Late-season pigweeds (Amaranthus spp) that escaped herbicide applications are considerable threats to future crop production. Just one escapee can release 10,000 to 100,000 seeds during harvest, which builds the weed seed in the soil. Effective herbicides for pigweeds are declining due to the emergence and rapid spread of herbicide resistance (HR) traits, combined with the lack of new herbicides being registered for their control. The two cross-pollinating species, waterhemp and Palmer amaranth, have a dynamic repertoire of resistance for eight modes of action, making them focal species for HR research. The self-pollinating species (Powell amaranth) also have HR populations, but are rarely studied despite contributing to yield loss. HR traits have been linked to genomic changes that can be detected via genotyping. To date, 14 states have surveyed pigweed populations to gain an understanding of HR genotype prevalence across the soybean belt. These surveys have found the distribution and type of HR pigweed populations vary across states, making it difficult to extrapolate recommendations across state lines. Through a public-private partnership, we propose North Dakota’s first statewide survey to screen pigweed populations for glyphosate, PPO-inhibitor, and ALS resistance. This statewide project will provide: 1) a distribution map of HR genotypes occurring in North Dakota, 2) a DNA archive for developing additional tests for the HR genotyping panel, and 3) comparative analysis of genotypes and whole-plant herbicide trials. Ultimately, locating HR populations and their patchiness within North Dakota can help fine-tune herbicide recommendations and customize integrated pest management at the field level.

1. Genetic survey of pigweed populations in North Dakota for common target-site genotypes that confer resistance to PPO-inhibitor, glyphosate, and ALS-inhibitor herbicides.
2. Compare whole-plant greenhouse and genotyping assays to determine the target-site basis for HR in pigweed escapees found in North Dakota fields.

Combining genotyping and greenhouse data will help describe the particular types of HR pigweed populations that exist in the state, including western North Dakota. The project will look at additional molecular tests for HR detection in pigweeds, which will remain available to the agricultural community once described. Results will be communicated through presentations to growers and potential updates to NDSU publications or other resources.

Updated November 30, 2021:

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Characterizing HR pigweeds will benefit farmers in a few ways. First, at the field level, pigweeds can be genotyped for HR traits at any stage of the life cycle, including outside the herbicide application window. If HR genotypes are discovered within a particular field, this can help with decisions on the best herbicides to include (or avoid) for management. Mixes of herbicides are used to combat pigweeds, but the returns of using particular mixes diminishes when local pigweeds contain HR for one or more herbicide in the mix. Second, at the regional level, locating HR pigweed populations in the state is necessary to track their spread and provide clarity on where more resources are needed for eradication efforts. Third, growers rely on weed scientists for recommendations on optimal chemical treatments. Some of these recommendations come from greenhouse or test plot research. Confirming HR traits through genotyping can speed up the selection process for seed sources for herbicide trials when greenhouse space and time are limited.