2,4-D choline resulted in a yield reduction in all three years. Micronaire, fiber length, and uniformity were negatively influenced by the 1/10X and 1X rates of 2,4-D choline at various timings in 2019, 2020, and 2021. In addition, short fiber content, neps, and seed coat neps increased where micronaire, fiber length, and uniformity were negatively impacted.

Turfgrass managers apply nonselective herbicides to control winter annual weeds during dormancy of warm-season turfgrass. Zoysiagrass subcanopies, however, retain green leaves and stems during winter dormancy, especially in warmer climates. The partially green zoysiagrass often deters the use of nonselective herbicides due to variable injury concerns in transition and southern climatic zones. This study evaluated zoysiagrass response to glyphosate and glufosinate applied at four different growing degree day (GDD)-based application timings during postdormancy transition in different locations, including Blacksburg, VA; Starkville, MS; and Virginia Beach, VA, in 2018 and 2019. GDD was calculated using a 5 C base temperature with accumulation beginning January 1 each year, and targeted application timings were 125, 200, 275, and 350 GDD5C. Zoysiagrass injury response to glyphosate and glufosinate was consistent across a broad growing region from northern Mississippi to coastal Virginia, but it varied by application timing. Glyphosate application at 125 and 200 GDD5C can be used safely for weed control during the postdormancy period of zoysiagrass, while glufosinate caused unacceptable turf injury regardless of application timing. Glyphosate and glufosinate exhibited a stepwise increase to maximum injury with increasing targeted GDD5C application timings. Glyphosate applied at 125 or 200 GDD5C did not injure zoysiagrass above a threshold of 30%, whereas glufosinate caused greater than 30% injury for 28 and 29 d when applied at 125 and 200 GDD5C, respectively. Likewise, glyphosate application at 125 or 200 GDD5C did not affect the zoysiagrass green cover area under the progress curve per day, whereas later applications reduced it. Glyphosate and glufosinate caused greater injury to zoysiagrass when applied at greater cumulative heat units and this was attributed to increasing turfgrass green leaf density, because heat unit accumulation is positively correlated with green leaf density. Accumulated heat unit-based application timing will allow practitioners to apply nonselective herbicides with reduced injury concerns.

Annual bluegrass is a troublesome weed in turfgrass, with reported resistance to at least 12 herbicide sites of action. The mitotic-inhibiting herbicide pronamide has both preemergence and postemergence activity on susceptible annual bluegrass populations. Previous studies suggest that postemergence activity may be compromised due to lack of root uptake, as well as target-site- and translocation-based mechanisms. Research was conducted to determine the effects of spray droplet spectra on spray coverage and control of annual bluegrass with pronamide, flazasulfuron, and a mixture of pronamide plus flazasulfuron. Herbicides were delivered to annual bluegrass plants having two to three leaves via five different spray spectra based on volume median diameters (VMD) of 200, 400, 600, 800, and 1,000 µm. Fluorescent tracer dye was added to each treatment solution to quantify the effects of herbicide and spray droplet spectra on herbicide deposition. In another experiment, the efficacy of 0.58, 1.16, and 2.32 kg pronamide ha−1; 0.022, 0.044, and 0.088 kg flazasulfuron ha−1, or a combination of the two, were assessed in iteration with droplet spectrum sprays of 400 and 1,000 µm on two pronamide-resistant and two pronamide-susceptible annual bluegrass populations. Spray droplet spectrum affected the deposition of pronamide and flazasulfuron, applied alone and in combination. Pronamide foliar deposition decreased with increasing droplet spectra. Pronamide efficacy was affected by droplet spectrum, with the largest (1,000 µm) exhibiting improved control. Flazasulfuron efficacy and pronamide plus flazasulfuron efficacy were not affected by droplet spectra. Pronamide plus flazasulfuron mixture controlled all four populations more effectively than pronamide alone, regardless of droplet spectra. A mixture of pronamide plus flazasulfuron applied with relatively large droplets may be optimal for annual bluegrass control, which offers valuable insights for optimizing herbicide application and combatting herbicide resistance. However, applications in this controlled-growth pot study may not mimic conditions in which thatch and turfgrass canopy limit the soil deposition of pronamide.

Shattercane is a problematic summer annual grass weed species in regions that produce grain sorghum. Three shattercane populations (DC8, GH4, and PL8) collected from sorghum fields from northwestern Kansas survived the field-use rate (52 g ha−1) of postemergence-applied imazamox. The main objectives of this research were to 1) confirm and characterize the level of resistance to imazamox in putative imazamox-resistant (IMI-R) shattercane populations, 2) investigate the underlying mechanism of resistance, and 3) determine the effectiveness of postemergence herbicides for controlling IMI-R populations. A previously known imazamox susceptible (SUS) shattercane population from Rooks County, KS, was used. All three putative populations exhibited a 4.1-fold to 6.0-fold resistance to imazamox compared with the SUS population. The ALS gene sequences from all IMI-R populations did not reveal any known target-site resistance mutations. A pretreatment with malathion, which inhibits cytochrome P450, followed by imazamox at various doses, reversed the resistance phenotype of the PL8 population. In a separate greenhouse study, postemergence treatments with nicosulfuron, quizalofop, clethodim, and glyphosate resulted in ≥96% injury to all IMI-R populations. The lack of known ALS target-site mutations and the reversal of resistance phenotype by malathion suggest the possibility of metabolism-based resistance to imazamox in PL8 shattercane population.

Glyphosate-resistant (GR) horseweed is a problematic weed for Michigan soybean growers. Additionally, rosette- and upright-horseweed growth types have been observed co-emerging during mid- to late summer in several Michigan fields. In the greenhouse, shade levels from 35% to 92% reduced rosette- and upright-horseweed biomass 31% to 99% compared with the upright growth type grown under 0% shade. Greater reductions in biomass occurred under 69% and 92% shade. Thus, increased shading by planting in narrow rows and/or planting green into cereal rye may improve horseweed suppression. A field experiment conducted over 3 site-years compared the effect of fall-planted cereal rye terminated with glyphosate 1 wk after planting (WAP; planting green) with a preemergence residual herbicide program (glyphosate + 2,4-D + flumioxazin + metribuzin) on horseweed control in soybean planted in three row widths (19, 38, and 76 cm). Planting green or applying a residual herbicide program across all row widths reduced horseweed biomass 86% to 91% and 95% to 99%, respectively, compared with soybean planted with no cover in 76-cm rows, 4 to 6 WAP. At soybean harvest, when a noneffective postemergence herbicide (glyphosate) was applied, horseweed biomass was 42% and 81% lower by planting green or applying a residual-herbicide program compared with no cover, respectively. Similarly, planting soybean in 19-cm rows reduced horseweed biomass compared with 38- and 76-cm rows. When an effective postemergence program was applied, similar horseweed biomass reductions were observed by planting green or applying a residual herbicide across all row widths. Additionally, soybean yield and economic returns were similar between planting green and applying a residual herbicide in 1 of 2 site-years. Integrating planting green and an effective postemergence herbicide program offers an alternative horseweed management strategy to applying a residual preemergence herbicide program.

Multiple herbicide-resistant (MHR) kochia is a serious concern in the U.S. Great Plains and warrants alternative herbicide mixtures for its control. Greenhouse and field experiments were conducted at Kansas State University research and extension centers near Hays and Garden City, KS, to investigate the interactions of 2,4-D, dichlorprop-p, dicamba, and halauxifen/fluroxypyr premix in various combinations for MHR kochia control. Two previously confirmed MHR (resistant to glyphosate, dicamba, and fluroxypyr) populations and a susceptible population were tested in a greenhouse study. Kochia at the Hays field site was resistant to glyphosate and chlorsulfuron, whereas the population at Garden City was resistant to glyphosate, dicamba, and fluroxypyr. Results from a greenhouse study indicated that 2,4-D, dicamba, dichlorprop-p, and a halauxifen/fluroxypyr premix provided 26% to 69% control of both MHR populations at 28 d after treatment (DAT). However, the control increased to 85% to 97% when these herbicides were applied in three-way mixtures. Synergistic interactions were observed when dicamba was mixed with dichlorprop-p, 2,4-D, dichlorprop-p + 2,4-D, and halauxifen/fluroxypyr + 2,4-D for shoot dry weight reductions (86% to 92%) of both MHR populations. Results from a field study also indicated synergistic interactions when dicamba was mixed with dichlorprop-p + 2,4-D, halauxifen/fluroxypyr + dichlorprop-p, and halauxifen/fluroxypyr + 2,4-D, resulting in 84% to 95% control of MHR kochia at 28 DAT across both sites. These results indicate that synergistic effects of mixing dicamba with other auxinic herbicides in two- or three-way mixtures can help control MHR kochia.

Only a limited number of herbicides are available to provide postemergence (POST) control of selective monocot weeds in grain sorghum crops. The herbicides currently labeled for use with grain sorghum have strict use restrictions, low efficacy on johnsongrass, or weed resistance issues. To introduce a new effective herbicide mode of action for monocot control, multiple companies and universities have been developing herbicide-resistant grain sorghum that would allow producers to use herbicides that inhibit either acetolactate synthase (ALS) or acetyl coenzyme A carboxylase (ACCase) for POST monocot control. An experiment was conducted in Fayetteville, AR, in 2020 and 2021, to determine the effectiveness of two ALS-inhibiting herbicides and nine ACCase-inhibiting herbicides on TamArk™ grain sorghum, conventional grain sorghum, and problematic monocot weed species. Grain sorghum and monocot weeds (johnsongrass, broadleaf signalgrass, barnyardgrass, and Texas panicum) were sprayed when TamArk grain sorghum reached the 2- to 3-leaf stage. TamArk grain sorghum was tolerant of all ACCase-inhibiting herbicides tested, exhibiting ≤10% injury at all evaluation timings, except clethodim and sethoxydim, and had no resistance to the ALS-inhibiting herbicides that were evaluated. Additionally, all ACCase inhibitors except diclofop and pinoxaden controlled all monocots tested by >91% at 28 d after application (DAA). Conversely, the two ALS inhibitors, imazamox and nicosulfuron, provided ≤81% control of broadleaf signalgrass 28 DAA but still controlled all other monocots by >95%. TamArk grain sorghum has low sensitivity to multiple ACCase-inhibiting herbicides and thus provides an effective POST option for monocot weed control. In addition, unwanted volunteer TamArk plants can be controlled with cledthodim, sethoxydim, nicosulfuron, or imazamox. Although the ALS-inhibiting herbicides imazamox and nicosulfuron were not useful on TamArk grain sorghum, they are effective options for monocot control on Igrowth™ and Inzen™ grain sorghum crops, respectively.

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.

Delaying cover crop termination until cash crop planting (i.e., planting green) is an emerging no-till practice. Improved management recommendations are needed for optimizing weed suppression benefits while minimizing other pest, fertility, and crop management risks when planting green in corn production systems. In a 2-yr field experiment, we evaluated the interaction between cereal rye residue management tactics (standing residue, roll-crimping, roll-crimping with row cleaners) and herbicide programs (1-pass preemergence [PRE], 2-pass postemergence [POST]) when planting green on weed recruitment spatial patterns and corn performance compared to standard termination (14 d preplant [DPP]) and ryelage harvest (14 DPP) practices. In a 2-yr on-farm experiment, we evaluated corn performance in response to the same residue management tactics. Cereal rye biomass production varied significantly across years in on-station experiments, with average (4.9 Mg ha−1) and anomalous (9.9 Mg ha−1) levels observed in 2020 and 2021, respectively. In 2020, planting green with an integrated roll-crimper/row cleaner system resulted in greater intrarow weed density compared with planting green into standing cereal rye. Interrow weed density was lower when roll-crimping was employed compared to early termination (14 DPP). Planting green into standing cereal rye resulted in greater mean corn height (V5 stage) compared to other treatments, but corn population and yield did not differ. In 2021, few differences in weed recruitment patterns were observed, but corn population and yield were significantly lower in planting green treatments compared to early termination. In both years, late-season weed biomass was lower in two-pass POST programs compared to one-pass PRE programs. On-farm trials showed that planting green into standing residue increases corn height and can reduce corn populations, which may lead to reduce yields. Our results suggest that management recommendations for optimizing herbicide application timing should consider intrarow and interrow weed recruitment dynamics associated with residue management tactics needed to optimize corn performance.

Off-target movement of 2,4-D and dicamba is sometimes to blame as the cause of symptoms observed in weeds growing in production fields. Pesticide regulatory authorities routinely sample tissues of weeds or crops from fields under investigation for potential illegal use of auxin herbicides. This research aimed to determine if analytical tests of herbicide residue on soybean or Palmer amaranth vegetation treated with dicamba or 2,4-D could be used to differentiate between rates applied and how the residue levels decay over a 1-mo interval. Four rates of each herbicide (1X, 0.1X, 0.01X, and 0.001X) were applied, with a 1X rate of dicamba and 2,4-D assumed to be 560 and 1,065 g ae ha−1, respectively. Experiments included dicamba- and 2,4-D-resistant soybean (Xtend® and Enlist® traits, respectively) and Palmer amaranth categorized by size (8 to 15 cm, 20 to 30 cm, and 35 to 50 cm in height). Analytical results show that herbicide residues were detected above detection limits of 0.04 µg g−1 for dicamba and 0.004 µg g−1 for 2,4-D, respectively, particularly for samples treated with a 1X and 0.1X rate of dicamba or 2,4-D. Nondetections were frequent, even as early as 2 to 3 d after treatment (DAT), with 0.01X and 0.001X rates of 2,4-D or dicamba. Residues declined rapidly on Xtend® soybean treated with dicamba and on Enlist® soybean treated with 2,4-D. The severity of auxin symptomology generally agreed with the ability to detect dicamba or 2,4-D residue in plant tissue for Palmer amaranth, whereas for soybean, this was not always the case. Hence detecting dicamba or 2,4-D residues in both Palmer amaranth and soybean vegetation, along with visible symptoms on both plants during investigations, would generally indicate an earlier direct application of the auxin herbicide rather than off-target movement being the cause of detection.

The goal of weed science extension efforts are to encourage and accelerate adoption of diverse, effective, and economical management tactics. To be most successful and efficient, extension personnel need to know how growers prefer to receive information, the format in which the information is delivered, and areas that future extension research should focus on. To this end, surveys were distributed at crop and forage extension meetings in Virginia. The results from 249 responses indicate that both crop and forage producers have similar preferences. Agribusiness personnel (e.g., co-ops, suppliers, vendors, crop consultants, sales representatives) had the greatest influence on herbicide-purchasing decisions and were the primary source of information for producers who make weed management decisions, and thus should be a target audience of extension. Respondents said that economic assessments, weed control data, and yield data are most likely to influence changes in their management practices and that they would prefer to receive that information through traditional extension formats (presentations, publications, and on-farm demonstrations). Generally, respondents also indicated that they wanted extension efforts to focus on evaluating new herbicides for weed control and crop safety in the future over alternative nonherbicidal weed control methods. Therefore, extension personnel are likely to be more successful by including herbicides in the practice of integrated weed management rather than relying solely on nonchemical approaches.

Several species of Echinochloa P. Beauv., introduced at multiple events, have established themselves as a persistent concern for U.S. rice production. In this review, we highlight the key biological characteristics of economically relevant Echinochloa in U.S. rice production, revisit their historical trajectory, and suggest research directions for their management with special reference to barnyardgrass. Ecologically differentiated Echinochloa species have a distinct association with rice culture methods that have been practiced for the last few decades, barnyardgrass being historically predominant in drill-seeded rice in the mid-South, and early watergrass and late watergrass in water-seeded California rice. However, the emerging evidence challenges the dogma that other Echinochloa species for specific regions are of less importance. Primarily managed by the water-seeding method of rice culture in the early years of the 20th century, Echinochloa species have persisted in the sophisticated U.S. rice culture through the evolution of resistance to herbicides in recent decades. Accumulating knowledge, including those of recent genomic insights, suggests the rapid adaptability of Echinochloa. The last decade has seen a (re)emergence of nonchemical methods as a key component of sustainable management, among which use of harvest weed seed control (HWSC) methods and cover crops in the mid-South and stale-drill seeding in California are being considered as potential methods for managing Echinochloa. In recent years, furrow-irrigated rice has rapidly supplanted a significant proportion of conventionally flooded rice in the mid-South, whereas the propensity for compromised continuous submergence is increasing in California rice. On the cusp of this shift, the question at the forefront is how this will affect Echinochloa interference in rice and how this change will dictate the management efforts. Future research will lead to the development of a clear understanding of the impact of the changing agroecosystems on Echinochloa species and their response to the prospective integrated control interventions.