Glyphosate-resistant Common Waterhemp Research Articles

Monitoring the Changes in Weed Populations in a Continuous Glyphosate- and Dicamba-Resistant Soybean System: A Five-Year Field-Scale Investigation

AbstractResearch was conducted from 2011 to 2015 to determine the effect of herbicide strategy on efficacy and evolution of herbicide resistance in weeds in a continuous glyphosate- and dicamba-resistant (GDr) soybean system. The nine herbicide strategies included sequential applications of glyphosate only, glyphosate plus dicamba with or without acetochlor, PRE application of residual herbicides with POST glyphosate or non-glyphosate herbicides, and their biennial rotation with one another. Giant foxtail and horseweed were the least problematic during all growing seasons. An increase in horseweed was observed by the end of the experiment especially in the plots where POST glyphosate was not used with PRE application of residual herbicides. Giant ragweed evolved resistance to glyphosate over a 4-yr period of selection with strategies that predominantly included PRE and POST glyphosate. Herbicide use strategies that included glyphosate-only and PRE application of residual herbicides fb POST glyphosate annually or in a biennial rotation were ineffective in controlling giant ragweed and glyphosate-resistant (GR) common waterhemp. Over the years, application of PRE herbicide mixtures before POST glyphosate application improved weed control and soybean yields compared with the glyphosate-only strategy. During all growing seasons, the greatest yield and reduction in total weed density before harvest was provided by herbicide use strategies that included glyphosate plus dicamba annually or in a biennial rotation regardless of the inclusion of acetochlor POST. Dicamba proved to be a valuable addition to improve the control of GR weeds. GDr soybean will provide growers with a new option for managing resistant weeds, but it needs to be used with caution, as multiple resistance in weeds, including waterhemp and giant ragweed, is already widespread.

Control of Glyphosate-Resistant Common waterhemp (Amaranthus rudis) in Three New Herbicide-Resistant Soybean Varieties in Ontario

Glyphosate-resistant (GR) common waterhemp (CW) is a localized weed in Ontario and one of the most problematic weeds in the US Corn Belt. First confirmed in Ontario in 2014, GR CW has now been confirmed in forty fields in three counties in Ontario as of 2015. Historically, the primary POST herbicides used for the control of CW in soybean were glyphosate, acifluorfen and fomesafen, but resistance to all three has been confirmed in many US states. Research was conducted in 2015 and 2016 to determine the control of GR CW with some of the new herbicide-resistant soybean technologies including glufosinate (LibertyLink), 2,4-D and glyphosate (Enlist), and isoxaflutole, mesotrione, and glufosinate (HPPD-resistant). Glyphosate-resistant CW was controlled (≥90%) all season with a two-pass weed control system across all herbicide-resistant soybean technologies evaluated. The two-pass weed control system in this research is defined as a PRE herbicide followed by a POST herbicide. At 12 WAA, the two-pass programs in LibertyLink, Enlist, and HPPD-resistant systems controlled GR CW up to 98, 98, and 92%, respectively, and reduced GR CW densities to 0 to 2% of the weedy control at 4 WAA. The two-pass programs provided greater GR CW control than PRE or POST herbicides alone. This study found that the use of two-pass weed control programs in glufosinate-resistant, glyphosate DMA/2,4-D choline-resistant and HPPD-resistant soybean can provide excellent control of GR CW, and can be valuable tools to reduce the selection intensity for herbicide-resistant weeds. Through the rotational use of different technologies, growers may be able to better manage their weed populations in reducing the risk of resistance when compared to the use of one herbicide repeatedly.

Control of Glyphosate-Resistant Common Waterhemp (Amaranthus tuberculatusvar.rudis) in Soybean in Ontario

Glyphosate-resistant (GR) common waterhemp is the fifth GR weed species confirmed in Canada, and the fourth in Ontario. As of 2017, GR common waterhemp has been confirmed in Lambton, Essex, and Chatham-Kent counties in Ontario. Greenhouse and field dose–response experiments revealed that GR common waterhemp in Ontario had a resistance level of 4.5 and 28, respectively, when compared with known glyphosate-susceptible populations. At 12 wk after application, pyroxasulfone/flumioxazin (240 g ai ha−1), pyroxasulfone/sulfentrazone (300 g ai ha−1), andS-metolachlor/metribuzin (1,943 g ai ha−1) controlled GR common waterhemp 97%, 92%, and 87%, respectively. Pyroxasulfone/sulfentrazone orS-metolachlor/metribuzin applied PRE followed by acifluorfen (600 g ai ha−1) or fomesafen (240 g ai ha−1) applied POST controlled GR common waterhemp 98% and performed better than PRE or POST alone. This research is the first to determine the resistance factor of GR common waterhemp in Ontario and identifies control strategies in soybean to mitigate the impact of common waterhemp interference in soybean crop production.

Pollen-mediated gene flow from glyphosate-resistant common waterhemp (Amaranthus rudis Sauer): consequences for the dispersal of resistance genes

Gene flow is an important component in evolutionary biology; however, the role of gene flow in dispersal of herbicide-resistant alleles among weed populations is poorly understood. Field experiments were conducted at the University of Nebraska-Lincoln to quantify pollen-mediated gene flow (PMGF) from glyphosate-resistant (GR) to -susceptible (GS) common waterhemp using a concentric donor-receptor design. More than 130,000 common waterhemp plants were screened and 26,199 plants were confirmed resistant to glyphosate. Frequency of gene flow from all distances, directions, and years was estimated with a double exponential decay model using Generalized Nonlinear Model (package gnm) in R. PMGF declined by 50% at <3 m distance from the pollen source, whereas 90% reduction was found at 88 m (maximum) depending on the direction of the pollen-receptor blocks. Amplification of the target site gene, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), was identified as the mechanism of glyphosate resistance in parent biotype. The EPSPS gene amplification was heritable in common waterhemp and can be transferred via PMGF, and also correlated with glyphosate resistance in pseudo-F2 progeny. This is the first report of PMGF in GR common waterhemp and the results are critical in explaining the rapid dispersal of GR common waterhemp in Midwestern United States.

Control of Glyphosate-Resistant Common Waterhemp (Amaranthus rudis) in Glufosinate-Resistant Soybean

Glyphosate-resistant (GR) common waterhemp has become a significant problem weed in Nebraska and several Midwestern states. Several populations of GR common waterhemp are also resistant to acetolactate synthase (ALS)-inhibiting herbicides, making them difficult to control with POST herbicides in GR soybean. Glufosinate-resistant (GFR) soybean is an alternate system for controlling GR common waterhemp, justifying the need for evaluating glufosinate-based herbicide programs. The objectives of this study were to compare POST-only herbicide programs (including one-pass and two-pass POST programs) with PRE followed by (fb) POST herbicide programs for control of GR common waterhemp in GFR soybean and their effect on common waterhemp density, biomass, and soybean yield. Field experiments were conducted in 2013 and 2014 near Fremont, NE in a grower’s field infested with GR common waterhemp. Glufosinate applied early- and late-POST provided 76% control of GR common waterhemp at 14 d after late-POST (DALPOST) compared with 93% control with a PRE fb POST program when averaged across treatments. The PRE application of chlorimuron plus thifensulfuron plus flumioxazin,S-metolachlor plus fomesafen or metribuzin, saflufenacil plus dimethenamid-P fb glufosinate provided ≥95% control of common waterhemp throughout the growing season, reduced common waterhemp density to ≤2.0 plants m─2, caused ≥94% biomass reduction, and led to 1,984 to 2,210 kg ha─1soybean yield. Averaged across treatments, the PRE fb POST program provided 82% common waterhemp control at soybean harvest, reduced density to 23 plants m─2at 14 DALPOST, and caused 86% biomass reduction and 1,803 kg ha─1soybean yield compared with 77% control, 99 plants m─2, 53% biomass reduction, and 1,190 kg ha─1yield with POST-only program. It is concluded that PRE fb POST programs with multiple effective modes of action are available for control of GR common waterhemp in GFR soybean.

Comparison of Herbicide Programs for Season-Long Control of Glyphosate-Resistant Common Waterhemp (Amaranthus rudis) in Soybean

The evolution of glyphosate and acetolactate synthase (ALS) inhibitor-resistant common waterhemp in the Midwestern United States has reduced the number of effective POST herbicide options for management of this problem weed in glyphosate-resistant soybean. Moreover, common waterhemp emerges throughout the crop growing season, justifying the need to evaluate herbicide programs that provide season-long control. The objectives of this study were to compare POST-only and PRE followed by (fb) POST herbicide programs for control of glyphosate-resistant common waterhemp in glyphosate-resistant soybean. Field experiments were conducted in 2013 and 2014 in Dodge County, NE, in a field infested with glyphosate-resistant common waterhemp. Programs containing PRE herbicides resulted in ≥83% control of common waterhemp and densities of ≤35 plantsm–2at 21 d after PRE (DAPRE). Post-only herbicide programs resulted in &lt;70% control and densities of 107 to 215 plants m–2at 14 d after early-POST (DAEPOST) treatment. PRE fb POST herbicide programs, including saflufenacil plus imazethapyr plus dimethenamid-P, sulfentrazone plus cloransulam, orS-metolachlor plus metribuzin, fb fomesafen plus glyphosate;S-metolachlor plus fomesafen fb acifluorfen plus glyphosate resulted in &gt;90% control of glyphosate-resistant common waterhemp throughout the growing season, reduced density to ≤7plantsm–2, ≥92% biomass reduction, and soybean yield &gt;2,200kg ha–1. Averaged across herbicide programs, common waterhemp control was 84%, and density was 15 plants m–2with PRE fb POST herbicide programs compared with 42% control, and density of 101 plants m–2with POST-only herbicide programs at harvest. Results of this study indicated that PRE fb POST herbicide programs with effective modes of action exist for season-long control of glyphosate-resistant common waterhemp in glyphosate-resistant soybean.

Glyphosate-Resistant Weed Control and Soybean Injury in Response to Different PPO-Inhibiting Herbicides

&lt;p&gt;In Nebraska, 5-enolpyruvyl shikimate-3-phosphate synthase (EPSPS) as well as acetolactate synthase (ALS)-inhibitor-resistant weeds occur in many soybean fields where herbicides from these modes-of-action have been frequently used in the past. Currently, the protoporphyrinogen oxidase (PPO)-inhibitors are the only effective herbicides for POST control of both glyphosate- and ALS-inhibitor-resistant weeds in soybean. Greenhouse experiments were conducted in 2014 to evaluate the efficacy of PPO-inhibitors applied POST for the control of three glyphosate-resistant (GR) weeds and potential for soybean injury, when applied at two growth stages. All herbicide treatments controlled 10- and 20-cm tall GR common waterhemp ≥ 95% at 21 DAT. GR giant ragweed and kochia were controlled 86 to 99% when treated at 10-cm height and 78 to 92% at 20-cm height by 21 DAT. Herbicide treatments reduced shoot biomass in the three GR weeds 88 to 100% when treated at 10-cm height and 73 to 100% when treated at 20-cm height, at 21 DAT. Soybean injury and shoot biomass data revealed that acifluorfen and lactofen were more injurious (≥ 17%), whereas fomesafen, and fomesafen plus glyphosate were relatively safer (&amp;lt; 10% injury). Overall, fomesafen and fomesafen plus glyphosate caused least injury to soybean and were more effective in controlling GR common waterhemp, giant ragweed, and kochia compared with acifluorfen and lactofen.&lt;/p&gt;

Growth Stage Affects Dose Response of Selected Glyphosate-Resistant Weeds to Premix of 2,4-D Choline and Glyphosate (Enlist Duo™ Herbicide*)

With the intent to control glyphosate–resistant and hard to control weeds, a formulation of 2,4-D choline (24.4%) and glyphosate (22.1%) (Enlist Duo™ herbicide) (Note 1) has been developed recently to be used post-emergence in corn and soybean tolerant to Enlist Duo™ in the United States. Dose response studies were conducted under greenhouse conditions for the evaluation of effective rates of Enlist Duo™ to control glyphosate-resistant common waterhemp (Amaranths rudis Sauer), giant ragweed (Ambrosia trifida L.), and kochia [Kochia scoparia (L.) Schrad] and to determine the effect of growth stage of these weeds on the efficacy of Enlist Duo™. Three parameter log-logistic models were used to develop dose response curves. Glyphosate-resistant giant ragweed was the most sensitive of the three weed species, followed by common waterhemp, and kochia. Based on the visual control or injury estimates, the Enlist Duo™ rates required for 90% control (ED90) of common waterhemp, giant ragweed, and kochia were 1179, 825, and 4,382 g ae ha -1 , respectively, for 10-cm tall plants compared to 2,480, 1,101, and 5,305 g ae ha -1 , respectively, for 20-cm tall plants at 21 days after treatment (DAT). The ED90 values calculated on the basis of percent shoot biomass reduction and visual control or injury estimates were usually similar at 21 DAT. The greenhouse studies indicate that Enlist Duo™ can effectively control less than or equal to 20-cm tall glyphosate-resistant giant ragweed and less than or equal to 10-cm tall glyphosate-resistant common waterhemp at the recommended rate (1,640 g ae ha -1 ).

Response of selected glyphosate-resistant broadleaved weeds to premix of fluthiacet-methyl and mesotrione (Solstice™) applied at two growth stages

Ganie, Z. A., Stratman, G. and Jhala, A. J. 2015. Response of selected glyphosate-resistant broadleaved weeds to premix of fluthiacet-methyl and mesotrione (Solstice™) applied at two growth stages. Can. J. Plant Sci. 95: 861–869. A premix of fluthiacet-methyl and mesotrione (1:17.5 ratio) has recently been registered for post-emergence (POST) broadleaf weed control in corn. The objective of this study was to determine the response of glyphosate-resistant common waterhemp, giant ragweed, and kochia to a premix of fluthiacet-methyl and mesotrione when applied to 10- and 20-cm-tall plants. Greenhouse dose response studies were conducted and log-logistic models were used to determine how the response varies between the weed species at two growth stages under various rates ranging from 21.8 to 218.5 g a.i. ha−1 (0.25× to 2.5×). The effective rate required to achieve &gt;90% control and shoot biomass reduction varied depending on the weed species and growth stage. The rates required for 90% control (ED90) of 10-cm-tall glyphosate-resistant common waterhemp, giant ragweed, and kochia were 78, 251, and 17 g a.i. ha−1, respectively, compared with 144, 489, and 79,349 g a.i. ha−1, respectively, for 20-cm-tall plants at 21 d after treatment (DAT). Based on visual control estimates at ED90 level, glyphosate-resistant common waterhemp was the most sensitive at both growth stages; kochia was sensitive at 10-cm height, while giant ragweed was the least sensitive at both growth stages. Irrespective of weed species or growth stage, the ED90 values calculated on the basis of shoot biomass reduction were mostly higher compared with visual control estimates. It is concluded that premix of fluthiacet-methyl plus mesotrione can be potentially used as a POST herbicide in corn for controlling glyphosate-resistant common waterhemp and kochia (≤ 10 cm tall) at the labeled rate (87 g a.i. ha−1).

Confirmation and Control of Glyphosate-Resistant Common Waterhemp (Amaranthus rudis) in Nebraska

Glyphosate-resistant common waterhemp is a difficult-to-control annual broadleaf weed that has become a serious management challenge for growers in Nebraska and other states in the United States. The objectives of this study were to confirm glyphosate-resistant common waterhemp in Nebraska by quantifying level of resistance in a dose-response study, and to determine the sensitivity and efficacy of POST soybean herbicides for controlling suspected glyphosate-resistant common waterhemp biotypes. Seeds of suspected glyphosate-resistant common waterhemp biotypes were collected from seven eastern Nebraska counties. Greenhouse dose-response experiments were conducted to evaluate the response of common waterhemp biotypes to nine rates of glyphosate (0 to 16×). Common waterhemp biotypes were 3- to 39-fold resistant to glyphosate depending on the biotype being investigated and the susceptible biotype used for comparison. Results of the POST soybean herbicides efficacy experiment suggested that glyphosate-resistant biotypes, except a biotype from Pawnee County, had reduced sensitivity to acetolactate synthase (ALS)–inhibiting herbicides (chlorimuron-ethyl, imazamox, imazaquin, imazethapyr, and thifensulfuron-methyl). Glufosinate and protoporphyrinogen oxidase (PPO)–inhibiting herbicides (acifluorfen, fluthiacet-methyl, fomesafen, and lactofen) provided ≥ 80% control of glyphosate-resistant common waterhemp at 21 d after treatment (DAT). This study confirmed the first occurrence of glyphosate-resistant common waterhemp in Nebraska, and also revealed reduced sensitivity to ALS-inhibiting herbicides in most of the biotypes tested in this study.

Interactions between Glyphosate,FusariumInfection of Common Waterhemp (Amaranthus rudis), and Soil Microbial Abundance and Diversity in Soil Collections from Missouri

Greenhouse and laboratory experiments were conducted on common waterhemp and soil collected from 131 soybean fields in Missouri that contained late-season common waterhemp escapes. The objectives of these experiments were to determine the effects of soil sterilization on glyphosate-resistant (GR) and -susceptible (GS) common waterhemp survival, to determine the effects of soil sterilization and glyphosate treatment on infection of GR and GS common waterhemp biotypes byFusariumspp., and to determine the soil microbial abundance and diversity in soils collected from soybean fields with differences in common waterhemp biotypes and herbicide and crop rotation histories. Common waterhemp biotypes were treated with 1.7 kg glyphosate ae ha−1or left untreated once plants reached approximately 15 cm in height. Common waterhemp survival was visually assessed at 21 d after glyphosate treatment (21 DAT). To determineFusariuminfection frequency, a single intact common waterhemp root was harvested from each treatment at 0, 3, 7, 14, and 21 DAT and surface sterilized, and 10 to 15–mm common waterhemp root sections were plated on Komada culture medium. After 14 d incubation, fungal colonies were selected from colonized roots and maintained on potato dextrose agar medium amended with antibiotics before identification. Speciation ofFusariumisolates was conducted through microscopic examination of fungal characters and confirmed by sequencing and analysis of ribosomal DNA. Soil samples from 131 different collections were subjected to phospholipid fatty acid (PLFA) analysis and were conducted utilizing gas chromatography to determine the soil microbial community abundance and structure. Common waterhemp plants grown in sterile soils had the highest common waterhemp survival, regardless of biotype. After treatment with glyphosate, survival of GS common waterhemp grown in nonsterile soil was only 29% 21 DAT, whereas survival of GS common waterhemp grown in nonsterile soil was only 10%. Similarly, GR common waterhemp survival was reduced from 83 to 61% following treatment with glyphosate when grown in nonsterile compared to sterile soil.Fusariumspp. were recovered from only 12% of the assayed roots (223 treatments withFusariumout of a total 1,920 treatments). The greatest occurrence ofFusariumroot infection in both GR and GS common waterhemp occurred in nonsterile soils following a glyphosate treatment. Few differences in total PLFA were observed in field soil collected from locations with either GR or GS common waterhemp, and regardless of herbicide or crop history. This research supports previous findings that plant species are more sensitive to glyphosate in nonsterile than sterile soils and indicates glyphosate may predispose plants to soil-borne phytopathogens. This research also suggests that continuous use of glyphosate does not significantly affect soil microbial abundance or diversity.

Glyphosate-Resistant Common Waterhemp (Amaranthus rudis) Confirmed in Texas

Glyphosate-resistantAmaranthusspecies are a recognized risk to U.S. agriculture. With affected cropland exceeding 1.2 million ha, this epidemic is particularly pertinent to agricultural regions that utilize an intensive glyphosate-based management program to control weedy pests. Before 2006, Texas had no identified glyphosate-resistant populations. Two independent common waterhemp populations exhibiting poor control by glyphosate were identified in Wharton County and Fort Bend County, TX in 2006 and 2008, respectively. The objective of the present research was to characterize the level of glyphosate resistance (50% lethal dose [LD50] and 50% reduction in growth rate [GR50]) in each population. Resistance levels in four putatively glyphosate-resistant common waterhemp biotypes selected from these two populations were compared with confirmed glyphosate-resistant and -susceptible common waterhemp populations under greenhouse conditions. The LD50value for the susceptible population (736 g ae ha−1) was equivalent to the 0.9× labeled rate of glyphosate, whereas the putatively resistant lines exhibited a broad range of resistance with LD50values ranging from 3.5 to 59.7× the labeled rate of glyphosate. The GR50value for the most resistant line was 2.5-fold greater than the susceptible biotype (317 g ae ha−1of glyphosate). These results confirm the first documented case of a glyphosate-resistant weed species in Texas.

Weed Control in Dicamba-Resistant Soybeans

Field experiments were conducted in 11 states to evaluate broadleaf weed management programs in dicamba-resistant soybeans which involved the use of preemergence and postemergence dicamba. Preemergence (PRE) dicamba at 0.25 lb ae/acre provided less than 60% control of smooth pigweed, giant ragweed, velvetleaf, palmer amaranth, waterhemp, and morningglory spp., but 97% control of common lambsquarters and horseweed at 3 weeks after treatment (WAT). Preemergence flumioxazin plus chlorimuron or sulfentrazone plus cloransulam provided 66 to 100% control of these weeds. Use of dicamba postemergence (POST) improved uniformity of control of velvetleaf, smooth pigweed, morningglory, and glyphosate-susceptible waterhemp. However, combining dicamba at 0.25 lb/acre with glyphosate resulted in 30% to 65% greater control of glyphosate-resistant palmer amaranth, glyphosate-resistant common waterhemp, glyphosate-resistant horseweed, and glyphosate-resistant giant ragweed compared to sequentially applied glyphosate.

Evaluation of herbicide programs for the management of glyphosate-resistant waterhemp ( Amaranthus rudis) in Maize

Field experiments were conducted in corn to evaluate the influence of herbicide treatments on glyphosate resistant waterhemp control in Platte County, Missouri during 2006 and 2007. Pre-emergence (PRE), pre-emergence followed by post-emergence (PRE fb POST), and post-emergence-only (POST-only) herbicide programs were evaluated for use in conventional, glyphosate-resistant, or glufosinate-resistant corn hybrids. All programs containing a PRE herbicide, conventional POST herbicide, or glufosinate resulted in greater than 98% control of glyphosate-resistant waterhemp and reduced seed production by at least 99%. Waterhemp densities were also reduced by programs that contained a PRE herbicide, conventional POST herbicide, or glufosinate. The highest waterhemp density and poorest control occurred with a sequential glyphosate program. Corn yields did not differ between herbicide treatments in 2006 however certain conventional PRE and PRE fb POST programs increased yields compared to the sequential glyphosate program in 2007. Results from this research indicate that several herbicide options are available for the control of glyphosate-resistant common waterhemp in corn. However, the lack of crop rotation at this field site for a period of at least 15 consecutive years and lack of residual herbicide applications typically utilized in corn likely contributed to the increased sensitivity of this waterhemp population to PRE-only corn herbicide programs.