Why does glyphosate not work on resistant weeds?

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As the glyphosate season is well underway, many farmers will turn on the sprayer valve and wonder how many weeds will survive this year. Extension weed specialists have seemingly increased their reported cases each year of either increased populations of resistant weeds, or new weeds in new states that just won’t be controlled with labeled applications of glyphosate. How is it that some weeds are easily controlled and others are not? We may have an answer.

Most farmers are familiar with and many of them are hosts to palmer amaranth and marestail. Both are high on the list of resistant weeds when it comes to glyphosate control.  Weed specialists and agronomists Lowell Sandell, Deana Namuth, Greg Kruger, and Mark Bernards at the University of Nebraska are concerned about the growing resistance and offer their analysis in the June 24th issue of the Nebraska Crop Watch. They remind everyone about the ramifications of growing resistance, and list those as:

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.

The way that glyphosate basically works is by disrupting the weed’s ability to created essential amino acids. Glyphosate disables an enzyme known as EPSP, which produces three different amino acids needed for the plant to be healthy. When the glyphosate is sprayed on the plant, the molecule works its way to growing tissues and halts the amino acid creation, and over the next 10-20 days it dies. (That is what is supposed to happen.) For weeds like palmer amaranth and marestail, the story has a different ending.

Palmer amaranth, which is known in some areas as red pigweed, and is closely linked to waterhemp, was first discovered resistant in Georgia. In susceptible plants, there is only one gene that controls the reproduction of EPSP, but in the palmer amaranth that is resistant, there are anywhere from 5 to 160 different genes that can re-create amino acids. The NE researchers say two plants of the same specie may look similar, but genetically can be very different. Because of the broad genetic variability, the continued use of glyphosate will kill off the plants with only 1 gene to copy the EPSP but allow those with many genes to survive and thrive.

When a field has a growing number of the resistant weeds, their genetics can spread by harvesting equipment or tillage equipment, or animals, wind and water. Since the palmer amaranth is either a male or female plant, the pollen from the male plants can travel as far as 1,000 feet from a resistant male to a susceptible female and pass along the genetics for resistance. 

Another problem weed is marestail, but the mechanism for resistance is not the same as the amaranth. In marestail the glyphosate is kept in one part of the plant, and the other parts are unaffected and continue to grow with the help of a dominant or semi-dominant gene. And because marestail is capable of both self pollination and cross pollination, the resistance can occur rapidly in a group of plants.

So what do you do? The NE researchers say the movements of resistant traits do not observe property lines, managing the resistance problem is the challenge for all farmers, and that means knowing the weeds in your field and implementing weed management programs that are both economically effective and capable of reducing the overall resistance. They recommend the use of a pre-emergent herbicide that is effective against most of the weeds in the field, followed by a post emergent herbicide that is effective against the same weeds. They say the use of multiple products will result in delayed evolution of herbicide resistance populations.

Summary:
A growing number of weeds have become resistant to glyphosate for different reasons. Amaranth is able to overcome the glyphosate’s designed destruction of an enzyme that makes amino acids, while marestail has a gene that corrals glyphosate in a single area of the plant to keep it away from reproductive areas. Resistant plants can either spread their pollen great distances or reproduce resistance themselves. A farmer’s options for overcoming the problem are the use of pre and post emergent herbicides that are alternatives to glyphosate.


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