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 retailer grew suspicious of the population in their Randolph County fields when 2,4-D, for which the weed currently displays few cases of resistance, didn’t kill the plant. Astute and sure that 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 2,4-D in two other states before this discovery: Illinois and Nebraska. 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.” This is the first and only documented case of resistance to six herbicides in a single plant, beating even Palmer amaranth. Waterhemp is more adaptable than Palmer when it comes to conferring multiple resistances, but Palmer is the more aggressive-growing weed. “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 to control this monster of a weed. 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 building a monster on your farm. Researchers are advising farmers and retailers to attack multi-resistant waterhemp with a diversified approach. This means using herbicidal, cultural, mechanical and biological control practices. “It’s just 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 recommends farmers consider tillage, prevent field-to-field and within-field movement of seeds on machinery, plant weed-free crop seed and manage field borders, to name a few of their best management practices. “I hope we’re not getting too accustomed to multiple resistances, that’s where waterhemp wins,” Bradley says. Apathy toward weed control and growing resistance puts weeds a step ahead.