Research Highlights

Watching Waterhemp for Resistance and Integrated Control Strategies

In this article, you’ll find details on:

• Comprehensive surveying and screening monitor waterhemp populations for distribution and herbicide resistance to help farmers plan to manage this weed, which has become a major problem in Ohio fields and across the Midwest.
• Research plots are exploring long-term synergies between a cereal rye cover crop and various herbicide programs for improved waterhemp control in soybean-corn rotations. 

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By Laura Temple

Waterhemp has become a major weed problem in Ohio soybean fields. Like in much of the Midwest, poor control of this pigweed species is becoming more common in central and western Ohio. 

“Waterhemp has really taken off,” says Alyssa Essman, assistant professor of weed science at Ohio State University. “In our preharvest surveys, we used to evaluate all pigweeds together, but we have broken them up because we’ve seen so much waterhemp in recent years.”

With support from the Ohio Soybean Council, Essman is delving into the details of waterhemp pressure. She is leading a team determining the distribution and frequency of waterhemp and screening populations found in soybeans late in the season for herbicide resistance. Her team is also exploring how cover crops and herbicides work together as management options.

“Farmers need best management practices and more control options for this problem weed,” she says. “This research is improving our ability to serve as a waterhemp management resource.”

Surveying and Visualizing Waterhemp Distribution

To monitor populations of waterhemp and other problem weeds in Ohio, Essman’s team drives through the top soybean-producing counties in the state just before harvest. Observing the weeds still in fields and producing seed at that time of year highlights potential problems for next season. 

“We conduct this preharvest weed survey from mid-September through early October,” she explains. “We look for patterns in weeds remaining in fields that indicate what the control issue might have been, like a sprayer skip or plugged nozzle versus signs that indicate poor herbicide control.”

In addition to waterhemp, her team provides a visual rating for control of marestail, giant and common ragweed, Palmer amaranth, redroot pigweed, volunteer corn, lambsquarters, velvetleaf and giant foxtail/grasses. The team translates those ratings into a Preharvest Weeds Survey Map that shows weed pressure at the county level to help farmers, extension educators and crop consultants plan for the next season. Survey data going back 10 years is also available through a recent collaboration with Ohio State’s Knowledge Exchange. 

Comprehensive Herbicide Resistance Screening

Essman’s team also uses the physical survey as an opportunity to collect waterhemp seed from fields with suspected herbicide resistance, taking these samples to the lab for screening.

“At this point, we assume all waterhemp populations in Ohio have resistance to ALS inhibitors and glyphosate,” she explains. ALS inhibitors, or Group 2 herbicides, include active ingredients like imazethapyr found in Pursuit, chlorimuron in Classic, thifensulfuron in Harmony and similar pre- and post-emergence herbicides. 

Essman’s team screens for resistance to four other herbicide classes commonly used in both soybeans and corn.

• Group 5 herbicides are a class of PSII inhibitors that includes atrazine and metribuzin.
• Group 14 herbicides include PPO inhibitors like sulfentrazone in Authority, flumioxazin in Valor and fomesafen in Flexstar.
• Group 15 herbicides inhibit very long-chain fatty acid synthesis. This group contains pre-emergence products like S-metolachlor in Dual brands and acetochlor in Harness.
• Group 27 herbicides are HPPD inhibitors, currently labeled for corn, though HPPD-tolerant soybeans, GT27 soybeans, are also available in some areas. Active ingredients in this category include isoxaflutole in Balance Flexx and Alite 27, as well as mesotrione in Callisto.

In addition, she collaborates with other researchers in a herbicide resistance screening funded by the United Soybean Board. Her team sends off a portion of the waterhemp seed collection to collaborating universities for resistance screening to the Group 4 synthetic auxins 2,4-D and dicamba, as well as glufosinate, the active ingredient in Liberty.

“Results have been variable,” Essman reports. “Some waterhemp populations are susceptible to everything we screen for. However, others show likely resistance to atrazine and Group 5 herbicides, which is concerning. We also saw some waterhemp regrowth issues with glufosinate in 2022, and we continue to monitor that closely.”

She hasn’t seen any resistance to the HPPD inhibitor mesotrione yet, and she says 2,4-D seems to be working well so far.

“The potential issue I am currently most nervous about is one waterhemp population that showed variable control with S-metolachlor a couple years ago,” she says. “It is harder to positively identify resistance to pre-emergence herbicides, so we are watching this potential problem carefully and have started an in-depth screening for resistance to this and other pre-emergence herbicides.”

Exploring Synergies between Cover Crops and Herbicide Programs

Essman’s research also includes proactive waterhemp management. Her team aims to quantify the effect of cover crops and herbicide programs on waterhemp density and seedbanks.

“Other research has shown that a simple cereal rye cover crop can reduce weed pressure,” she explains. “Our trial explores how that works in conjunction with herbicide programs over multiple years in a soybean-corn rotation.”

It started with a cereal rye cover crop in the fall of 2023, followed by soybeans in 2024. The plot will rotate to corn in 2025. The team took seedbank measurements prior to starting the trials, with plans to repeat those measurements at the end of the project, ideally five years later.

The trial plots compare the effects of different cover crop termination timings. In soybeans, early termination is about one week before planting and late termination is two weeks after planting. In corn, early termination is two weeks before planting and late termination is just a week before planting, giving the residue less time to dry down.

The plots also compare four different herbicide programs that will be used in both crops:

• Pre-emergence herbicides only.
• A simple pre- and post-emergence program that is more cost-conscious.
• A more comprehensive pre- and post-emergence program that includes residual control and more herbicide modes of action.
• A control with no herbicide applications.

“We will look at the association between cover crop biomass and waterhemp density, herbicide efficacy and crop yield,” Essman says. “A great way to follow our progress on this work is to look at Ohio State’s Crop Observation and Recommendation Network outlet, the C.O.R.N. newsletter.”

Additional Resources:

Waterhemp information – GROW website

Waterhemp research articles – SRIN page

How to Use ‘Many Little Hammers’ to Management Waterhemp – GROW article

Meet the principal researcher on this project: Alyssa Essman

Published: Sep 16, 2024