Why does glyphosate not work on resistant weeds?
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.