Control of acetolactate synthase inhibitor/glyphosate-resistant Palmer amaranth (Amaranthus palmeri) in isoxaflutole/glufosinate/glyphosate-resistant soybean

Palmer amaranth is the most problematic and troublesome weed in agronomic cropping systems in the United States. Acetolactate synthase (ALS) inhibitor and glyphosate-resistant (GR) Palmer amaranth has been confirmed in Nebraska and it is widespread in several counties. Soybean resistant to isoxaflutole/glufosinate/glyphosate has been developed that provides additional herbicide site of action for control of herbicide-resistant weeds. The objectives of this study were to evaluate herbicide programs for control of ALS inhibitor/GR Palmer amaranth and their effect on Palmer amaranth density and biomass, as well as soybean injury and yield in isoxaflutole/glufosinate/glyphosate-resistant soybean. Field experiments were conducted in a grower\u27s field infested with ALS inhibitor and GR Palmer amaranth near Carleton, Nebraska, in 2018 and 2019. Isoxaflutole applied alone or mixed with sulfentrazone/pyroxasulfone, flumioxazin/pyroxasulfone, or imazethapyr/saflufenacil/pyroxasulfone provided similar control (86%-99%) of Palmer amaranth 21 d after PRE (DAPRE). At 14 d after early-POST (DAEPOST), isoxaflutole applied PRE and PRE followed by (fb) POST controlled Palmer amaranth by 10% to 63% compared to 75% to 96% control with glufosinate applied EPOST in both years. A PRE herbicide fb glufosinate controlled Palmer amaranth 80% to 99% 21 d after late-POST (DALPOST) in 2018, and reduced density 89% to 100% in 2018 and 58% to 100% in 2019 at 14 DAEPOST. No soybean injury was observed from any of the herbicide programs tested in this study. Soybean yield in 2019 was relatively higher due to higher precipitation compared with 2018 with generally no differences between herbicide programs. This research indicates that herbicide programs are available for effective control of ALS inhibitor/GR Palmer amaranth in isoxaflutole/glufosinate/glyphosate-resistant soybean

Biology, Gene Flow, and Management of Glyphosate-Resistant Common Waterhemp (\u3ci\u3eAmaranthus rudis\u3c/i\u3e Sauer) in Nebraska

Common waterhemp is the most troublesome weed in the midwestern United States. Growers from Nebraska reported failure to control common waterhemp following sequential applications of glyphosate in glyphosate-tolerant corn and soybean, which led to moderate to severe yield loss; justifying the need to confirm resistance and study the biology and management of common waterhemp. The objectives of this research were: 1) to confirm the presence of glyphosate-resistant (GR) common waterhemp biotypes in Nebraska and to evaluate their sensitivity to herbicides belonging to alternative sites-of-action; 2) to evaluate the response of common waterhemp to water stress; 3) to quantify pollen-mediated gene flow from GR common waterhemp under field conditions; and 4) to evaluate different herbicide programs for season-long control of glyphosate-resistant common waterhemp in glyphosate-tolerant soybean. Greenhouse dose-response studies conducted to evaluate the response of suspected GR common waterhemp biotypes collected from seven eastern Nebraska counties (Antelope, Dodge, Fillmore, Lancaster, Pawnee, Seward, and Washington) revealed that the biotypes were 3- to 39-fold resistant to glyphosate. The GR biotypes also showed a reduced sensitivity to acetolactate synthase (ALS)-inhibiting herbicides (chlorimuron-ethyl, imazamox, imazaquin, imazethapyr, and thifensulfuron-methyl). Experiments conducted to evaluate the response of common waterhemp to water stress suggested that degree and duration of water stress can adversely affect the growth and seed production of common waterhemp. Highest plant height (≥ 150 cm), growth index (≥ 3.8 × 105 cm3), and seed production (\u3e 34,000 seeds plant-1) were recorded with 100% pot water content applied at 2-d intervals. Pollen-mediated gene flow studies from GR to GS biotypes were conducted under field conditions using a Nelder wheel design. Frequency of gene flow was found to be highest (up to 0.77) at the closer distances (0 to 0.1 m); whereas gene flow frequency declined by 50% at \u3c 2.5 m and 90% at distances \u3c 90 m from the pollen source. Field experiments conducted for management of GR common waterhemp in soybean showed that preemergenece (PRE) followed by postemergence (POST) herbicide programs with multiple sites-of-action provided season-long control of GR common waterhemp and resulted in the highest soybean yield compared to the POST-only herbicide programs. Adviser: Amit J. Jhal

Biology, gene flow, and management of glyphosate-resistant common waterhemp (Amaranthus rudis Sauer) in Nebraska

Common waterhemp is the most troublesome weed in the midwestern United States. Growers from Nebraska reported failure to control common waterhemp following sequential applications of glyphosate in glyphosate-tolerant corn and soybean, which led to moderate to severe yield loss; justifying the need to confirm resistance and study the biology and management of common waterhemp. The objectives of this research were: 1) to confirm the presence of glyphosate-resistant (GR) common waterhemp biotypes in Nebraska and to evaluate their sensitivity to herbicides belonging to alternative sites-of-action; 2) to evaluate the response of common waterhemp to water stress; 3) to quantify pollen-mediated gene flow from GR common waterhemp under field conditions; and 4) to evaluate different herbicide programs for season-long control of glyphosate-resistant common waterhemp in glyphosate-tolerant soybean. Greenhouse dose-response studies conducted to evaluate the response of suspected GR common waterhemp biotypes collected from seven eastern Nebraska counties (Antelope, Dodge, Fillmore, Lancaster, Pawnee, Seward, and Washington) revealed that the biotypes were 3- to 39-fold resistant to glyphosate. The GR biotypes also showed a reduced sensitivity to acetolactate synthase (ALS)-inhibiting herbicides (chlorimuron-ethyl, imazamox, imazaquin, imazethapyr, and thifensulfuron-methyl). Experiments conducted to evaluate the response of common waterhemp to water stress suggested that degree and duration of water stress can adversely affect the growth and seed production of common waterhemp. Highest plant height (≥ 150 cm), growth index (≥ 3.8 × 105 cm3), and seed production (\u3e 34,000 seeds plant-1) were recorded with 100% pot water content applied at 2-d intervals. Pollen-mediated gene flow studies from GR to GS biotypes were conducted under field conditions using a Nelder wheel design. Frequency of gene flow was found to be highest (up to 0.77) at the closer distances (0 to 0.1 m); whereas gene flow frequency declined by 50% at \u3c 2.5 m and 90% at distances \u3c 90 m from the pollen source. Field experiments conducted for management of GR common waterhemp in soybean showed that preemergenece (PRE) followed by postemergence (POST) herbicide programs with multiple sites-of-action provided season-long control of GR common waterhemp and resulted in the highest soybean yield compared to the POST-only herbicide programs

Episode 16: Week Management in Fall and What to do with Weed Escapes

Runtime 30:3

Biologically effective rates of a new premix (atrazine, bicyclopyrone, mesotrione, and S-metolachlor) for pre-emergence or post-emergence control of common waterhemp (Amaranthus rudis Sauer) in corn

A premix of atrazine, bicyclopyrone, mesotrione, and S-metolachlor was recently approved for broad-spectrum weed control in corn in the United States. Greenhouse and field experiments were conducted in 2015 and 2016 to evaluate the response of common waterhemp to various rates of the premix applied pre-emergence or post-emergence in corn. In greenhouse dose-response bioassays, pre-emergence application of the premix at 975 g ai ha─1 provided 90% control (visual estimates) of common waterhemp at 28 DAT. The ED90 values for post-emergence applications were 1,157 and 1,838 g ai ha─1 at 21 DAT when applied to 8 to 10, and 15 to 18 cm tall common waterhemp, respectively. Under field conditions, the premix applied pre-emergence at the labeled rate (2,900 g ai ha─1) provided 98 and 91% control of common waterhemp at 14 and 63 d after treatment (DAT), respectively. The ED90 values for the in-field post-emergence dose-response bioassay were 680 and 2,302 g ai ha─1 at 14 DAT for the 8 to 10 and 15 to 18 cm tall common waterhemp, respectively. The root mean square error (RMSE) and the model efficiency coefficient (EF) values indicated a good fit for the prediction models. Spearman’s correlation coefficient (rs) showed that corn yield was positively correlated (rs ≥ 0.55; PThe accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

A follow‐up survey to assess stakeholders’ perspectives on weed management challenges and current practices in Nebraska, USA

Abstract Stakeholders across the state of Nebraska, USA, were surveyed in 2019–2020 to assess problem weeds and weed management practices in agronomic crops. A total of 420 complete responses were obtained across four Nebraska districts (Northeast, Panhandle, Southeast, and West Central). Accumulated across the state, 65.5% of farmed or scouted crop ground in Nebraska was under no‐till production, with the major crops being corn and soybean representing 39.3% and 30.7% of agronomic crop production area, respectively. Palmer amaranth, horseweed, waterhemp, kochia, and giant ragweed were ranked the most problematic weeds. In a 2014–2015 survey, Palmer amaranth was the sixth most problematic weed. The most used preplant herbicides were 2,4‐D, glyphosate, and dicamba in the 2019–2020 survey. Atrazine applied alone or in mixture with acetochlor, bicyclopyrone, clopyralid, mesotrione, or S‐metolachlor were the most applied pre‐emergence (PRE) herbicides in corn, whereas the most applied PRE herbicides in soybean were metribuzin/sulfentrazone, flumioxazin/pyroxasulfone, and chloransulam‐methyl/sulfentrazone. Like the previous survey, glyphosate was the most frequent choice of survey respondents as a post‐emergence (POST) herbicide in glyphosate‐resistant corn and soybean, while 2,4‐D was the most applied POST herbicide in grain sorghum and wheat. Most of the respondents (77%) were aware of the new multiple herbicide‐resistant crops, and 86% listed physical drift and volatility of dicamba/2,4‐D as a primary concern. Twenty‐three percent of survey respondents identified integrated pest management as a primary research and extension priority for profitable agronomic crop production in Nebraska

Late-season surveys to document seed rain potential of Palmer amaranth (Amaranthus palmeri) and waterhemp (Amaranthus tuberculatus) in Texas cotton.

Weed escapes are often present in large production fields prior to harvest, contributing to seed rain and species persistence. Late-season surveys were conducted in cotton (Gossypium hirsutum L.) fields in Texas in 2016 and 2017 to identify common weed species present as escapes and estimate seed rain potential of Palmer amaranth (Amaranthus palmeri S. Watson) and waterhemp [A. tuberculatus (Moq.) J.D. Sauer], two troublesome species with high fecundity. A total of 400 cotton fields across four major cotton-producing regions in Texas [High Plains (HP), Gulf Coast (GC), Central Texas, and Blacklands] were surveyed. Amaranthus palmeri, Texas millet [Urochloa texana (Buckley) R. Webster], A. tuberculatus, ragweed parthenium (Parthenium hysterophorus L.), and barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] were reported as the top five weed escapes in cotton fields. Amaranthus palmeri was the most prevalent weed in the HP and Lower GC regions, whereas A. tuberculatus escapes were predominantly observed in the Upper GC and Blacklands regions. On average, 9.4% of an individual field was infested with A. palmeri escapes in the Lower GC region; however, 5.1 to 8.1% of a field was infested in the HP region. Average A. palmeri density ranged from 405 (Central Texas) to 3,543 plants ha-1 (Lower GC). The greatest seed rain potential by A. palmeri escapes was observed in the upper HP region (13.9 million seeds ha-1), whereas the seed rain potential of A. tuberculatus escapes was the greatest in the Blacklands (12.9 million seeds ha-1) and the upper GC regions (9.8 million seeds ha-1). Seed rain from late-season A. palmeri and A. tuberculatus escapes is significant in Texas cotton, and effective management of these escapes is imperative for minimizing seedbank inputs and impacting weed species persistence

Pollen-mediated gene flow from glyphosate-resistant common waterhemp (\u3ci\u3eAmaranthus rudis\u3c/i\u3e 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 donorreceptor 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 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