Waterhemp is the most problematic weed in Minnesota impacting several thousand acres of soybean in the state. When competing with the row crops, the herbicide-resistant waterhemp populations can reduce the crop yields substantially and the contaminations of waterhemp seeds in soybean can decrease the market value for the produce. Moreover, waterhemp populations resistant to multiple herbicide chemistries including some of the important herbicides such as Classic, Flexstar, and Glyphosate have already been reported in Minnesota. The multiple herbicide-resistant waterhemp populations are reducing the value of recently developed multiple herbicide-tolerant soybean traits such as RR2 Xtend (dicamba + glyphosate-tolerant), Enlist E3 (2,4-D choline + glyphosate + glufosinate), and XtendFlex (dicamba + glyphosate + glufosinate). Waterhemp can emerge throughout the growing season and sequential application of soil residual herbicides are recommended for optimum control of this weed; however, some of these herbicides may cause carryover issues and injure subsequent sugarbeet crops.

This proposed research will include a survey of suspected herbicide-resistant waterhemp populations in Minnesota soybean. Growers and stakeholders will be contacted to submit the waterhemp seed samples that survive the pre-emergence and post-emergence herbicide treatments. Then, they will be screened for resistance to soybean herbicides (both pre-emergence and post-emergence) in the greenhouse located at the University of Minnesota’s St. Paul campus. If needed, the genetic tests will be performed on selective populations at the Molecular Weed Science Lab at Colorado State University to confirm the results obtained in the greenhouse research. The individual growers will be informed about the herbicide resistance status of their submitted waterhemp samples and the results will be presented in different Extension meetings and web-posts.

The second goal of this project is to evaluate the impact of integrated weed management in soybean on waterhemp control and soybean yield in soybean-sugarbeet rotation. The research will be conducted at Clay County and Renville County, MN, and the treatments will include different herbicide management options (low-input and high-input) along with soybean row-spacing (15- and 22-inches) and waterhemp seed removal at harvest.

The second goal of this research will be conducted at two locations in the state to represent different environmental conditions. Also, this approach will serve the soybean growers in different parts of Minnesota. We will have multiple field days, and plot tours at each site to demonstrate the research work. In several surveys, soybean growers have indicated that weed management is among their top priorities, and in this proposal, we are addressing some of the timely weed management issues that need immediate attention. This research will promote sustainable waterhemp management and enhance soybean yield and profitability in Minnesota.

Background:
Waterhemp is the most troublesome weed in Minnesota soybean. In a survey conducted across Minnesota at Private Pesticide Applicator Recertification Workshops in 2020, ~50% of the respondents have ranked waterhemp as the number one problem weed in row crops in the state. Moreover, in 2016, waterhemp populations resistant to more than two herbicide sites of action (also known as multiple herbicide-resistant) were reported from several soybean fields in south-central Minnesota. New multiple herbicide-tolerant soybeans (e.g., RR2Xtend, Enlist E3, and LibertyLink GT27, etc.) are providing opportunities to apply several herbicides that are not traditionally considered soybean herbicides as pre-emergence or post-emergence treatments in soybean. However, to preserve these herbicide-tolerant technologies, it is important to know the resistance status of major weed species in the state and select the herbicides and application timing accordingly.

In 2020, the University of Minnesota’s Extension Weed Science group initiated a herbicide-resistant weed screening project (read the MN Crop News article: https://blog-crop-news.extension.umn.edu/2020/09/herbicide-resistant-weed-screening.html). As a part of this project, we have requested the growers and other stakeholders to submit suspected herbicide-resistant weed samples that survived the pre-emergence or post-emergence herbicide applications. We were able to reach out to a large group of stakeholders by disseminating the information about this project using various platforms, such as MN Crop News, Goffer Coffee Shop podcast, Minne-Line, radio spots, farm magazines, and Twitter, etc. In 2020, we received more than 70 weed samples from different parts of MN, and more than 90% of those samples were from soybean fields and about 70% of those samples were waterhemp. We have started screening these populations for multiple herbicide resistance. We will continue this research project in 2021 with a major focus on the screening of waterhemp populations for multiple herbicide resistance (Goal 1).

Waterhemp is a highly competitive weed species and can cause total crop loss if left uncontrolled (Sarangi et al. 2017). Favorable biological attributes such as rapid growth rate and smaller seed size have provided an opportunity for waterhemp to persist as the most successful weed in major cropping systems in the Midwest. Moreover, waterhemp has an extended emergence period and Werle et al. (2014) classified this weed as a late-emerging species. Therefore, the sequential application of pre-emergence herbicides was recommended for season-long control of this weed species (Soltani et al. 2009). However, due to the carryover issues to subsequent sugarbeet crop, growers have limited soil residual herbicide choices to use in soybean in a soybean-sugarbeet rotation that may lead to poor waterhemp control in soybean and an increase in weed seedbank for the subsequent sugarbeet.

The harvest weed seed control is providing an opportunity to target weed seeds at crop harvest and the inclusion of this tactic to weed management practices in soybean can reduce the weed seedbank significantly (Schwartz-Lazaro et al. 2017). This project will evaluate different herbicide programs (low-input and high-input) along with other non-chemical tactics, such as row-spacing and weed seed removal in soybean, to promote sustainable waterhemp control in soybean-sugarbeet rotation (Goal 2).

Goal 1: Survey of herbicide-resistant waterhemp populations in Minnesota soybean
Objective 1: Confirm multiple herbicide-resistant waterhemp populations
Objective 2: Check the resistance level and sensitivity of waterhemp populations to different soybean herbicides
Objective 3: Confirm the genetic basis for herbicide resistance in waterhemp populations
Goal 2: Evaluate multi-tactic waterhemp control options in soybean in a soybean-sugarbeet rotation

Objective 1: Evaluate waterhemp control, density, and biomass production in response to multi-tactic weed management tactics
Objective 2: Evaluate the impact of weed management practices on soybean yield and economics
Objective 3: Compare the effect of soybean weed management programs on waterhemp control and density in subsequent sugarbeet

Goal 1: Survey of herbicide-resistant waterhemp populations in Minnesota soybean
Waterhemp escapes following pre-emergence and post-emergence herbicide applications in soybean will be surveyed during the summer and Fall of 2021, and the waterhemp seedheads will be collected from at least 20 random plants in each field. The survey sites will either be selected randomly or based on the weed escape information from individual growers. With the help of UMN Extension Educators, independent crop consultants, and the growers, waterhemp samples will be collected from all soybean producing counties in Minnesota. Depending on their availability, the history (herbicide use, crops, and tillage) of the surveyed fields will be obtained from the individual farm owners.

The waterhemp seedheads will be cleaned at UMN’s Plant Growth Facility at St. Paul campus, and the seeds will be kept in a refrigerator for two months in airtight polyethylene bags to overcome the dormancy. The composite seed samples will be planted in plastic trays in the greenhouse at UMN’s St. Paul campus. Seedlings from individual populations (collected from individual soybean fields) will be grown separately and soybean herbicides will be sprayed at 1× and 3× of the labeled doses to check the sensitivity of waterhemp populations to different herbicides. Preemergence herbicides will be sprayed at the same day of planting, and postemergence herbicides will be applied at 4 inches weed height. A non-treated control will be included for comparison. The herbicides will be applied using a single-tip spray chamber located at the UMN’s St. Paul campus.

Waterhemp seedlings emerged after pre-emergence herbicide application will be counted and the numbers will be compared with the non-treated control. For post-emergence treatments, waterhemp injury/control will be estimated visually using a 0 to 100% scale, where 0% meaning no injury and 100% meaning complete death of the plants. Weed biomass (dry) from the treated trays will be measured and compared with the nontreated control.

Selective waterhemp populations that survived the 3× doses of at least three herbicides, will be selected for the whole-plant dose-response bioassays. That population will be sprayed with the eight doses (0, 0.125×, 0.25×, 0.5×, 1×, 2×, 4×, and 16×) of selective herbicides. Injury and aboveground dry biomass data will be collected. Leaf tissue samples from selective waterhemp populations will be collected and sent to the Molecular Weed Science Lab at Colorado State University for testing the molecular mechanisms for herbicide resistance.

Spatial maps to illustrate the distribution of herbicide-resistant waterhemp populations in Minnesota will be developed using ArcGIS software. The dose-response data analysis will be performed using a four-parameter log-logistic function (Knezevic et al. 2007) in R (R Statistical Software, Vienna, Austria). Model goodness-of-fit will also be tested using root-mean-square error (RMSE) and model efficiency coefficient using the method described by Sarangi et al. (2016).

Goal 2: Evaluate multi-tactic waterhemp control options in soybean in a soybean-sugarbeet rotation
Field experiments will be conducted in 2021 on two private farmlands (contractors’/ growers’ land) in Clay County and Renville County, MN. Locations were selected to represent two different environments. The fields where this experiment will be conducted known to have Acetolactate synthase (ALS)-inhibiting herbicide-resistant waterhemp populations. The Enlist E3 soybeans (2,4-D choline, glyphosate, and glufosinate-tolerant) will be planted. The treatments will be laid out in a split-plot design with soybean row spacing (15- and 22-inches) as the main plot factor and weed control treatment (listed in Table 2) as the subplot, and the treatments will be replicated four times. The subplot size will be 10 × 30 feet, and the herbicide applications will be made using CO2-pressurized backpack sprayers equipped with recommended nozzles.

Pre-emergence herbicides will be applied at the day of soybean planting, and the early-, and mid-post-emergence herbicide applications will be made at V2, and V5 soybean stages, respectively. Verdict will be applied pre-plant at 15 days prior to soybean planting. The weed control treatments are classified into three groups: low-input, high-input, and integrated weed management (listed in Table 2). The integrated weed management treatment will include herbicide treatments along with the waterhemp seed removal (manually) at soybean harvest to simulate the harvest weed seed control (HWSC). The waterhemp control (estimated visually),density, and biomass will be recorded throughout the season. Soybean yield data will be obtained and the gross-profit margins (economics) will be calculated for each treatment. Data will be analyzed using PROC GLIMMIX in SAS.

Though we are requesting the funding for one-year of research in this proposal, we will continue to monitor the research plots for waterhemp density, emergence, and control in the subsequent sugerbeet. Standard herbicide options (preemergence followed by Roundup PowerMax as postemergence) will be used in sugarbeet.

Update:

View uploaded report

The proposed research project will evaluate and document the spatial distribution of herbicide-resistant waterhemp (the most problematic weed) in the state. Also, it will address some of the key management issues for waterhemp control in soybean. It is somewhat difficult to control waterhemp in a soybean-sugarbeet rotation and the need for integrated weed management is often realized. The goals outlined in this proposal are timely and need immediate attention. Additionally, this research work will be conducted at multiple locations to represent different environments that will serve a large group of growers and other stakeholders in the state. Our ultimate goal is to obtain information on best management practices (BMPs) that will make weed management sustainable and increase soybean yield and profitability.