Weed control in a volatile year: Management options, timing and caution

“Every season is unique, and this one is setting up to be unpredictable, including when it comes to weed pressure,” says Tony Lenz, technical agronomist for Stine^® Seed Company. “Fortunately, in my region of southern Minnesota and western Iowa, pre-emerge applications seem to be working well, and the weather has allowed most farmers to get their post-emerge down. But in areas that have been cool and wet, and where a pre-emerge wasn’t applied, post-emerge applications are going to be critical.”

A full-system approach

With the rise of herbicide-resistant weeds — including giant and common ragweed, Palmer amaranth, waterhemp and marestail, to name a few — having multiple modes of action has never been more important. This is especially true as more weeds are showing resistance to one or more herbicides, making a diversified approach not just beneficial but necessary.

To start, Lenz notes that a strong pre-emerge program is critical in combating weeds in any season.

“If you began with a no-till fall or spring burndown or pre-emerge, you’re probably pretty happy right now,” he says. “I highly recommend it. Having multiple modes of action across both your pre- and post-emerge programs gives you the best chance at staying ahead of resistant weeds.”

Lenz says it’s important to keep in mind that weed management doesn't stop once the chemistry is applied. A more comprehensive approach may be necessary for long-term control.

Experts from the University of Nebraska–Lincoln recommend 10 core strategies for an effective integrated weed management program, including diversifying herbicide modes of action, using overlapping residuals, rotating crops, adding cover crops, improving crop competitiveness, using occasional tillage, scouting regularly, tailoring tactics to specific weed species, consulting experts and cleaning equipment between fields.

Putting those strategies into practice starts with building the right herbicide program.

For example, soybean growers may benefit from using a program such as Enlist E3^®, which incorporates three different modes of action through the Enlist weed control system — 2,4-D choline, glyphosate and glufosinate. Combine it with a Group 15 residual herbicide or a separate pass with a Group 1 grass herbicide (the most common being Clethodim) to add another mode of action for managing volunteer corn.

“Layering your herbicide program with other practices such as tillage, cover crops and crop rotation helps ensure the efficacy of future herbicide programs,” says Lenz. “That said, while having multiple herbicide modes of action is a great tool for weed management, you can’t overuse those chemistries because that’s where you get herbicide-resistant weeds. It’s not sustainable to continue doing the same thing over and over; you really need a full-system approach, or the problem will only get worse.”

Knowing the risks of overreliance on any one chemistry is half the battle. The other is execution.

The weed window is narrow — hit them early

For growers moving forward with post-emerge applications this summer, remember that timing and accuracy are of the essence. For best results, Lenz recommends targeting weeds as soon as possible.

“You typically want to apply a post-emerge when weeds are pop-can size or smaller — around 4–6 inches,” says Lenz. “Weed size can impact herbicide efficacy, so when field conditions are right, a timely post-emerge can help suppress weeds, keeping them from getting taller and competing with the crop for critical nutrients. This is especially important before the crop reaches its key reproductive stages.”

Lenz cautions that in areas where it’s still too wet to get into the fields — or where fields have experienced extremely dry conditions already this season — weeds could quickly push past the labeled height recommendations for post-herbicide applications.

“Growers need to consider multiple modes of action to manage the taller, harder-to-kill weeds,” he says. “If you applied a pre-emerge before planting, the good news is you have more flexibility to delay the post-emerge application. But for growers relying solely on a post-emerge application, the window is a bit tighter and the risk is higher.”

Lenz notes it’s important to plan ahead before spraying to fully assess the situation and the chemistries you intend to use.

“You really need to review your chemical labels to determine weed height and best-use recommendations,” he stresses.

Proceed with caution

Applying when weeds are at or under label height restrictions is good practice, but each herbicide chemistry comes with its own set of rules beyond just timing.

“We stress to always read and follow label directions for good reason,” says Lenz. “From nozzle selection and weed height to added surfactants, boom pressure and appropriate tank-mix partners, herbicide application is a full-program approach. You have to get every detail right to not only effectively control weeds but to also be a good steward of the chemistries.”

Not heeding the warnings and recommendations on labels could be costly, and something as routine as neglecting to clean spray booms between applications can lead to crop damage. This is especially true when hot temperatures are at play or when solution is allowed to sit in the tank for extended periods of time.

Drift, volatilization and herbicide carryover can also pose challenges when post-emerge herbicides are used.

“If the wind is blowing a certain direction at a particular speed or if heat and humidity are a concern, weather has a big impact on how a herbicide moves,” says Lenz. “While many chemistries offer low or no volatility features — like the Enlist weed control system — you have to apply them at the labeled rate to capture that benefit.”

In a crop rotation year, farmers also need to think about the tolerances of next year’s crop. For example, if a post-emerge herbicide is applied and is followed by a very dry season, that chemistry may not break down in the soil as effectively, leaving residue that can injure the following year’s crop.

“Late post-emergence applications made close to the reproductive stage or within the pre-harvest interval may run the risk of carrying over into the next year’s crop,” says Lenz. “Before you apply, it’s important to think through the current growth stage of your crop, what you’re planting next year and how it will respond to the chemistries you’re using today.”