Corn that is resistant to aryloxyphenoxypropionate, known commercially as Enlist™ corn, enables the use of quizalofop-p-ethyl (QPE) as a selective postemergence (POST) herbicide for control of glufosinate/glyphosate-resistant corn volunteers. Growers usually mix QPE with 2,4-D choline or glufosinate or both to achieve broad-spectrum weed control in Enlist corn. The objectives of this study were 1) to evaluate the efficacy of QPE applied alone or mixed with 2,4-D choline and/or glufosinate to control glufosinate/glyphosate-resistant corn volunteers in Enlist corn and 2) to determine the effect of application time (V3 or V6 growth stage of volunteer corn) of QPE-based treatments on volunteer corn control and Enlist corn injury and yield. Field experiments were conducted in Clay Center, NE, in 2021 and 2022. Quizalofop-p-ethyl (46 or 93 g ai ha−1) applied at the V3 or V6 growth stage controlled volunteer corn by ≥88% and ≥95% at 14 and 28 d after treatment (DAT), respectively. QPE (46 g ai ha−1) mixed with 2,4-D choline (800 g ae ha−1) produced 33% less than expected control of V3 volunteer corn in 2021, and 8% less than expected control of V6 volunteer corn in 2022 at 14 DAT. Volunteer corn control was improved by 7% to 9% using the higher rate of QPE (93 g ai ha−1) in a mixture with 2,4-D choline (1,060 g ae ha−1). QPE mixed with glufosinate had an additive effect and interactions in any combinations were additive beyond 28 DAT. Mixing 2,4-D choline can reduce QPE efficacy on glufosinate/glyphosate-resistant corn volunteers up to 14 DAT when applied at the V3 or V6 growth stage; however, the antagonistic interaction did not translate into corn yield loss. Increasing the rate of QPE (93 g ai ha−1) while mixing with 2,4-D choline can reduce antagonism.

Herbicide resistance coupled with a dearth of selective herbicide options has increased the complexity of annual bluegrass control in hybrid bermudagrass putting greens. Cumyluron, endothall, and methiozolin are herbicides that control annual bluegrass by inhibiting novel sites of action compared with the herbicides currently used for turfgrass management in the United States. However, peer-reviewed literature contains no information on hybrid bermudagrass putting green tolerance to these herbicides. Sixteen field studies were established on eight golf greens in Midlothian, VA, in 2021 and 2022 to evaluate effects of cumyluron, endothall, methiozolin, pronamide, and trifloxysulfuron on bermudagrass spring transition. The 16 studies were split equally between initiation during full dormancy versus mid-spring transition. Methiozolin applied at 500 and 1,000 g ai ha−1 typically increased the heat units (growing degree days with a base temperature of 15 C) required for hybrid bermudagrass to visibly achieve 90% green coverage (T90) when applied to fully dormant hybrid bermudagrass. This delay in green coverage was more pronounced at sites where hybrid bermudagrass vigor was seemingly reduced via abiotic stressors. Endothall was generally more injurious than all other treatments when applied to hybrid bermudagrass during mid-transition. Endothall applied at 840 g ai ha−1 injured hybrid bermudagrass for 0 to 9 d over a threshold of 30% (DOT30), depending on location. In two site-years characterized by increased abiotic stress, methiozolin applied at 1,000 g ai ha−1 caused 44 DOT30. Cumyluron never injured hybrid bermudagrass by more than 30% or delayed T90 regardless of application timing. These results indicate that methiozolin should be applied only within labeled rates to actively growing hybrid bermudagrass putting greens, cumyluron can be safely applied at 6,450 g ai ha−1 to dormant or actively growing bermudagrass greens, and endothall applications should be limited to dormant bermudagrass greens unless transient phytotoxicity is acceptable.

Herbicides are often used to terminate cover crops. Producers would like to use herbicides that work quickly, are effective, and do not increase the risk of selecting herbicide-resistant weeds. Eight experiments were conducted to determine whether mixing glyphosate (900 g a.e. ha−1) with rimsulfuron (15 g a.i. ha−1), mesotrione (100 g a.i. ha−1), or rimsulfuron + mesotrione enhances winter rye control and to ascertain whether using urea ammonium nitrate (UAN) as the herbicide carrier improves and accelerates herbicide efficacy. Winter rye control was assessed 1, 2, 3, and 4 wk after application (WAA) and biomass was measured 4 WAA. The addition of rimsulfuron, mesotrione, or rimsulfuron + mesotrione to glyphosate did not enhance winter rye control. Similarly, using UAN as the herbicide carrier did not improve or accelerate herbicide efficacy. Glyphosate alone provided the greatest level of winter rye control. The addition of rimsulfuron, mesotrione, or rimsulfuron + mesotrione to glyphosate did not increase the level or speed of control. However, mixing glyphosate with rimsulfuron, mesotrione, or rimsulfuron + mesotrione adds other modes of action without compromising winter rye control.

The increasing development of herbicide resistance in weeds found in rice cropping systems has encouraged researchers to evaluate alternate herbicides to prevent and manage herbicide-resistant weed biotypes. Metribuzin is a photosynthetic-inhibiting herbicide that controls various important grass and broadleaf weeds. Several crops, including soybean, wheat, peas, and potato, have shown differential varietal responses to metribuzin. To determine whether rice has differential varietal responses to metribuzin for potential utilization in a rice breeding program, greenhouse experiments were conducted to evaluate the responses of 142 long-, medium-, and short-grain rice genotypes to the herbicide. Metribuzin was applied at 0, 22, 44, 88, 176, and 352 g ai ha−1 when rice plants were in the 3- to 4-leaf stage. Crop response regarding phytotoxicity, height reduction, and biomass reduction was evaluated. Metribuzin caused significant injury to all rice genotypes tested, but short-grain rice genotypes were, on average, more susceptible than medium- and long-grain rice genotypes. Short-grain rice genotypes generally had greater height reduction and produced less biomass than long-grain or medium-grain rice genotypes. Crop visual injury ratings were correlated with plant height reductions and biomass reductions. The results indicate that the level of metribuzin tolerance in rice is inadequate for commercial use; however, further research is needed to develop higher levels of herbicide resistance by mutagenized rice cultivars.

Smutgrass is a non-native perennial weed that is problematic because of its poor palatability to cattle and its difficulty to control once established. Limited literature exists to explain the effectiveness of herbicides other than hexazinone for smutgrass control and forage injury. This study aimed to evaluate seasonal applications of labeled herbicides used on forage for maximum smutgrass control. The second objective was to evaluate preemergent herbicides and hexazinone for their ability to control smutgrass germinating from seed. Hexazinone, nicosulfuron + metsulfuron-methyl, and glyphosate + imazapic were the most effective postemergence treatments, while quinclorac exhibited little activity on smutgrass. Common bermudagrass forage fully recovered from all treatments by 3 mo after treatment. Hexazinone, nicosulfuron + metsulfuron methyl, glyphosate, and imazapic were applied postemergence to smutgrass in spring, summer, and fall. Summer applications of hexazinone resulted in the greatest level of control, while spring treatments provided the least control. Applications of hexazinone or glyphosate resulted in the most effective smutgrass control. However, fall applications resulted in the least forage injury. Results of the study of preemergence herbicides indicate that treatments with indaziflam and hexazinone provide adequate control of germinating smutgrass seedlings in the greenhouse at 0.25×, 0.5×, and 0.75× of the lowest recommended labeled rate for seedling grass control. Indaziflam treatments prevented the emergence of any visible smutgrass seedling tissue, compared to hexazinone, which fully controlled the germinating seedlings by 21 d after treatment, whereas pendimethalin significantly reduced seedling numbers at the 0.5× and 0.75× rates.

Few herbicides are registered for goosegrass control in creeping bentgrass turfgrass. Topramezone controls goosegrass and is labeled for use on creeping bentgrass, but potential injury risks lead many turf managers to frequently apply it at a low-dose. This application practice increases the likelihood that topramezone treatments will be mixed with fungicide treatments. Previous research found that fungicides can reduce the activity of some herbicides, but their effects on topramezone efficacy are unknown. Four studies were established between Blacksburg, VA, and North Brunswick, NJ, in 2021 to determine whether chlorothalonil reduces goosegrass control from topramezone. In controlled environment dose-response studies the amount of topramezone needed to reduce goosegrass biomass by 50% increased from 3.04 g ha−1 to 5.27 g ha−1 when chlorothalonil (7,400 g ha−1) was added to the mixture. In field experiments, topramezone at 3.7 and 6.1 g ha−1 controlled goosegrass by 50% and 63%, respectively, at 42 d after treatment when averaged across herbicide admixtures. The addition of chlorothalonil alone and chlorothalonil plus acibenzolar-S-methyl to topramezone reduced goosegrass control from 73% to 52% and 45%, respectively, when averaged across topramezone rate. From these studies we can conclude that chlorothalonil has the potential to reduce goosegrass control when topramezone is applied at the maximum allowable rate (6 g ae ha−1) or less. This is the first report of fungicides acting to reduce herbicidal weed control efficacy in turfgrass systems.

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.

This article reviews the first textbooks focused on weed identification published in the United States. We go on to discuss those species considered the most troublesome weeds in agriculture. Common and scientific names written in the original texts have been cross referenced to current common and scientific names.

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.

Russian thistle is one of the most important broadleaf weeds in the semiarid U.S. Pacific Northwest. It consumes soil water after wheat harvest, compromising the yield of the following crop. The objectives of this work were to determine the impact of post–wheat harvest herbicide application timing on Russian thistle control and of stubble height on Russian thistle postharvest control and plant dispersal. For the first objective, experiments were conducted at the Columbia Basin Agricultural Research Center, Adams, OR (CBARC), and the Lind Dryland Research Station, Lind, WA (LDRS), in 2020 and 2021. Herbicides evaluated included paraquat, glyphosate, and either bromoxynil + pyrasulfotole (CBARC) or bromoxynil + metribuzin (LDRS). The different post–wheat harvest application timings were 24 h and 1, 2, and 3 wk after harvest. For the second objective, two stubble heights (short and tall) were compared for their impact on control at CBARC and in a production field near Ione, OR. Paraquat provided the greatest control in all scenarios, with no differences in application timings or stubble height. Impacts of application timings were not clear for glyphosate or bromoxynil mixtures. For glyphosate treatments, control in short stubble was 11% greater than in tall stubble in both years. Control was also greater in short stubble for the bromoxynil + pyrasulfotole application in 2020. However, Russian thistle plant dispersal was greater in short stubble at both locations. At CBARC, plant dispersal in short stubble was 58%, compared to 18% in tall stubble. Near Ione, plant dispersal in flattened stubble was 88%, compared to 43% in nonflattened short stubble. Leaving tall stubble at harvest should be considered to reduce Russian thistle plant dispersal if the infestation is going to be left untreated after harvest; otherwise, short stubble might result in better Russian thistle control when using systemic herbicides, such as glyphosate.

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.

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.

Successful cover crop (CC) establishment in the fall is important to maximize CC production, which is critical for achieving many objectives of CCs. Competition from winter weeds may reduce CC establishment and biomass production. A preplant herbicide, such as paraquat, at the time of CC planting in the fall will reduce winter weed pressure resulting in better establishment and growth. An experiment was conducted between 2019 and 2021 to test this hypothesis by evaluating a no-CC check, cereal rye, hairy vetch, crimson clover, and cereal rye + hairy vetch drilled with and without paraquat applied at planting (mid-October to mid-November) following either a corn or soybean crop. Visible weed suppression ratings were collected in mid-April, and total CC and weed biomass was collected in late April. More CC biomass was accumulated following corn than soybean, regardless of preplant herbicide application, because corn is typically harvested before soybeans. Therefore, CCs should be planted early to accumulate more biomass. Weed suppression varied by weed species from all factors, but in general weed suppression was best from a CC mixture containing cereal rye and paraquat applied at planting. If weed suppression is the main goal of the CC, then a preplant herbicide at CC planting is recommended. However, if CC weed suppression goals can be achieved through biomass accumulation, no preplant herbicide is needed. This information is useful for producers to achieve various CC objectives while managing costs.

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.

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.

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.

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.

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.

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.