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.

What are the ramifications if additional glyphosate-resistant weed populations develop?

1. Greater yield loss due to weed competition.
2. The subsequent cost of additional herbicides needed to achieve adequate control.
3. A potential reduction in no-till acres, since successful no-till is predicated on effective weed control using herbicides.
4. 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.

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).

Gaines et al. (2010) showed that glyphosate-resistant Palmer amaranth have an extraordinarily high copy number of the EPSPS gene (5 to more than 160 copies), relative to a glyphosate-susceptible population.

A high EPSPS copy number means the plant can produce enough EPSPS to support necessary amino acid production even in the presence of glyphosate. Shikimate does not build up, and amino acid synthesis occurs in spite of the uptake and translocation of glyphosate.

While individual plants of a weed species may look similar (phenotypically similar), there can be a tremendous amount of genetic variability within a population.

Because of this broad genetic variability, there is a chance of selecting for a resistant individual under persistent, high selection pressure (for example, through multiple applications of glyphosate alone).

In a situation of high selection pressure, resistant plants can dominate a field population in only four to five generations (years in the case of most weeds) once plant genetics are selected that confer resistance to that selection pressure (Jasieniuk et al. 1996).

Once a resistant weed population evolves, the genetics can be moved in a number of ways. Seed can be moved by tillage and harvesting equipment, animals, or wind and water. It also can be moved via pollen (genetic exchange).

Palmer amaranth is a dioecious species, meaning plants are either male or female. This means that plants are obligate out-crossers (cross-pollinators), resulting in the exchange of genetic traits each year. Gaines et al. (2010) demonstrated that the increased EPSPS gene copy number is a heritable trait when plants are cross-bred.

The concern for producers is that this genetic basis for glyphosate resistance can spread over any distance that the pollen can travel. Sonoskie et al. (2011) have reported movement of resistance traits via pollen up to 1000 feet from a known resistant male plant to susceptible female plants.

Glyphosate Resistance in Marestail

Currently, marestail is the species presenting the largest glyphosate-resistant weed management problem in Nebraska. The mechanism of glyphosate resistance is not the same in all resistant weed species.

In contrast to Palmer amaranth, glyphosate resistance in marestail is likely due to reduced or altered translocation of glyphosate (Feng, et al, 2004; Koger and Reddy, 2005). The glyphosate is sequestered in part of the plant, allowing unaffected plant parts to continue to grow and produce seed.

This process is thought to be controlled by a single dominant or semi-dominant gene (Zelaya, et al, 2004). Because marestail is capable of both self or cross pollination, passing on resistance traits (from pollen movement) can occur rapidly in a population.

These examples illustrate the complexity of evolved glyphosate resistance in weed species. While both of these plant be considered non-target site resistance, the physiological methods by which these species avoid death by glyphosate are different.

Management Tips

Because the movement of glyphosate resistant traits in weed populations (via pollen or seed movement) cares nothing about property ownership, fence lines, or man-made boundaries, resistance management is the responsibility of the entire ag community.

This underscores the necessity of knowing the weeds present in your fields and then implementing weed management programs that are both economically effective and will reduce the potential development of glyphosate resistance.

Nebraska producers have not experienced the problems with glyphosate-resistant Palmer amaranth that those in southern states have. However, if there is over-reliance on glyphosate for weed control, it is likely that resistance will evolve in local populations. Palmer amaranth is present in the southern tiers of Nebraska counties, and becomes more prevalent as you move west.

Proper resistance management is a proactive approach to managing weeds. In fields infested by Palmer amaranth or any weed species mentioned at the start of this article, manage to prevent the development of herbicide resistance.

Use at least one PRE herbicide that is effective on all the weeds present in the field, followed by a POST application, if warranted, of a second herbicide that is effective on the same weeds. Herbicide resistance management principles apply to all cropping systems, including Roudup Ready, Liberty Link, conventional crops, and any herbicide-resistant technologies that will be commercialized in the future.

Using multiple herbicide resistance traits in your crop rotation to “change-up” your herbicide program will result in delayed evolution of herbicide-resistant weed populations and greater value and longevity of the weed control tools currently available to producers.

Take-Home Message

• Glyphosate resistance in at least one Palmer amaranth population is due to elevated production of the EPSPS enzyme from an increased EPSPS gene copy number in resistant individuals.

• Glyphosate resistance in marestail is likely due to reduced or altered glyphosate translocation.

• Use at least two different herbicides that are effective on Palmer amaranth each year. Make sure the herbicides have different mechanisms of action. Do not use the same two herbicides every year. This strategy also applies to fields with marestail, kochia, ragweeds, and waterhemp.

Lowell Sandell, Extension Educator, Weed Science, Lincoln
Deana Namuth Covert, Plant Science Educator
Greg Kruger, Extension Cropping Systems Specialist, West Central REC
Mark Bernards, Extension Weeds Specialist, Lincoln