Waterhemp Control Research Articles

Biologically Effective Dose of Diflufenican for the Control of Multiple Herbicide-Resistant Waterhemp in Soybean

Waterhemp biotypes have evolved resistance to Weed Science Society of America (WSSA) Herbicide Groups 2, 5, 9, 14, and 27 in Ontario Canada, are present in 15 counties, spanning a distance of 800 km across southern Ontario, and cause an average soybean yield loss of 42%. Five field experiments were established in growers’ fields in southwestern Ontario to determine the biologically effective doses of diflufenican (Group 12) applied preemergence (PRE) to control multiple herbicide-resistant waterhemp in soybean. The calculated diflufenican doses to elicit 50, 80, and 95% control of MHR waterhemp were 71, 164, and 304 g ai ha-1 at 2 weeks after herbicide application (WAA); 50, 115, and 214 g ai ha-1 at 4 WAA; and 69, 158, and 294 g ai ha-1 at 8 WAA, respectively. The calculated diflufenican doses that caused a 50, 80, and 95% reduction in MHR waterhemp density were 28, 70, and Non-est. g ai ha-1 and the doses that caused a 50, 80, and 95% reduction in MHR waterhemp biomass were 44, 109, and Non-est. g ai ha-1, respectively. The calculated diflufenican doses that resulted in 50, 80, and 95% of the soybean yield of the industry standard herbicide (flumioxazin/pyroxasulfone) were 3, 12, and 57 g ai ha-1, respectively. Diflufenican (180 g ai ha-1) PRE controlled MHR waterhemp 89, 92, and 85%; metribuzin (300 g ai ha-1) PRE controlled MHR waterhemp 94, 84, and 69%; and flumioxazin/pyroxasulfone (105/134 g ai ha-1) PRE controlled MHR waterhemp 100, 99, and 98% at 2, 4, and 8 WAA, respectively. Diflufenican, metribuzin, and flumioxazin/pyroxasulfone applied PRE reduced MHR waterhemp density 96, 84, and 100% and biomass 93, 67, and 99%, respectively at 8 WAA. Diflufenican, metribuzin, and flumioxazin/pyroxasulfone applied PRE caused 9, 0, and 4% visible soybean injury, respectively but the injury was transient and caused no adverse effect on seed moisture content or seed yield of soybean. This study concludes that diflufenican and metribuzin applied PRE provide comparable MHR waterhemp control; however, control was lower than flumioxazin/pyroxasulfone.

Mapping of flumioxazin tolerance in a snap bean diversity panel leads to the discovery of a master genomic region controlling multiple stress resistance genes.

Effective weed management tools are crucial for maintaining the profitable production of snap bean (Phaseolus vulgaris L.). Preemergence herbicides help the crop to gain a size advantage over the weeds, but the few preemergence herbicides registered in snap bean have poor waterhemp (Amaranthus tuberculatus) control, a major pest in snap bean production. Waterhemp and other difficult-to-control weeds can be managed by flumioxazin, an herbicide that inhibits protoporphyrinogen oxidase (PPO). However, there is limited knowledge about crop tolerance to this herbicide. We aimed to quantify the degree of snap bean tolerance to flumioxazin and explore the underlying mechanisms. We investigated the genetic basis of herbicide tolerance using genome-wide association mapping approach utilizing field-collected data from a snap bean diversity panel, combined with gene expression data of cultivars with contrasting response. The response to a preemergence application of flumioxazin was measured by assessing plant population density and shoot biomass variables. Snap bean tolerance to flumioxazin is associated with a single genomic location in chromosome 02. Tolerance is influenced by several factors, including those that are indirectly affected by seed size/weight and those that directly impact the herbicide's metabolism and protect the cell from reactive oxygen species-induced damage. Transcriptional profiling and co-expression network analysis identified biological pathways likely involved in flumioxazin tolerance, including oxidoreductase processes and programmed cell death. Transcriptional regulation of genes involved in those processes is possibly orchestrated by a transcription factor located in the region identified in the GWAS analysis. Several entries belonging to the Romano class, including Bush Romano 350, Roma II, and Romano Purpiat presented high levels of tolerance in this study. The alleles identified in the diversity panel that condition snap bean tolerance to flumioxazin shed light on a novel mechanism of herbicide tolerance and can be used in crop improvement.

Biologically effective dose of diflufenican applied preemergence for the control of multiple herbicide-resistant waterhemp in corn

Abstract Waterhemp is a dioecious species which contributes to a wide genetic diversity that has enabled it to evolve resistance to several commonly used Herbicide Groups (G) in North America. Five field trials were established in Ontario to ascertain the biologically effective doses (BED) of diflufenican, a new Group 12 herbicide applied preemergence (PRE) for control of multiple herbicide-resistant (MHR) waterhemp in corn. Based on regression analysis, the predicted diflufenican doses to elicit 50, 80, and 95% MHR waterhemp control were 99, 225, and 417 g ai ha-1 at 2 WAA; 73, 169, and 314 g ai ha-1 at 4 WAA, and 76, 215, and “–“ (the effective dose was beyond the set of doses in this study) g ai ha-1 at 8 WAA, respectively. The diflufenican doses that caused a 50, 80, and 95% decrease in MHR waterhemp density were 42, 123, and ”–“ g ai ha-1, and MHR waterhemp biomass were 72, 167, and 310 g ai ha-1, respectively at 8 WAA. Diflufenican PRE at 150 g ai ha-1 controlled MHR waterhemp 64, 79, and 73%; isoxaflutole + atrazine PRE at 105 + 1060 g ai ha-1 controlled MHR waterhemp 98, 98, and 97%; and S-metolachlor/mesotrione/bicyclopyrone/atrazine PRE at 1259/140/35/588 g ai ha-1 controlled MHR waterhemp 100, 100, and 99% at 2, 4, and 8 WAA, respectively. Diflufenican PRE reduced MHR waterhemp density and biomass by 83%; in contrast, isoxaflutole + atrazine and S-metolachlor/mesotrione/bicyclopyrone/atrazine reduced MHR waterhemp density and biomass by 99%. All treatments evaluated caused either no or minimal corn injury and provided yield comparable to weed-free control. Results indicate that diflufenican applied PRE alone does not provide superior MHR waterhemp control than the commonly used herbicides isoxaflutole + atrazine or S-metolachlor/mesotrione/bicyclopyrone/atrazine; however, there is potential for the utilization of diflufenican as a part of an Integrated Weed Management (IWM) strategy for the control of MHR waterhemp control in corn.

Control of multiple herbicide-resistant waterhemp (Amaranthus tuberculatus) with acetochlor-based herbicide mixtures in corn

Abstract Waterhemp is a summer annual, broadleaf weed with high fecundity, short seed longevity in the soil, and wide genetic diversity. Populations have evolved resistance to five herbicide modes of action (Groups 2, 5, 9, 14, and 27), which are present across southern Ontario; this has increased the challenge of controlling this competitive weed species in corn, the most important grain crop produced worldwide, and the highest value agronomic crop in Ontario. Acetochlor is a Group 15 soil-applied residual herbicide that has activity on many grass and broadleaf weeds but has yet to be registered in Canada. The objective of this study was to ascertain whether mixtures of acetochlor with flumetsulam, dicamba, atrazine, isoxaflutole/diflufenican, or mesotrione + atrazine applied preemergence would increase the control of multiple herbicide-resistant (MHR) waterhemp in corn. Five field trials were conducted between 2022 and 2023. No corn injury was observed. Acetochlor applied alone controlled MHR waterhemp 97% 12 weeks after application (WAA). All herbicide mixtures controlled MHR waterhemp similarly at ≥98% 12 WAA; there were no differences among herbicide mixtures. Flumetsulam, dicamba, and atrazine provided lower MHR waterhemp control than all other herbicide treatments and did not reduce density or biomass. Acetochlor reduced waterhemp density 98%, while the acetochlor mixtures reduced density similarly at 99 to 100%. This study concludes that the acetochlor mixtures evaluated provide excellent waterhemp control; however, control was not greater than acetochlor alone. Herbicides herbicide mixtures should be used as a best management practice to mitigate the evolution of herbicide resistance.

Preplant Herbicide Programs for the Control of Multiple-Herbicide-Resistant Waterhemp in No-Till Corn

Multiple-herbicide-resistant (MHR) waterhemp has been confirmed in 18 Ontario counties. A total of four experiments were conducted in commercial fields with MHR waterhemp in 2020 and 2021 to evaluate preplant (PP) herbicide tank mixtures for control of MHR waterhemp in no-till corn. There was minimal visible corn injury from the herbicide treatments evaluated. At 2 WAA, all herbicide tank mixtures provided greater than 90% control of MHR waterhemp except for pyroxasulfone/carfentrazone + atrazine which controlled MHR waterhemp 85%. At 12 WAA, S-metolachlor/mesotrione/atrazine, S-metolachlor/mesotrione/bicyclopyrone/atrazine, and diflufenican + atrazine + flufenacet controlled MHR waterhemp 86, 91, and 98%, respectively; all other herbicide tank mixtures provided 65 to 83% control. At 8 WAA, S-metolachlor/mesotrione/bicyclopyrone/atrazine and diflufenican + atrazine + flufenacet reduced MHR waterhemp density 97 and 100%, respectively similar to the weed-free control; all other herbicide tank mixtures reduced MHR waterhemp density 64 to 96%. At 8 WAA, S-saflufenacil/dimethenamid-P + mesotrione, S-metolachlor/mesotrione/atrazine, tembotrione + dicamba + flufenacet, metolachlor/mesotrione/bicyclopyrone/atrazine, and diflufenican + atrazine + flufenacet reduced MHR waterhemp biomass 96, 97, 98, 98, and 100%, respectively; all other herbicide tank mixtures reduced MHR waterhemp biomass 72 to 93%. MHR waterhemp interference reduced corn yield 80% in this study. All herbicide tank mixtures resulted in corn yield that was similar to the weed-free control. Among the herbicide tank mixtures evaluated S-metolachlor/mesotrione/bicyclopyrone/atrazine and diflufenican + atrazine + flufenacet provided the greatest control of MHR waterhemp in no-till corn.

Control of multiple-herbicide-resistant waterhemp with acetochlor-based herbicide mixtures in soybean

Abstract 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.

Efficacy of dicamba and dicamba/tembotrione with and without ammonium sulfate for broadleaf weed control

Abstract Mixing ammonium sulfate (AMS) can increase dicamba volatility. Therefore, AMS cannot be used with dicamba products in dicamba-resistant soybean. However, most dicamba products applied in corn are labeled to mix with AMS. The objectives of this study were to evaluate broadleaf weed control with dicamba (DiFlexx®) and dicamba/tembotrione (DiFlexx® DUO) applied alone or with AMS or AMS substitute and their effect on broadleaf weed density and biomass. Field experiments were conducted in Illinois, Missouri, and Nebraska in 2018 and 2019. In Illinois and Nebraska, mixing AMS + crop oil concentrate (COC) with dicamba applied at 1,120 g ae ha−1 increased the control of Palmer amaranth and waterhemp (Amaranthus species) from 78% to 92% and velvetleaf from 73% to 96% compared with dicamba applied alone 14 d after application (DAA); however, Missouri data showed no difference. Mixing AMS + COC with dicamba/tembotrione at 597 and 746 g ai ha−1 did not improve broadleaf weed control 14 DAA at any site compared with dicamba/tembotrione applied alone. Control of Amaranthus species was increased from 82% with dicamba applied at 840 g ae ha−1 to 96% when mixed with AMS + COC 28 DAA in Illinois; however, control was similar to dicamba applied at 1,120 g ae ha−1. Broadleaf weed control did not differ among dicamba or dicamba/tembotrione 28 and 56 DAA in Missouri and Nebraska. Broadleaf weed density decreased from 64 plants m−2 to 24 plants m−2 with dicamba at 1,120 g ae ha−1 with AMS + COC 14 DAA in Nebraska; however, no differences were observed in broadleaf weed density or biomass 56 DAA in any state. The results suggest that dicamba or dicamba/tembotrione can be applied without AMS or AMS substitute, especially at higher rates, without losing broadleaf weed control efficacy.

Evaluation of two‐pass herbicide programs for broad‐spectrum weed control in conventional tillage non‐transgenic corn production in Wisconsin atrazine prohibition areas

AbstractAchieving effective weed control is crucial for Wisconsin conventional corn growers farming in atrazine prohibition areas. Upon request from our extension clientele, a field study was conducted at Arlington and Brooklyn, WI, in 2021 and 2022 (four site‐years). The study aimed to evaluate the weed control efficacy of comprehensive two‐pass (PRE followed by POST) herbicide program treatments containing products with a wide range of active ingredients and sites of action from various manufacturers without atrazine and glyphosate, in conventional tillage non‐transgenic corn production. Nine and eight out of the 11 two‐pass herbicide programs investigated provided >90% giant foxtail (Setaria faberi Herrm), common ragweed (Ambrosia artemisiifolia L.), velvetleaf (Abutilon theophrasti Medik) and waterhemp [Amaranthus tuberculatus (Moq.) J.D. Sauer] control in 2021, respectively. However, only three treatments provided >90% waterhemp control and the greatest giant foxtail control in 2022 was 79%. Greater end‐of‐season broadleaf and grass weed control was observed in 2021 compared to 2022 due to lower weed pressure and timely early‐season rainfall in 2021. The two‐pass programs, acetochlor/mesotrione fb dicamba/tembotrione (94%–100%) and acetochlor/flumetsulan/clopyralid fb nicosulfuron + dicamba/diflufenzopyr (85%–100%), provided the most consistent broadleaf control across all site‐years. These results demonstrate that a well‐designed and timely deployed two‐pass herbicide program can provide effective control of common broadleaf and grass weeds in conventional corn production assuming moderate weed pressure and timely rainfall for soil residual herbicide activation, otherwise effective weed control, particularly of grass weeds, in the absence of atrazine and glyphosate can be challenging. These results are also translatable to support weed management decisions for glyphosate‐resistant corn hybrids in non‐atrazine prohibition areas.

Norflurazon and metribuzin as components of postemergence tank mixtures with HPPD-inhibiting herbicides for multiple-resistant waterhemp control

Field and greenhouse studies investigated the phytoene desaturase (PDS) and photosystem II (PSII)-inhibiting herbicides, norflurazon and metribuzin, as components of two- and three-way postemergence (POST) tank mixtures for controlling multiple herbicide-resistant (MHR) waterhemp (Amaranthus tuberculatus). Studies were conducted at two locations (Champaign County, Illinois, U.S.A (CHR) and McLean County, Illinois, U.S.A. (MCR)) during 2019 and 2020. One of three commercial HPPD-inhibiting herbicides (mesotrione, tembotrione or topramezone) was added to a tank mixture with norflurazon, a reduced rate of metribuzin, or both to determine if greater MHR waterhemp control with tank mixtures could be achieved compared to individual herbicides and assess the potential benefit of norflurazon. Field results indicated the addition of norflurazon to HPPD-inhibiting herbicides resulted in greater waterhemp control (14% increase on average) compared to individual herbicides applied alone. Additionally, metribuzin in combination with HPPD-inhibiting herbicides consistently increased control of MHR waterhemp populations compared to the herbicides applied alone. However, only the three-way tank mixture containing tembotrione at the MCR site reduced biomass relative to two-way treatments containing tembotrione, metribuzin or norflurazon. Greenhouse experiments characterized mixtures of norflurazon with metribuzin and the HPPD-inhibiting herbicides, mesotrione and topramezone. Waterhemp biomass was significantly reduced when metribuzin and HPPD inhibitors were combined in comparison to individual components. Two-way tank mixtures containing norflurazon with mesotrione or metribuzin also resulted in significant biomass reductions compared to herbicides applied alone in MHR waterhemp. However, lower control levels were noted in MHR waterhemp when norflurazon was mixed with either topramezone or topramezone plus metribuzin compared to topramezone alone. In summary, norflurazon and metribuzin can be effective tank mixture components with HPPD-inhibiting herbicides for MHR waterhemp control. However, more research on these tank mixtures is warranted to optimize weed control and crop selectivity, which includes investigating active ingredient ratios, application timings and droplet spectrum effects on coverage.

Preemergence herbicide premixes reduce the risk of soil residual weed control failure in corn

AbstractWidespread 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.

Multiple herbicide-resistant waterhemp control with Group 15 herbicides

AbstractWaterhemp has evolved resistance to seven herbicide modes of action in the United States and to five in Canada, which limits weed control options for producers. The objective of this research was to quantify the level and duration of residual control of multiple herbicide-resistant (MHR) waterhemp with five Group 15 herbicides (acetochlor, dimethenamid-p, flufenacet, pyroxasulfone, andS-metolachlor) applied preemergence in a non-crop area. Four field trials were conducted over a 2-yr period (2021, 2022) in southwestern Ontario, Canada. By 4 wk after application (WAA) 91% of waterhemp had emerged in the nontreated control area. The numerical control of waterhemp with all Group 15 herbicides, with the exception of pyroxasulfone, was greatest at 4 WAA, then control declined. Flufenacet provided the lowest waterhemp control; dimethenamid-p andS-metolachlor provided intermediate control, and acetochlor and pyroxasulfone provided the highest control. Waterhemp control with pyroxasulfone peaked at 6 WAA with 99% and declined to 77% at 12 WAA. Flufenacet (low and high rates) was predicted to reduce waterhemp emergence by 50% for 42 to 44 d after application (DAA). Dimethenamid-p,S-metolachlor, and acetochlor (both formulations and three rates) were predicted to reduce waterhemp emergence by 80% for 36, 43, and 33 to 51 DAA, respectively; in contrast, pyroxasulfone was predicted to reduce waterhemp emergence by 80% for 82 DAA. This study concludes that of the Group 15 herbicides evaluated, flufenacet provides the lowest and shortest residual control of waterhemp, and pyroxasulfone provides the highest and longest residual control of waterhemp.

Layered‐residual herbicide programs for waterhemp control in glufosinate‐resistant soybean

Core Ideas Glufosinate is an important POST herbicide for waterhemp control in glufosinate‐resistant soybean. The layered‐residual herbicide approach improved season‐long waterhemp control. Glufosinate POST combined with layered soil residual herbicides improved season‐long waterhemp control. Glufosinate is an important POST herbicide for waterhemp control in glufosinate‐resistant soybean. The layered‐residual herbicide approach improved season‐long waterhemp control. Glufosinate POST combined with layered soil residual herbicides improved season‐long waterhemp control.

Evaluation of foliar‐applied post‐emergence corn–soybean herbicides on giant ragweed and waterhemp control in Wisconsin

AbstractGiant ragweed (Ambrosia trifida L.) and waterhemp (Amaranthus tuberculatus [Moq.] J.D.Sauer) are highly competitive and the most troublesome weeds in corn (Zea mays L.) and soybean (Glycine max Merr.) cropping systems in Wisconsin. Field experiments were conducted to evaluate the efficacy of multiple foliar‐applied POST‐emergence herbicides on giant ragweed at Janesville, WI (2020 and 2021), and on groups 2‐ and 9‐resistant waterhemp at Brooklyn, WI (2019 and 2020), respectively. Several herbicides from site of action groups 2, 4, 5, 6, 9, 10, 14, and 27 were evaluated. Herbicide treatments were applied when giant ragweed and waterhemp reached ~10 cm in height. Herbicide efficacy (0‐100% visual control) was evaluated 14 days after treatment (DAT). Group 2 herbicides provided poor (<45%) control of giant ragweed and waterhemp 14 DAT. Several herbicides encompassing different sites of action provided >90% giant ragweed control. Certain herbicides from groups 4, 10, 14, and 27 provided the best control (74‐87%) of waterhemp, but no herbicide evaluated in this study provided = 90% waterhemp control. Weed management programs containing multiple effective herbicide sites of action PRE‐ and POST‐emergence accompanied by non‐chemical strategies are recommended to extend the spectrum of chemical control and reduce the risk of selection of further herbicide‐resistant biotypes for both giant ragweed and waterhemp in Wisconsin and beyond.

Multiple-herbicide-resistant waterhemp control in glyphosate/glufosinate/2,4-D-resistant soybean with one- and two-pass weed control programs

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

Impact of Acetochlor Rate and Application Timing on Multiple-Herbicide-Resistant Waterhemp Control in Corn and Soybean

Documented 6-way (Groups 2, 4, 5, 9, 14, and 27) and 5-way (Groups 2, 5, 9, 14, and 27) multiple-herbicide-resistant (MHR) waterhemp have been confirmed in the US and Canada, respectively causing corn and soybean yield losses > 70%. The objective of this study was to determine the effect of acetochlor application timing and rate on non-emerged MHR waterhemp control in corn and soybean. Acetochlor is not yet registered in Canada, but it could be useful component of an integrated MHR waterhemp control program. Two studies, one in corn and one in soybean, were conducted in southwestern Ontario, Canada from 2020 to 2022. Three rates of acetochlor were applied preplant (PP), preemergence (PRE) and postemergence (POST) to non-emerged waterhemp. In corn, acetochlor [Emulsifiable Concentrate (EC)] applied at 1,225, 2,100 and 2,950 g ai ha-1 controlled MHR waterhemp 81, 85, and 90%, respectively, at 8 weeks after POST application (WAC). Acetochlor EC applied POST or PRE provided better control than when applied PP at 4, 8, and 12 WAC. In soybean, acetochlor [Capsule suspension (CS)] applied at 1,050, 1,375, and 1,700 g ai ha-1 controlled MHR waterhemp 63, 70, and 74%, respectively, at 8 WAC. The timing of acetochlor CS application did not affect MHR waterhemp control. Acetochlor applied at the low, medium, and high rate reduced waterhemp density by 87, 89, and 92% in corn, and by 82, 84, and 87% in soybean, respectively. The high rate of acetochlor provides acceptable control of MHR waterhemp in corn; control in soybean was inadequate.

Multiple herbicide resistance in waterhemp (Amaranthus tuberculatus) accessions from Wisconsin

AbstractA 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.

The impact of electrocution treatments on weed control and weed seed viability in soybean

AbstractField experiments were conducted in 2020 and 2021 to determine the effectiveness of electrocution on several weeds commonly encountered in Missouri soybean production using an implement known as The Weed Zapper™. In the first study, the effectiveness of electrocution on waterhemp, cocklebur, giant and common ragweed, horseweed, giant and yellow foxtail, and barnyardgrass was determined. Electrocution was applied when plants reached average heights and/or growth stages of 30 cm, 60 cm, flowering, pollination, and seed set. Electrocution was applied once or twice, at two different tractor speeds. Electrocution was more effective at the later plant growth stages. Pearson correlation coefficients indicated that control of weed species was most related to plant height and amount of plant moisture at the time of electrocution. When plants contained seed at the time of electrocution, viability was reduced from 54% to 80% among the species evaluated. A second study determined the effect of electrocution on late-season waterhemp plants, and also soybean injury and yield. Electrocution timings took place throughout reproductive soybean growth stages. The control of waterhemp escapes within the soybean trial ranged from 51% to 97%. Yield of soybean electrocuted at the R4 and R6 growth stages was similar to that of the nontreated control, but soybean yield was reduced by 11% to 26% following electrocution at all other timings. However, the visual injury and yield loss observed in these experiments likely represents a worst-case scenario because growers who maintain a clear height differential between waterhemp and the soybean canopy would not need to maintain contact with the soybean canopy. Overall, results from these experiments indicate that electrocution as part of an integrated weed-management program could eliminate late-season herbicide-resistant weed escapes in soybean, and reduce the number and viability of weed seed that return to the soil seedbank.

Influence of Broadcast Nozzle Design and Weed Density on Dicamba Plus Glyphosate Deposition, Coverage, and Efficacy in Dicamba-Resistant Soybean

Dicamba injury to sensitive soybean and other broadleaf crops due to drift is a major issue. Dicamba label restrictions have been created to mitigate the off-target movement of dicamba. One restriction is the mandated use of low-drift nozzles to spray dicamba; these nozzles produce large droplet spectrums and minimize the production of driftable fines. Experiments were conducted to evaluate herbicide coverage, deposition, and efficacy as influenced by spray nozzle design and density of waterhemp, goosegrass, and large crabgrass in dicamba-resistant soybean. Dicamba plus glyphosate was applied to 5- to 10-cm-tall weeds with a Turbo TeeJet (TT11005) nozzle and two drift reduction nozzles approved for dicamba applications: Turbo TeeJet Induction (TTI11005) and Pentair Ultra Lo-Drift (ULD12005). Weed densities were categorized into different levels and established in a 0.25-m2 quadrat prior to postemergence application. Deposition of herbicide spray solution onto targeted weeds was not different despite coverage differences observed on Kromekote spray cards. Coverage of herbicide solution was consistently lower with the low-drift TTI11005 nozzle as compared to the TT11005 nozzle. Herbicide efficacy on waterhemp plants was the lowest at the highest waterhemp densities of 54 plants per m2 with the drift-reducing TTI11005 nozzle, although weed control was not lowered at any density when applications were made with the ULD nozzle as compared to the TT11005 nozzle. Additionally, herbicide efficacy was reduced as large crabgrass density increased. Overall, the use of a drift-reducing nozzle can be successful for waterhemp control and Poaceae control postemergence in soybean when weed densities are suppressed or reduced through methods such as the use of a residual preemergence herbicide or cereal rye cover crop.

Multiple Genomic Regions Govern Tolerance to Sulfentrazone in Snap Bean (Phaseolus Vulgaris L.)

The availability of effective weed management tools against waterhemp (Amaranthus tuberculatus) is crucial to maintain profitable production of snap bean (Phaseolus vulgaris L.). Preemergence herbicides enable the crop to gain a size advantage over the weed, but the few preemergence herbicides registered in snap bean have poor control of waterhemp. Sulfentrazone, a protoporphyrinogen oxidase (PPO) -inhibiting herbicide, provides significant control of waterhemp and other problematic weeds. However, crop tolerance to the herbicide is poorly known. To quantify snap bean tolerance to sulfentrazone and investigate the underlying tolerance mechanism(s), a genome-wide association mapping study was conducted using field-collected data on a snap bean diversity panel. Response to a preemergence application of sulfentrazone was measured using plant population density and shoot biomass variables. High levels of crop tolerance were found in several entries including Bush Romano 71, Navarro, and Flamata. Snap bean tolerance to sulfentrazone is associated with multiple genomic regions, indicating the trait is likely a non-target site resistance (NTSR). Seed size is partially responsible for the tolerance, thus the genetic factors conditioning sulfentrazone tolerance are likely a combination of those driven indirectly by seed size/weight and those acting directly on the metabolism of the herbicide or ameliorating its damage. In this study, several cytochrome P450 and ABC transporter genes were localized in associated genomic regions. Alleles conditioning snap bean tolerance to sulfentrazone identified in the diversity panel shed light on herbicide metabolism and could be used in snap bean improvement.

The interaction of pyroxasulfone and flumioxazin applied preemergence for the control of multiple-herbicide-resistant waterhemp (Amaranthus tuberculatus) in soybean

AbstractSix field experiments were conducted to investigate any interaction between pyroxasulfone and flumioxazin on soybean tolerance and control of multiple-herbicide-resistant (MHR) waterhemp in soybean during 2016 and 2017 in Ontario, Canada. There was a synergistic increase in soybean injury with the co-application of pyroxasulfone and flumioxazin at all rates evaluated at 2 wk after emergence (WAE), the two highest rates evaluated (134/106 and 268/211 g ai ha–1) at 4 WAE, and the highest rate (268/211 g ai ha–1) evaluated at 8 WAE. Soybean injury with all pyroxasulfone and flumioxazin treatments was transient and had no adverse effect on soybean grain yield. Pyroxasulfone applied preemergence at 45, 89, 134, and 268 g ai ha–1 controlled MHR waterhemp up to 72%, 89%, 92%, and 95%, respectively. Flumioxazin applied preemergence at 35, 70, 106, and 211 g ai ha–1 controlled MHR waterhemp up to 78%, 90%, 93%, and 96%, respectively. Pyroxasulfone/flumioxazin applied preemergence at 45/35, 89/70, 134/106, and 268/211 g ai ha–1 controlled MHR waterhemp up to 92%, 96%, 98%, and 100%, respectively. There were no significant antagonistic or synergistic interactions for the control of MHR waterhemp with pyroxasulfone/flumioxazin at rates evaluated except at 268/211 g ai ha–1, which provided a synergistic increase in MHR waterhemp control at 4 WAE. The MHR waterhemp biomass and density reductions followed a trend similar trend to visible control. Pyroxasulfone/flumioxazin at 268/211 g ai ha–1 caused a synergistic response in biomass reduction (9% difference). Based on these results, there is an additive increase in MHR waterhemp control and potential for a synergistic increase in soybean injury with the co-application of pyroxasulfone plus flumioxazin.