It’s a thing of nightmares—a towering, aggressive weed that defies modern management by evading death by herbicide. Just south of Moberly, Mo., researchers confirmed waterhemp with resistance to six herbicide modes of action. The farmer and his retailer grew suspicious of the population in the Randolph County field when 2,4-D, to which the weed displays few cases of resistance, didn’t kill the plant. Sure the applications were performed correctly, the retailer reached out to University of Missouri Extension Weed Scientist Kevin Bradley. “Not getting control with 2,4-D was significant and worth telling us,” Bradley adds. “We get all kinds of requests every year, but he also indicated it to us because he wanted to test and see if it really was resistant like he thought.”
Waterhemp had only displayed resistance to 2,4-D in two other states, Illinois and Nebraska, before this discovery. This population showed resistance to the following: 2,4-D, atrazine, chlorimuron, fomesafen, glyphosate and mesotrione. The only herbicides that killed the resistant population were dicamba and glufosinate, Bradley says. “This is one case where pre-emergent herbicides become critical because we have so few post-emergent herbicides that work. “I don’t want to rely only on post-emergent anything,” he adds. “This is a cautionary tale—this can happen. It won’t happen everywhere, but if we keep treating herbicides wrong over and over, we’ll learn just how prone the species is to multiple resistances.” The Randolph County field is the first and only documented case of resistance to six herbicides in a single plant, beating out even Palmer amaranth. Waterhemp is known to be more adaptable than Palmer when it comes to conferring multiple resistances, but Palmer tends to grow more aggressively. “We haven’t found it outside of that area yet, which is a good thing,” Bradley says. “I hope it stays that way, but I can’t say with confidence it will. If there’s anything I’ve learned about waterhemp it’s don’t underestimate it.” The farmer and retailer who discovered the weed are following best management practices, but with wind, ducks, machinery and any number of other vehicles to carry seeds, there’s no telling where it could end up. Learn from this case to avoid creating a monster on your farm. Researchers are advising farmers and retailers to attack multiresistant waterhemp with a diversified approach. This means using all control practices—herbicidal, cultural, mechanical and biological. “It’s another opportunity to remind people of what we’ve been saying for years: use multiple modes of action, rotate herbicides and use pre-emergent herbicides,” Bradley says. In addition, Take Action on Weeds encourages farmers to consider tillage to prevent field-to-field and even within-field movement of seeds on machinery, planting weed-free crop seed and managing field borders, just to name a few of their recommended best management practices. “I hope we’re not getting too accustomed to multiple resistances—that’s when waterhemp will win,” Bradley says. Apathy toward weed control and growing resistance puts weeds a step ahead.