A new contender is fighting for top honors as the ‘most troublesome weed.’ Waterhemp has once again proven it can evolve quickly and through mechanisms of resistance researchers didn’t expect. “Our initial theory was that waterhemp would mimic corn as it does for the other two HPPD-inhibitors, but no, it found a different way,” said Dean Riechers, University of Illinois Department of Crop Sciences weed scientist in a recent news release. “We don’t know how or why, but it has a different mechanism from what corn naturally has. Bottom line is that you can’t use any of the three HPPD-inhibitors to control this population.” The population is found in McLean County, Ill. It’s resistant to mesotrione, tembotrione and topramezone, the latter is the active ingredient that troubles researchers the most. Instead of mimicking corn’s natural resistance pathway it has created one of its own—and is displaying resistance to populations that have never been exposed to this particular ingredient.
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 and also showed alarming resilience to topramezone. “The greenhouse experiment showed the Nebraska population did have resistance to a herbicide it had never been exposed to,” Riechers explained. “Did the other two herbicides select for topramezone resistance?” He, and other experts think so. The question now is whether or not each herbicide has its own resistance gene or if there are genes that other herbicides can select for. Because the McLean County, Ill. population showcases a different resistance pathway than corn it might be more difficult to control chemically. “Right now, you could spray any of these three HPPD-inhibitor [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 know 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 row crop farmers face every year. This highlights the importance of a multi-pronged approach to weed management. “We’re finding out more and more about what these waterhemp populations can do for detoxification, and it’s disheartening,” he said. “Take alternative steps to limit the spread of these resistant plants or prevent it [resistance] from happening in the first place.”