Palmer amaranth and common waterhemp are two especially problematic pigweed species due to their competitive ability, fecundity, extended period of emergence, and facility to acquire herbicide resistance traits. The Weed Science Society of America rated common waterhemp as the #1 most common as well as the #1 most troublesome weed in soybeans and Palmer amaranth as the #1 most common as well as the #1 most troublesome weed among all broadleaf crops, fruits, and vegetables (Van Wychen, 2019). Palmer amaranth was first identified in North Dakota in 2018 and has been confirmed in 19 counties within the state since. Waterhemp was found in the Red River Valley in the 1990s and has spread throughout the eastern part of North Dakota over the last two decades. These two species cause economic harm directly through yield loss and indirectly due to increased herbicide costs resulting from resistance issues.
This field research will measure the soybean yield loss associated with increasing densities of Palmer amaranth and waterhemp. This research will also identify the economic threshold of controlling Palmer amaranth and common waterhemp in soybeans. By quantifying the yield interference of Palmer amaranth and common waterhemp in soybeans grown in North Dakota, this research will highlight the importance of controlling these pigweeds and mitigating their spread.

The objectives of this research are to 1) quantify the influence of Palmer amaranth and waterhemp densities on soybean yield loss in North Dakota and 2) determine the economic threshold for controlling Palmer amaranth and waterhemp in North Dakota soybeans.

This research will measure the soybean yield loss due to Palmer amaranth and common waterhemp interference in North Dakota. This research will also determine the economic threshold of Palmer amaranth and common waterhemp control in soybeans in North Dakota. This research will be presented at a regional weed science society meeting, and an open access peer-reviewed journal article will be published. These field trials will also be demonstrated at a NDSU Weed Science Field Day in summer 2023 and presented at NDSU’s Wild World of Weeds Workshop in January 2024 and throughout the winter meeting season. A progress report will be submitted to NDSC on or before December 1, 2023, and a final financial report, Technical Report, and Executive Summary will be submitted to NDSC on or before their June 2024 due dates.

Update:
Materials and Methods

Research was conducted at two experimental sites in 2023: a Palmer amaranth-infested field near Valley City, ND and a waterhemp-infested near Fargo, ND. The experiment was a randomized complete block design with four blocks and six Palmer amaranth/waterhemp densities (i.e., treatments): 0, 0.5, 1, 2, 4, and 8 plants m-1 of row. An additional treatment, which included an early transplant of the 4 plants m-1 row weed density, was also included. The experimental areas were cultivated prior to planting, and soybeans were planted with a 76-cm row spacing. Plots were four rows wide by 7.6 m in length.

Soybean variety AG09XFO was planted to a 4 cm depth at 156,000 seeds acre-1 on May 22 and June 12 at the waterhemp (Fargo) and Palmer amaranth (Valley City) locations, respectively. Waterhemp and Palmer amaranth plants were initiated in the greenhouse and then transplanted into research plots using 36 mm Jiffy-7 peat pellets. Waterhemp was transplanted on May 26 (early) and June 22. Palmer amaranth was transplanted on June 16 (early) and July 12. The waterhemp and Palmer amaranth seed used for this experiment was collected from their respective research sites in 2022. Throughout the growing season, plots were hand weeded weekly.

Eight random soybean plants were collected to measure yield components (pods plant-1 and seeds pod-1) from the waterhemp site on September 28 and Palmer amaranth site on October 20. Waterhemp biomass was collected on September 28, and soybeans from the waterhemp site were harvested using a small-plot combine (Zurn Harvesting, Werther, Germany) on October 2. Palmer amaranth biomass was collected on October 20, and soybeans from the Palmer amaranth site were cut using a walk-behind sickle bar mower and threshed using a mobile thresher (ALMACO, Nevada, Iowa) on October 25. Data were analyzed using PROC GLIMMIX in SAS (version 9.4).

Preliminary Results

Soybean yield varied by treatment at both the waterhemp (P = 0.0250) and Palmer amaranth sites (P = 0.0009). In both cases, the early-transplanted 4 plants m-1 pigweed density reduced yield compared to the 0 plants m-1 pigweed density (Figures 1 and 2). This research emphasizes the influence of waterhemp and Palmer amaranth emergence timing on soybean yield, with a 24% and 67% yield loss associated with early-emerging waterhemp and Palmer amaranth, respectively.

In Process

Soybean yield components are still be counted; waterhemp and Palmer amaranth biomass data is still being processed.

View uploaded report

Demonstrating the direct economic harm and calculating the economic threshold of control of these two pigweed species will inform North Dakota soybean producers and help guide their weed control decision-making.