Battling Weed Resistance in Corn and Soybeans

Does this sound familiar? A field looks clean and weed-free; you have an occasional weed escape, but nothing worth noting. Then, one season, part of that field explodes with weeds that just won’t go away.

More than 230 cases of herbicide-resistant weeds have been confirmed in corn and soybeans across the United States. Though herbicide technology has improved, weeds remain hard to control.

Common problems include widespread resistance to atrazine, ALS inhibitors and glyphosate, according to David Belles, Ph.D., herbicide technical product lead for Syngenta.

“Due to herbicide resistance, waterhemp and Palmer amaranth have significantly spread to new areas,” he says. “These weeds have adapted to become tolerant to herbicides and they can outcompete other weeds. Plus, they adapt well to different environments, making them difficult to control.”

While weeds run rampant for a variety of reasons, growers are also managing more acres with less help. Volatile weather patterns narrow windows for field work, leading to the need to plant crops, side-dress fertilizer and apply herbicides at the same time.

“Growers move as fast as they can,” Belles says. “But unpredictable weather and equipment problems slow them down. Tough weeds keep growing regardless of conditions and when growers get a little behind, weed control problems can snowball.”

Proactive Management Fights Weed Resistance

To help prevent rampant weed problems, Belles advises growers to rely on residual weed control and sound application practices.

“Be proactive rather than reactive and manage fields like they already have resistance,” he says. “Once a weed problem starts, it’s hard to get it back under control.”

He shares recommendations based on years of helping growers manage resistant weeds.

Start clean

Eliminating weeds before planting prevents competition when the crop is most vulnerable. Take care of early season pressure with either tillage or a burndown herbicide application. Belles says that burndowns should contain at least two effective modes of action against problem weeds.

“If atrazine resistance is present in the field, the herbicide application should contain two other modes of action that will control the atrazine-resistant species,” he explains.

Give the crop a strong start

Crop competition helps manage weeds. Planting for quick, even emergence and stand in a clean field allows the crop to use nutrients and moisture first. A healthier crop also canopies more quickly, which provides better shade for later-emerging weeds.

Starting strong includes being ready to complete fieldwork when conditions allow. Aim to get equipment ready and inputs delivered to take advantage of suitable days for planting, weed control and other critical spring tasks.

Overlap residual applications in a two-pass herbicide program

A full labelled rate of a preemergence herbicide extends its residual activity — and the window for the second pass — as long as possible to account for weather, labor and equipment issues. It should be followed by an early post-emergence application that adds residual herbicide before the first application runs out.

“The goal is to apply more residual before weeds emerge,” Belles says. “Keeping residual herbicides in the root zone of the soil allows them to work when conditions are right for weed seeds to germinate.”

That’s when weeds are most susceptible. Germinating weed seedlings generally can’t metabolize herbicide active ingredients.

He recommends making the second herbicide application before weeds emerge, when corn and soybeans are just a few inches tall. If weeds come up before that application, adjust to include a product with activity on growing weeds combined with residual control.

Follow best application practices to maximize herbicide effectiveness

Using the right herbicide rate, additives and spray volume helps ensure good coverage of the soil and emerged weeds. Higher spray volumes provide better coverage; for example, spray 15 to 20 gallons per acre rather than 5 to 10 gallons per acre.

“Use high quality herbicide suppliers and formulations that maintain active ingredient concentration,” Belles says. “Ensure products are compatible with other herbicides and inputs in tank mixtures.” Always read and follow product labels and instructions, including using full rates.

He encourages growers to consider everything that goes into the spray tank, including fertilizer or biologicals.

“Think about the efficacy of weed control first,” he says. “Even just a little antagonism in the tank can impact herbicide performance.”

Use cultural practices where they fit

Mechanical control, hand-weeding, cover crop residue or technologies like at-harvest seed destructors all provide the equivalent of another mode of action in a weed management strategy. While these efforts may not make sense on every acre, using them strategically can help prevent weeds from going to seed, reducing the weed seed bank and pressure over time.

Use Herbicides as Directed for Full Potential

Applying herbicides at the proper application rate is absolutely essential to tackle herbicide resistance. “Premixes simplify weed control by combining ideal rates of active ingredients,” says Shawn Hock, corn herbicide product lead for Syngenta. “Nothing helps increase the spread of weed resistance like using low or non-labeled herbicides or only one effective site of action in a tank mix.”

A corn herbicide like Acuron^® includes four active ingredients and three sites of action, including the unique HPPD inhibitor bicyclopyrone, for built-in resistance management and added control of waterhemp and Palmer amaranth, according to Hock.

For flexibility in areas where growers decide what to plant as spring unfolds, Acuron GT corn herbicide provides an early post-emergence option.

Storen^® corn herbicide contains four different active ingredients: S-metolachlor, mesotrione, bicyclopyrone and pyroxasulfone, from three herbicide sites of action when atrazine is added, giving it the opportunity to provide maximum residual performance.

At full rates, both Acuron and Storen help control resistant weeds, especially when applied preemergence.

In U.S. corn and soybean fields, weed scientists have documented 131 cases of resistance to ALS inhibitors, 88 cases of atrazine resistance, 140 cases of glyphosate resistance and 24 cases of resistance to PPO inhibitors. In contrast, they’ve confirmed just 11 cases of HPPD resistance. Though this system doesn’t account for unreported or unconfirmed cases, HPPD resistance is limited.

According to Belles, HPPD resistance coincided with increased use of mesotrione and other HPPD herbicides. Post-emergence HPPD applications have become less effective on large weeds, but they still effectively control weeds preemergence and very small weeds post-emergence.

Because of their formulations and doses, Acuron and Storen can help control even HPPD-resistant weeds.

“In these cases, it’s about horsepower,” Hock says. “These herbicides overcome resistance with potency.”

In soybeans, he encourages starting with a preemergence application of Tendovo^® herbicide. It’s one of the few soybean products with three active ingredients for improved consistency, crop safety and length of residual control. As part of the post-emergence tank mix, Dual Magnum^® herbicide provides overlapping residual.

“A good corn and soybean weed control program protects yield for good short-term return-on-investment (ROI) potential,” Hock says. “It also provides the best shot at decreasing the weed seedbank, providing long-term ROI potential for resistance management.”