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 fourteen 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. That is, it will calculate the weed density at which the costs associated with yield loss equal and begin to exceed the cost of weed control, where the cost of weed control is justified. 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, assuming a second site-year, 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 2022 and presented at NDSU’s Wild World of Weeds Workshop in January 2023. A progress report will be submitted to NDSC on or before December 1, 2022, and a final financial report, Technical Report, and Executive Summary will be submitted to NDSC on or before June 30, 2023.

Update:

View uploaded report

Update:

View uploaded report

In year 1 of this experiment, Palmer amaranth was estimated to cause up to a 57% soybean yield loss when Palmer amaranth was transplanted 24 days after soybean planting. Palmer amaranth density reduced the number of soybean pods per plant but not seeds per pod. Further, as Palmer amaranth density increased, so did Palmer amaranth biomass. Soybean yield was apparently reduced as waterhemp biomass increased.

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. Once yield interference trials have been conducted, the critical period of weed control can be investigated.