Modeling pollen-mediated gene flow from glyphosate-resistant to -susceptible giant ragweed (\u3ci\u3eAmbrosia trifida\u3c/i\u3e L.) under field conditions

A field experiment was conducted to quantify pollen mediated gene flow (PMGF) from glyphosateresistant (GR) to glyphosate-susceptible (GS) giant ragweed under simulated field conditions using glyphosate resistance as a selective marker. Field experiments were conducted in a concentric design with the GR giant ragweed pollen source planted in the center and GS giant ragweed pollen receptors surrounding the center in eight directional blocks at specified distances (between 0.1 and 35 m in cardinal and ordinal directions; and additional 50 m for ordinal directions). Seeds of GS giant ragweed were harvested from the pollen receptor blocks and a total of 100,938 giant ragweed plants were screened with glyphosate applied at 2,520 g ae ha−1 and 16,813 plants confirmed resistant. The frequency of PMGF was fit to a double exponential decay model selected by information-theoretic criteria. The highest frequency of gene flow (0.43 to 0.60) was observed at ≤0.5 m from the pollen source and reduced rapidly with increasing distances; however, gene flow (0.03 to 0.04) was detected up to 50 m. The correlation between PMGF and wind parameters was inconsistent in magnitude, direction, and years

Overlapping Residual Herbicides for Control of Photosystem (PS) II- and 4-Hydroxyphenylpyruvate Dioxygenase (HPPD)-Inhibitor-Resistant Palmer amaranth (\u3ci\u3eAmaranthus palmeri\u3c/i\u3e S. Watson) in Glyphosate-Resistant Maize

A Palmer amaranth (Amaranthus palmeri S. Watson) biotype has evolved resistance to photosystem (PS) II- (atrazine) and 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides (mesotrione, tembotrione, and topramezone) in maize seed production field in Nebraska, USA. The objectives of this study were to determine the effect of soil residual pre-emergence (PRE) herbicides followed by (fb) tank-mixture of residual and foliar active post-emergence (POST) herbicides on PS-II- and HPPD-inhibitor-resistant Palmer amaranth control, maize yield, and net economic returns. Field experiments were conducted in a grower’s field infested with PS II- and HPPD-inhibitor-resistant Palmer amaranth near Shickley in Fillmore County, Nebraska, USA in 2015 and 2016. The contrast analysis suggested that saflufenacil plus dimethenamid-P or pyroxasulfone plus saflufenacil applied PRE provided 80–82% Palmer amaranth control compared to 65 and 39% control with saflufenacil and pyroxasulfone applied alone at 3 weeks after PRE (WAPRE), respectively. Among the PRE fb POST herbicide programs, 95–98% Palmer amaranth control was achieved with pyroxasulfone plus safluefenacil, or saflufenacil plus dimethenamid-P applied PRE, fb glyphosate plus topramezone plus dimethenamid-P plus atrazine, glyphosate plus diflufenzopyr plus dicamba plus pyroxasulfone, glyphosate plus diflufenzopyr plus pendimethalin, or glyphosate plus diflufenzopyr plus dicamba plus atrazine applied POST at 3 weeks after POST (WAPOST) through maize harvest. Based on contrast analysis, PRE fb POST programs provided 77–83% Palmer amaranth control at 3 WAPOST through maize harvest compared to 12–15% control with PRE-only and 66–84% control with POST-only programs. Similarly, PRE fb POST programs provided 99% biomass reduction at 6 WAPOST compared to PRE-only (28%) and POST-only (87%) programs. PRE fb POST programs provided higher maize yield (13,617 kg ha−1) and net return (US $1,724 ha−1) compared to the PRE-only (2,656 kg ha−1; US$285 ha−1) and POST-only (11,429 kg ha−1; US $1,539 ha−1) programs. The results indicated that effective control of multiple herbicide-resistant Palmer amaranth can be achieved with PRE fb POST programs that include herbicides with overlapping residual activity to maintain season-long control

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

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 (\u3c 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

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

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 (\u3c 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

Water Use Characteristics of Weeds: A Global Review, Best Practices, and Future Directions

Weeds usually penalize crop yields by competing for resources, such as water, light, nutrients, and space. Most of the studies on the crop-weed competition domain are limited to assessing crop-yield losses due to weed pressure and other crop-weed interactions, overlooking the significant uptake of soil-water by weeds that exacerbates global water constraints and threatens the productivity and profitability. The objective of this review was to synthesize globally available quantitative data on weed water use (WU) sourced from 23 peer-reviewed publications (filtered from 233 publications via a multi-step protocol of inclusion criteria) with experimental investigations across space (3 continents), time (1927–2018), weed species (27 broadleaf and 7 grasses) and characteristics, cropping systems (5), soil types (ranging from coarse-textured sand to fine-textured clay soils), determination techniques, experimental factors (environment, management, resource availability, and competition), and aridity regimes (ranging from semi-arid to humid climate). Distributions of weed WU data reported via eight different metrics were assessed for variability and mean WU. A lack of the best experimental and reporting practices in weed WU research was identified that undermined the robustness, transferability, and application of the WU data. Mandatory protocols and the best practices typically followed in the agricultural water management research were described and recommended for weed scientists to avoid pitfalls in quantifying and presenting weed WU. A model of mixed plant community evapotranspiration (ET) was adapted to model weed-crop-soil system evaporation and transpiration in a crop canopy infested with multiple (n) weed species. Finally, potential cross-disciplinary questions across the domains of crop science, weed science, agricultural water management, irrigation science and engineering, and environmental changes were proposed to direct and prioritize future research efforts in the crop-weed-water arena

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

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 Nebrask

Herbicide-Resistant Palmer amaranth (Amaranthus palmeri S. Wats.) in the United States — Mechanisms of Resistance, Impact, and Management

Palmer amaranth, a dioecious summer annual species, is one of the most troublesome weeds in the agronomic crop production systems in the United States. In the last two decades, continuous reliance on herbicide(s) with the same mode of action as the sole weed management strategy has resulted in the evolution of herbicide-resistant (HR) weeds, including Palmer amaranth. By 2015, Palmer amaranth biotypes had been confirmed resistant to acetolactate synthase (ALS)-inhibitors, dinitroanilines, glyphosate, hydroxyphenylpyruvate dioxygenase (HPPD)-inhibitors, and triazine herbicides in some parts of the United States along with multiple HR biotypes. Mechanisms of herbicide-resistance in Palmer amaranth are discussed in this chapter. Preplant herbicide options including glufosinate, 2,4-D, and dicamba provide excellent Palmer amaranth control; however, their application is limited before planting crops, which is often not possible due to unfavorable weather conditions. Agricultural biotechnology companies are developing new multiple HR crops that will allow the post-emergence application of respective herbicides for management of HR weeds, including Palmer amaranth. For the effective in-crop management of Palmer amaranth, and to reduce the potential for the evolution of other HR weeds, growers should apply herbicides with different modes of action in tank-mixture and should also incorporate cultural practices including inversion tillage and cover crops along with herbicide programs

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

Pollen-mediated gene flow from herbicide-resistant yellow corn to non-genetically engineered food-grade white corn

Nebraska is the number one producer of food-grade white corn in the United States. Pollen-mediated gene flow (PMGF) from genetically engineered high alpha-amylase corn, known as Enogen corn, to food-grade white corn can have undesirable outcomes. Alpha-amylase can convert starch in white corn to sugar during or after its processing, degrading the quality of processed products. Thus, proximity to Enogen corn puts white corn production at risk. The objectives of this study were to evaluate the PMGF from herbicide-resistant yellow corn to food-grade white corn and assess the significance of wind speed and direction. Field experiments were set up using a Nelder-wheel design in 2021 and 2022 in Nebraska, with yellow field corn as pollen donor in the center and white corn surrounding it as pollen receptor. At the end of the season, samples of white corn cobs were collected up to 50 and 70 m from four cardinal and ordinal directions, respectively. PMGF was detected by counting the number of yellow kernels on the white cobs. More than 4 million kernels were screened and the highest frequency of PMGF (0.0621–0.1950) was detected at the nearest distance (1 m). PMGF decreased exponentially with distance; however, it was still observed (0.0020–0.0032) at the greatest distance (70 m) evaluated in this study. Wind profile played a significant role in PMGF. Wind frequency (r = 0.58 ≤ 0.86) and wind run (r = 0.36 ≤ 0.95) were moderately to strongly correlated with PMGF in 2021 and 2022, respectively. The results are concerning for white corn growers due to the coexistence of Enogen corn and food-grade white corn