Waterhemp and other tough weeds are forcing farmers to rethink how they use herbicides in high-residue cropping systems, from heavy corn stalks to thick cereal rye covers.
Extension weed scientists say they increasingly hear from growers who did “everything right” with applying their preemergence products yet still see waterhemp push through and survive. Increasingly, one of the challenges is those fields carry a lot more residue than they used to.
“Every year, we have some situations where we get less than expected control of weeds for various reasons, and I’ve come around to appreciate the impact that residue can have on our success,” says Tom Peters, Extension agronomist and weed control specialist for North Dakota State University and the University of Minnesota.
Oftentimes, the assumption has been that rainfall will wash herbicides off the residue and down into the soil, where they can do their job. Peters says that belief does not hold up in reality.
“I would argue that some of our performance challenges have been related to those herbicides sticking to the residues,” contends Peters, who made his comments during the 2026 Field Notes program.
That problem is on the increase as farmers are dialing back their tillage passes, planting into more corn and soybean residue and seeding more cover crops.
Reduction In Control Assessed
During graduate work with the University of Minnesota, Eric Yu, now a regional crops Extension educator, measured just how much product residue can intercept herbicides. In cover crop plots, he and his colleagues placed water-sensitive cards below cereal rye crops, applied a preemergence herbicide and then evaluated the results.
“We were seeing about a 50% reduction in the amount of product that reaches the soil compared to our control plots,” Yu says. “Yet despite that 50% reduction, we were seeing still significant weed control, specifically waterhemp control.”
The message, Yu says, is not that residue makes the use of pre products pointless. It is that farmers need to account for residue when they design their weed-control programs — and still keep a strong preemergence herbicide in the plan.
Peters agrees. Even when residue cuts the amount of product reaching the soil, pres are still the foundation of a good program, especially as waterhemp increases in resistance to postemergence herbicides.
“Start the season with pre products, observe your results and then decide what the best postemergence program is,” he advises.
For farmers managing crops in high-residue systems, Peters and Yu point to several practical steps:
- Prioritize Soil Contact: Ensure herbicides are actually reaching the soil surface. In cases of extreme residue, it may be necessary to manage or move stalks and straw ahead of planting.
- Adjust Product and Rate: Work with agronomists to select products and rates that can withstand some interception while still delivering enough active ingredient to the soil to be effective. Using full labeled rates is increasingly a best-practice solution for control and to reduce selection pressure for further herbicide resistance.
- Tighten the Timing Window: Because residue can blunt the effectiveness of a pre product, escapes are more likely. Small waterhemp is much easier to control; once the weed reaches the 4- to 5-inch range, control becomes significantly more difficult.
Researchers Evaluate 21 Herbicides
A group of University of Wisconsin-Madison scientists recently studied which herbicides make it to the ground and provide residual waterhemp control in high-residue farming systems. The controlled-environment study evaluated 21 single-active-ingredient corn and/or soybean herbicides compatible with high-biomass cereal rye. Here are the results, courtesy of GROW:
Corn herbicides identified as effective for waterhemp control and compatible with high-biomass cereal rye in this study included:
- Acetochlor (Harness – Group 15)
- Dimethenamid-P (Outlook – Group 15)
- Pyroxasulfone (Zidua – Group 15)
- S-metolachlor (Dual II Magnum – Group 15)
- Atrazine (Group 5)*
- Isoxaflutole (Balance Flexx – Group 27)
- Mesotrione (Callisto – Group 27)
*waterhemp population used in this study is still susceptible to atrazine applied preemergence.
Soybean herbicides identified as effective for waterhemp control and compatible with high-biomass cereal rye in this study included:
- Dimethenamid-P (Outlook – Group 15)
- Pyroxasulfone (Zidua – Group 15)
- S-metolachlor (Dual II Magnum – Group 15)
- Flumioxazin (Valor – Group 14)
- Fomesafen (Flexstar – Group 14)
- Metribuzin (Group 5)
The Wisconsin researchers say soybean growers should pay close attention to application timing restrictions. Flumioxazin-containing products for instance must be applied within three days of soybean planting, while metribuzin must be applied prior to soybean emergence. The remaining soybean herbicides listed above can be applied preemergence or early postemergence, offering flexibility for growers who plant early and delay cereal rye termination until after soybean emergence.
A standard program in planting green systems where the cereal rye is terminated after soybean emergence may include glyphosate for cereal rye termination, combined with soil residual herbicides fomesafen plus a Group 15 herbicide (e.g., pyroxasulfone, S-metolachlor, or dimethenamid-P) and a Group 2 herbicide such as imazethapyr (Pursuit), cloransulam (FirstRate), or chlorimuron (Classic) for broad spectrum weed control.
