New pre-emergent chemistry offers hope for growers battling the most aggressive resistant weeds in North America.
The numbers don’t lie: resistant Palmer amaranth and waterhemp are becoming the most expensive weeds in American agriculture. What started as isolated resistance issues just a decade ago has now spread across millions of acres, pushing growers into costly multi-pass programs that often still fail to deliver clean fields.
"Last summer, I had an agronomist in the Dakotas tell me some of the farmers in his area were spending more than $100 per acre on herbicides and still not controlling waterhemp," says Brian Hefty, fourth-generation farmer, agronomist, and CEO of Hefty Seed Company1.
The Resistance Reality
Both Palmer amaranth and waterhemp have evolved resistance to multiple herbicide modes of action, with some populations showing resistance to five or six different chemistry groups. This "stacked resistance" leaves growers with fewer effective options each season, while these aggressive weeds continue to spread.
Palmer amaranth, now confirmed in over 30 states, can produce up to 1 million seeds per plant and grow up to 3 inches per day under optimal conditions. Waterhemp, equally problematic across the Corn Belt, has an extended emergence window from April through August, making it nearly impossible to control with a single application.
Economic Impact Beyond Herbicide Costs
The true cost extends far beyond increased herbicide expenses. Late-season weed escapes force costly rescue treatments, emergency tillage operations, and harvest delays. University research shows that even modest weed escapes—just one Palmer amaranth plant per 30 feet of row—can reduce soybean yields by 15-20%.
University of Illinois weed scientist Aaron Hager says, “It’s unnerving when we move into this realm of metabolic resistance, because we have zero predictability at all,” He adds, “We have no idea what still works and what doesn’t work — and that’s the challenge.”2
The Pre-Emergent Advantage
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While post-emergent herbicides continue to lose effectiveness, pre-emergent chemistry remains the most reliable foundation for resistance management. Stopping weeds before they emerge eliminates the selection pressure that drives resistance development, while providing season-long residual control.
The challenge has been finding pre-emergent chemistry with sufficient activity against these tough amaranth species. Traditional residual herbicides often provide inconsistent control, especially under variable weather conditions or against resistant populations.
New Chemistry, New Hope
Recent advances in pre-emergent herbicide chemistry are offering renewed optimism for growers facing severe resistance pressure. New active ingredients with novel modes of action are specifically targeting Palmer amaranth and waterhemp, providing the kind of early-season control that can break the resistance cycle.
These breakthrough technologies focus on stopping resistant weeds before emergence, when they're most vulnerable, while integrating seamlessly with existing weed management programs. For growers who’ve felt the strain with their herbicide programs year after year, this represents a critical new tool for sustainable weed management.
Taking Action
The fight against resistant Palmer amaranth and waterhemp requires new strategies, new chemistry, and a commitment to integrated weed management.
One of the most promising advancements is Convintro™, a new pre-emergent herbicide featuring diflufenican—a novel Group 12 mode of action not previously available in U.S corn and soy growers. Its long residual activity and compatibility with existing pre-emergent programs make it a valuable addition to preventing resistant weeds.
While Convintro may cause some temporary leaf markings in corn and soybeans, trials have shown that these effects do not have a significant impact on yields. For growers seeking proactive, sustainable solutions, Convintro represents a critical tool in gaining ground against resistant weeds.
