You gave your corn and soybean fields a good shower of Roundup earlier this year, but during harvest you found some healthy weeds that refused to cooperate with your herbicide program. Your combine did a great job of spreading the seed, and that weed patch will be waiting for you next year with reinforcements. You think, “Why does this have to happen to me?!?”

Roundup is supposed to kill everything but your corn and beans. That’s what the label says, but weed scientists are keeping close track of numerous patches of various kinds of weeds in the Cornbelt that resist a glyphosate shower. How that happens was explained by weed specialist Bob Hartzler recently in a research report presented at the Iowa State Integrated Crop Management Conference. And what he said in his presentation will not only help you understand how weeds resist glyphosate, but what you can do to prevent that headache.

Glyphosate was used for 20 years before weeds developed any resistance, and during that time is provided a high level of effectiveness, application flexibility, a large margin of crop safety, and safety for applicators and the environment. Glyphosate was successful because it would disrupt the weeds metabolic process, could not be metabolized by the weed, and was efficiently sent to the growing point in the weed. Glyphosate locks onto an enzyme known as EPSPS and disables it. EPSPS is involved in the pathway in the weed that carries growth regulators and many other chemicals, amino acids, and organic chemicals needed to survive. Weeds that are resistant to a chemical are able to metabolize and break it down, before it reaches the target site. That is why corn can survive atrazine and soybeans survive a spray of pursuit.

Roundup Ready soybeans introduced in 1996 contained the EPSPS enzyme, but it was modified by a gene from a bacterium that was insensitive to glyphosate. Roundup Ready corn contains a gene from a resistant variety of corn and the new version of EPSPS works fine.

Shortly after Roundup Ready soybeans were introduced, an Australian weed, known as rigid ryegrass was found to be resistant of the process that disables EPSPS in the weed. Then in 2000, some horseweeds in Delaware were found to be surviving glyphosate from year to year. Since then, 12 weed species have been identified as resistant to glyphosate. Some of the weeds have a mechanism that is resistant to glyphosate and several have multiple mechanisms. Weed scientists have found the resistant horseweeds to only have resistance at a specific time, and when glyphosate was applied at the two leaf stage it could be controlled. However, when the rosette stage appeared, so did the resistance. The plant may have sent the glyphosate chemical to an area where it was of no harm, or it could have produced more EPSPS, or it could have increased it branching. It is believed that the horseweed did not allow the glyphosate to migrate out of the leaves, and the leaves died, but not the rest of the plant. At the two leaf stage, the plant could not achieve that process.

In Italian ryegrass, one resistant patch demonstrated the same process as the horseweed, with 80% of the glyphosate remaining in the leaf, but in another resistant patch, only 51% remained in the leaf, indicating it had found a different way to survive. In that case one of the 425 amino acids that comprise the EPSPS enzyme was restructured after the glyphosate spray which allowed it to survive, and that escape mechanism has been found in other resistant weeds. In the past two years resistance has been found in other Cornbelt weeds, such as waterhemp, giant and common ragweed, and common lambsquarters. Their resistance mechanism has not yet been confirmed.

Weed scientists are still trying to find out why certain weeds have developed resistance to glyphosate, but anticipate different mechanisms to be found. Hartzler says, “Glyphosate resistance is different from previous herbicide resistance issues faced in the Midwest due to both the multiple resistance mechanisms and the relative level of resistance. The use of correct glyphosate rates has been promoted as the critical factor in managing glyphosate resistance. Since most (resistant weeds) possess a relatively low resistance level, use of rates that allow significant numbers of weed escapes undoubtedly would enhance the rate that resistance evolves within a weed population.” But he says those weeds with resistance have a high resistance level and using the labeled rate will only create more weeds that have resistance. And control of those weeds will require alternate strategies, with limited reliance on glyphosate.

Summary:
Glyphosate resistance is now a fact of life for some Cornbelt farmers who have weed patches that cannot be controlled with an application of labeled rates of Roundup. Those weeds have developed one of several mechanisms that allows them to escape the impact of glyphosate, and possibly it may have more than one mechanism of resistance. Weed scientists expect more weed species to develop resistance, more resistant weed patches to appear, and more challenges to farmers who have to find alternative management strategies.