Resistance: Palmer amaranth vs. marestail
The simplicity and effectiveness of weed control in glyphosate-resistant corn, soybean, cotton, and sugarbeets have led to extensive adoption of this technology in the U.S. However, relying primarily on a single herbicide has resulted in the selection of glyphosate-resistant populations in a number of important weed species.
Weed species with populations resistant to glyphosate in the Midwest include marestail, waterhemp, common ragweed, giant ragweed, and kochia. In the southern U.S., glyphosate-resistant Palmer amaranth populations are widespread and glyphosate-resistant Johnsongrass populations have been confirmed. All of these species are present in Nebraska, although Johnsongrass is not common in production fields. Resistant populations of these species are likely to develop in Nebraska where producers rely primarily on glyphosate for weed control.
What are the ramifications if additional glyphosate-resistant weed populations develop in Nebraska?
- Greater yield loss due to weed competition.
- The subsequent cost of additional herbicides needed to achieve adequate control.
- A potential reduction in no-till acres, since successful no-till is predicated on effective weed control using herbicides.
- An increased pesticide load in the environment resulting from the use of additional herbicides necessary to control glyphosate-resistant weed populations.
This article describes how glyphosate kills plants, mechanisms by which Palmer amaranth and marestail have evolved glyphosate resistance, and management to avoid glyphosate resistant weed development.
Glyphosate Action in Plants
Glyphosate controls plants by disrupting essential amino acid synthesis. (View an animation of this process.) Glyphosate binds to and disables EPSP synthase. EPSPS is an enzyme that catalyzes reactions in the shikimate pathway which forms three amino acids. When a susceptible plant is sprayed, glyphosate is absorbed by the plant and translocated to actively growing tissues, where it inhibits the necessary aromatic amino acid synthesis. In a susceptible plant, shikimate levels start to increase after application, indicating that glyphosate is working as it should. The plant undergoes a relatively slow death over the next 10-20 days.
Source: Bruce Mackellar, Michigan State UniversityFigure 1. Glyphosate-resistant Palmer amaranth recently discovered in Michigan. Glyphosate Resistance in Palmer Amaranth
Palmer amaranth is an economically important weed species in row crop production, especially in the southern U.S. In 2006, Culpepper et al. reported the first glyphosate-resistant Palmer amaranth population in Georgia. More recently published work describes a genetic basis for glyphosate resistance in Palmer amaranth. In susceptible plants, there is a low gene copy number that encode for production of EPSP synthase (1 gene copy).