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.