Anew contender is fighting for top honors as the “most troublesome weed.” Waterhemp has once again proven it can evolve quickly and in ways researchers didn’t expect. The newest resistant waterhemp population was found in McLean County, Ill. It was resistant to active ingredients mesotrione, tembotrione and topramezone, the latter of which troubles researchers the most. The waterhemp developed resistance to topramezone in a way that’s never been found before. Researchers brought in waterhemp plants from Nebraska that had only been exposed to mesotrione and tembotrione for testing. The waterhemp had virtually no reaction to those two active ingredients but showed alarming signs of resistance to topramezone.
“The greenhouse experiment showed the Nebraska population did have resistance to a herbicide it had never been exposed to,” explained Dean Riechers, a University of Illinois Department of Crop Sciences weed scientist in a news release. Which bears the question—did other similar herbicides select for resistance in topramezone even though the weeds have never been exposed to it? Riechers and other experts think so. The question now is whether each herbicide resistance is unique by herbicide, or if one gene can confer resistance to multiple herbicides. Because the McLean County, Ill. waterhemp population showcases a different resistance pathway than corn, it might be more difficult to chemically control. “Right now, you could spray any of these three HPPD-inhibitors [active ingredients] on corn, not kill the corn, but potentially kill the weeds. But if the weeds are using a different mechanism to detoxify the chemical, you’d have to develop a different kind of herbicide that doesn’t use the same metabolic pathways,” Riechers said. “[And that] might be effective on weeds but who knows if the corn would tolerate it.” This discovery proves yet again that nature will find a way, and waterhemp is among the most adaptable weeds. As a result, a multipronged approach to weed management is as important as ever. “We’re finding out more and more about what these waterhemp populations can do for detoxification, and it’s disheartening,” Riechers said. “Take alternative steps to limit the spread of these resistant plants or prevent it [resistance] from happening in the first place.”