Postemergence (POST) herbicides that control troublesome weeds during hybrid bermudagrass establishment via sprigs are limited due to potential turfgrass phytotoxicity and herbicide-resistant weeds. Research experiments were conducted in Blacksburg, VA, and Hope, AR, in 2016 and 2023 to evaluate herbicide programs to control goosegrass and smooth crabgrass and the response of hybrid bermudagrass sprigs to POST herbicides applied 3 to 5 wk after establishment (WAE). Another study was conducted to assess the tolerance of ‘Latitude 36’, ‘Tahoma 31’, and ‘TifTuf’ hybrid bermudagrass sprigs to POST herbicides applied 4 to 5 WAE. Thiencarbazone + foramsulfuron + halosulfuron did not injure hybrid bermudagrass > 6% across four cultivars and a total of 10 site-yr but reduced goosegrass and smooth crabgrass cover equivalent to the best-performing treatments. Topramezone + metribuzin injured turfgrass > 25% at 14 d after treatment (DAT), but tank mixing with thiencarbazone + foramsulfuron + halosulfuron reduced injury by 5% to 22%. Quinclorac injured hybrid bermudagrass 17% to 58%, depending on site, which was more than most other treatments. Mesotrione-, quinclorac-, or topramezone-based programs injured hybrid bermudagrass and also reduced turfgrass cover, the dark green color index, and the normalized difference vegetation index, but turfgrass recovered by 28 DAT. Results suggest that turfgrass managers have a variety of herbicides that can control smooth crabgrass and goosegrass during hybrid bermudagrass sprig establishment, but the margin of selectivity is relatively low for mesotrione, quinclorac, and topramezone and may be dependent on herbicide rate or hybrid bermudagrass cultivar.

Amaranthus species are problematic weeds in snap bean production systems. They reduce crop yields, and their stem fragments contaminate harvested pods. Knowledge of snap bean tolerance to different preemergence herbicides is limited; however, knowing this tolerance is essential for planning a reliable weed management system, breeding herbicide-tolerant cultivars, and registering herbicides for use on minor crops such as snap bean. Field trials were conducted in 2021 and 2022 to determine the tolerance of eight snap bean cultivars to preemergence herbicides with activity on Amaranthus species, including dimethenamid-P, flumioxazin, lactofen, metribuzin, saflufenacil, and sulfentrazone. Snap bean plant density (number of plants per square meter), plant biomass (grams per plant), and canopy biomass (grams per square meter) 21 d after treatment were used to assess crop tolerance to a range of herbicide rates. Linear mixed-effects regression models were fitted to quantify the relationships between preemergence herbicide rate and snap bean cultivar tolerance. Results indicated a high margin of crop safety with dimethenamid-P and lactofen for weed control in snap bean, and a low margin of crop safety with metribuzin and saflufenacil. Results indicated differential cultivar tolerance to flumioxazin and sulfentrazone, which could be driven by genetic variability among cultivars.

Amicarbazone, atrazine, and metribuzin behavior was examined in a field setting in Tennessee and in a laboratory setting using soils collected from Illinois and Tennessee. Fields planted to corn were sampled from 0 to 8 cm depth, and the samples were analyzed using methanolic extraction followed by tandem mass spectrometry analysis to determine residual herbicide concentrations. Conditions were favorable for herbicide degradation, including warm temperatures and adequate rainfall. All herbicide half-lives were <10 d. Laboratory research using soils with known atrazine-use histories showed that amicarbazone did not exhibit enhanced microbial degradation due to previous atrazine use. Apparent amicarbazone and metribuzin persistence levels implied that early-season weed control would be expected, but carryover to injure sensitive rotational crops would not be anticipated under these environmental conditions. Dissipation under field conditions of amicarbazone and metribuzin was not affected by being applied to separate plots or by coapplication to the same plots.

Waterhemp has evolved resistance to Group 2, 5, 9, 14, and 27 herbicides in Ontario, Canada, making control of this challenging weed even more difficult. Acetochlor is a Group 15, chloroacetanilide herbicide that has activity on many small-seeded annual grasses and some small-seeded annual broadleaf weeds, including waterhemp. The objective of this study was to ascertain if acetochlor mixtures with broadleaf herbicides (dicamba, metribuzin, diflufenican, sulfentrazone, or flumioxazin), applied preemergence (PRE), increase multiple-herbicide-resistant (MHR) waterhemp control in soybean. Five trials were conducted over 2 yr (2021 and 2022). The acetochlor mixtures caused ≤7% soybean injury, except acetochlor + flumioxazin, which caused 19% soybean injury. Acetochlor applied PRE controlled MHR waterhemp 89% at 4 wk after application (WAA). Dicamba, metribuzin, diflufenican, sulfentrazone, or flumioxazin controlled MHR waterhemp 59%, 67%, 58%, 64%, and 86%, respectively, at 4 WAA. Acetochlor applied in a mixture with dicamba, metribuzin, diflufenican, sulfentrazone, or flumioxazin provided good to excellent control of MHR waterhemp; control ranged from 91% to 98% but was similar to acetochlor applied alone. Acetochlor alone reduced MHR waterhemp density and biomass 98% and 93%; acetochlor + flumioxazin reduced waterhemp density and biomass by an additional 2% and 7%, respectively. This research concludes that acetochlor applied in a mixture with flumioxazin was the most efficacious mixture evaluated for MHR waterhemp control.

Industrial hemp is a multipurpose crop cultivated for fiber, seed, human food, and animal feed. Hemp legalization in Texas creates a considerable potential to increase its acreage in semi-arid conditions; however, knowledge is limited on growing hemp optimally in Texas. Best management practices, including weed control, require evaluation for profitable hemp production. As little is known about the herbicide tolerance of hemp, field studies were conducted to test several soil-residual herbicides with different modes of action for phytotoxicity to two hemp cultivars, ‘Yuma’ and ‘Jinma’. The experimental units were randomized three times in a blocked split-plot design with hemp cultivars in the main plots and soil-residual herbicides in the subplots. Ethalfluralin, the mixture of sulfentrazone and S-metolachlor, prometryn, and S-metolachlor, resulted in 60% to 90% and 73% to 100% weed control as compared to the nontreated control in 2021 and 2022, respectively. The highest hemp germination, stand count, and plant height were observed with ethalfluralin and S-metolachlor herbicides; however, no significant differences were observed for hemp germination and plant height compared to the nontreated control. S-metolachlor, ethalfluralin, fomesafen, and prometryn resulted in similar hemp biomass compared to the nontreated control. Overall, the results indicate that hemp is tolerant to ethalfluralin, prometryn, and S-metolachlor, and these soil-residual herbicides were effective for weed control in hemp. The mixture of bicyclopyrone plus S-metolachlor, metribuzin plus S-metolachlor, and mesotrione should be avoided, as they caused significant injury to hemp plants. Future research is needed to test the efficacy of different preemergence and postemergence herbicides that can be used in industrial hemp grown under different environments, making sure the delta-9-tetrahydrocannabinol content of the hemp is below the legal content restrictions.

Preemergence applications of mesotrione, an herbicide that inhibits 4-hydroxyphenolpyruvate dioxygenase (HPPD), have recently gained regulatory approval in soybean varieties with appropriate traits. Giant ragweed is an extremely competitive broadleaf weed, and biotypes resistant to acetolactate synthase inhibitors (ALS-R) can be particularly difficult to manage with soil-residual herbicides in soybean production. This study investigated control of giant ragweed from preemergence applications of cloransulam (32 g ai ha–1), metribuzin (315 g ai ha–1), and S-metolachlor (1,600 g ai ha–1) in a factorial design with and without mesotrione (177 g ai ha–1) at two different sites over 2 yr. Treatments with mesotrione were also compared with two commercial premix products: sulfentrazone (283 g ai ha–1) and cloransulam (37 g ai ha–1), and chlorimuron (19 g ai ha–1), flumioxazin (69 g ai ha–1), and pyroxasulfone (87 g ai ha–1). At 42 d after planting, control and biomass reduction of giant ragweed were greater in treatments with mesotrione than any treatment without mesotrione. Giant ragweed biomass was reduced by 84% in treatments with mesotrione, whereas treatments without mesotrione did not reduce biomass relative to the nontreated. Following these preemergence applications, sequential herbicide treatments utilizing postemergence applications of glufosinate (655 g ai ha–1) plus fomesafen (266 g ai ha–1) and S-metolachlor (1,217 g ai ha–1) resulted in at least 97% control of giant ragweed at 42 d after planting, which was greater than sequential applications of glufosinate alone in 3 of 4 site-years. Preemergence applications of mesotrione can be an impactful addition to soybean herbicide programs designed to manage giant ragweed, with the potential to improve weed control and delay the onset of herbicide resistance by providing an additional effective herbicide site of action.

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.

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.

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.

Preemergence (PRE) herbicides are often banded over the entire top of raised beds for broadleaf and grass control in plasticulture vegetable production systems. However, broadleaf and grass weeds may emerge from the planting holes and tears in the plastic mulch. Banded application results in herbicides applied where no holes occur, and therefore, where they are not needed. Our objective is to identify herbicides that do not harm transplanted crops when directed at transplant holes after transplant (POST) with the aim to reduce off-target applications. Therefore, we evaluated tomato and pepper tolerance to PRE herbicides applied to transplant holes 2 wk after transplant and the subsequent effects on crop tolerance and weed density. Halosulfuron, S-metolachlor, metribuzin, and pendimethalin did not injure tomato transplants, reduce height, or reduce yield. Fomesafen caused some tomato injury (7%) but had no effect on other measured parameters in Trial I. All PRE herbicides injured peppers by ≥19%, although no effect on yield was observed. Overall, halosulfuron, S-metolachlor, metribuzin, and pendimethalin can be safely applied to tomato transplant holes 2 wk after transplant with no significant crop injury nor effects on final yield, but none of the evaluated herbicides are safe for use on pepper crops.

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

Italian ryegrass is a major weed in winter cereals in the south-central United States. Harvest weed seed control (HWSC) tactics that aim to remove weed seed from crop fields are a potential avenue to reduce Italian ryegrass seedbank inputs. To this effect, a 4-yr, large-plot field study was conducted in College Station, Texas, and Newport, Arkansas, from 2016 to 2019. The treatments were arranged in a split-plot design. The main-plot treatments were (1) no narrow-windrow burning (a HWSC strategy) + disk tillage immediately after harvest, (2) HWSC + disk tillage immediately after harvest, and (3) HWSC + disk tillage 1 mo after harvest. The subplot treatments were (1) pendimethalin (1,065 g ai ha−1; Prowl H2O®) as a delayed preemergence application (herbicide program #1), and (2) a premix of flufenacet (305 g ai ha−1) + metribuzin (76 g ai ha−1; Axiom®) mixed with pyroxasulfone (89 g ai ha−1; Zidua® WG) as an early postemergence application followed by pinoxaden (59 g ai ha−1; Axial® XL) in spring (herbicide program #2). After 4 yr, HWSC alone was significantly better than no HWSC. Herbicide program #2 was superior to herbicide program #1. Herbicide program #2 combined with HWSC was the most effective treatment. The combination of herbicide program #1 and standard harvest practice (no HWSC; check) led to an increase in fall Italian ryegrass densities from 4 plants m−2 in 2017 to 58 plants m−2 in 2019 at College Station. At wheat harvest, Italian ryegrass densities were 58 and 59 shoots m−2 in check plots at College Station and Newport, respectively, whereas the densities were near zero in plots with herbicide program #2 and HWSC at both locations. These results will be useful for developing an improved Italian ryegrass management strategy in this region.

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.

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.

The full spectrum of herbicide resistance in a weed can vary according to the mechanistic basis and cannot be implied from the selective pressure. Common ragweed (Ambrosia artemisiifolia L.) is an important weed species of horticultural crops that has developed resistance to linuron based on either target site– or non–target site resistance mechanisms. The objective of the study is to characterize the cross-resistance to metribuzin of linuron-selected biotypes of A. artemisiifolia with target site– and non–target site resistance and determine its genetic basis. Crosses were made between two types of linuron-resistant biotype and a linuron-susceptible biotype, and the progeny were further backcrossed with susceptible plants to the third backcross (BC3) generation to determine their responses to both herbicides compared with parental lines. The target site–based linuron-resistant biotype was cross-resistant to metribuzin, and resistance to both herbicides was maintained at the same level in the BC3 line. In contrast, the linuron-selected biotype with a non–target site resistance mechanism was not cross-resistant to metribuzin. In addition, the BC3 lines deriving from the non–target site resistant parents had very low-level resistance. While the target site–resistance trait is maintained through multiple crosses, non–target site based resistance would be lost over time when selection is absent or insufficient to retain all genes involved in resistance as a complex trait. This would imply A. artemisiifolia biotypes with different mechanisms would need to be managed differently over time.

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

Laboratory and greenhouse studies were conducted to evaluate the effects of chemical treatments applied to Palmer amaranth seeds or gynoecious plants that retain seeds to determine seed germination and quality. Treatments applied to physiologically mature Palmer amaranth seed included acifluorfen, dicamba, ethephon, flumioxazin, fomesafen, halosulfuron, linuron, metribuzin, oryzalin, pendimethalin, pyroxasulfone, S-metolachlor, saflufenacil, trifluralin, and 2,4-D plus crop oil concentrate applied at 1× and 2× the suggested use rates from the manufacturer. Germination was reduced by 20% when 2,4-D was used, 15% when dicamba was used, and 13% when halosulfuron and pyroxasulfone were used. Use of dicamba, ethephon, halosulfuron, oryzalin, trifluralin, and 2,4-D resulted in decreased seedling length by an average of at least 50%. Due to the observed effect of dicamba, ethephon, halosulfuron, oryzalin, trifluralin, and 2,4-D, these treatments were applied to gynoecious Palmer amaranth inflorescence at the 2× registered application rates to evaluate their effects on progeny seed. Dicamba use resulted in a 24% decrease in seed germination, whereas all other treatment results were similar to those of the control. Crush tests showed that seed viability was greater than 95%, thus dicamba did not have a strong effect on seed viability. No treatments applied to Palmer amaranth inflorescence affected average seedling length; therefore, chemical treatments did not affect the quality of seeds that germinated.

Late-season control of Palmer amaranth in postharvest wheat stubble is important for reducing the seedbank. Our objectives were to evaluate the efficacy of late-season postemergence herbicides for Palmer amaranth control, shoot dry biomass, and seed production in postharvest wheat stubble. Field experiments were conducted at Kansas State University Agricultural Research Center near Hays, KS, during 2019 and 2020 growing seasons. The study site had a natural seedbank of Palmer amaranth. Herbicide treatments were applied 3 wk after wheat harvest when Palmer amaranth plants had reached the inflorescence initiation stage. Palmer amaranth was controlled by 96% to 98% 8 wk after treatment and shoot biomass as well as seed production was prevented when paraquat was applied alone or when mixed with atrazine, metribuzin, flumioxazin, 2,4-D, sulfentrazone, pyroxasulfone + sulfentrazone, or flumioxazin + metribuzin, and with glyphosate + dicamba, glyphosate + 2,4-D, saflufenacil + 2,4-D, glufosinate + dicamba + glyphosate, and glufosinate + 2,4-D + glyphosate. Palmer amaranth was controlled by 89% to 93% with application of glyphosate, glufosinate, dicamba + 2,4-D, saflufenacil + atrazine, and saflufenacil + metribuzin resulting in Palmer amaranth shoot biomass of 15 to 56 g m−2 and production of 1,080 to 7,040 seeds m−2. Palmer amaranth control was less than 86% with application of dicamba, 2,4-D, dicamba + atrazine, and saflufenacil resulting in Palmer amaranth shoot biomass of 38 to 47 g m−2 and production of 3,110 to 6,190 seeds m−2. Palmer amaranth was controlled 63% and 72%, shoot biomass was 178 and 161 g m−2, and seed production was 35,180 and 39,510 seeds m−2, respectively, with application of 2,4-D + bromoxynil + fluroxypyr, and bromoxynil + pyrasulfotole + atrazine. Growers should use these effective postemergence herbicide mixes for Palmer amaranth control to prevent seed prevention postharvest in wheat stubble.

Glyphosate resistance in weed species has presented immense challenges for farmers in Ontario. The co-application of burndown plus residual herbicides provides control of glyphosate-resistant (GR) horseweed in soybean. Pyraflufen-ethyl/2,4-D is a premixed herbicide formulation sold under the tradename Blackhawk®. Five field experiments were conducted over a 2-yr period (2019, 2020) in fields in southwestern Ontario to ascertain the biologically effective dose of pyraflufen-ethyl/2,4-D, applied alone, or mixed with metribuzin, for GR horseweed control when applied preplant to soybean. Soybean visible injury for all treatments was <15%. At 8 wk after application (WAA), the calculated doses of pyraflufen-ethyl/2,4-D for 50%, 80%, and 95% control of GR horseweed were 390, 1,148, and >2,108 g ha−1, respectively. The addition of metribuzin to pyraflufen-ethyl/2,4-D reduced the doses of pyraflufen-ethyl/2,4-D for 50%, 80%, and 95% control of GR horseweed to 19, 46, and 201 g ha−1, respectively. Pyraflufen-ethyl/2,4-D + metribuzin controlled GR horseweed by 97%, which is comparable to the current industry standards. Based on these results, pyraflufen-ethyl/2,4-D + metribuzin (527 + 400 g ha−1) applied preplant can be used for GR horseweed control in soybean.