Late-season Weeds Can Reduce Crop Yield and Quality.

Late-season weeds can reduce crop yield and quality.

Warm, wet summer weather is good for crop and ideal for late-season weed development, which makes a fall burndown important.

“A fall burndown is especially important to control late-season aggressive weeds. Weeds that escaped spring herbicide applications or emerged later in the season will continue to be a problem as harvest approaches,” says southeast Iowa DuPont Crop Protection Technical Sales Agronomist Jeremy Veach.

Adding to grower concerns, marestail, waterhemp, giant ragweed and Palmer amaranth are becoming more difficult to manage with traditional weed-control programs.

Do You Know Your Weeds?

Waterhemp is one of the toughest weeds to control due to its ability to emerge throughout the growing season. A higher percentage of plants tend to emerge later in the season compared with other summer annual weeds, like giant ragweed. Late emergers often escape control, adding more seeds to the soil. No-till and reduced tillage farming favors the spread of waterhemp because seeds remain near the soil surface where they germinate more easily than with conventional tillage systems.

Identifying and treating weeds when they are small is critical for both good control and resistance management, but that’s when identification is most difficult. Palmer amaranth and waterhemp (PDF) can be especially tricky to tell apart. In general, Palmer amaranth has wider, broader leaves than waterhemp, which has longer, more tapered leaves. Also, look for the V-shaped purple or white watermark occasionally found on Palmer amaranth leaves.

Timely scouting, using herbicides with multiple modes of action, and applying cultural practices to enhance the competitiveness of crops are best practices to control waterhemp.

Control Giant Ragweed Early

Giant ragweed germinates early and grows fast, so it easily competes with crops for light, nutrients and moisture. Giant ragweed plants towering over tasseled corn in some fields this season points out plants that weren’t controlled by postemergence herbicide passes earlier this year.

Experts recommend applying a postemergence herbicide before giant ragweed plants are 4 inches tall for best control. Too often, growers miss the window for controlling giant ragweed as they wait for other weeds to emerge. A season-long program that uses overlapping residual herbicides with multiple modes of action will help fill those gaps.

Late-Emerging Marestail

Marestail tops the list of problem weeds for Midwest growers. Seeds usually germinate in fall or spring, but marestail can germinate in late summer with the right conditions.

Marestail plants that germinate in the fall overwinter as rosettes and begin growing again in the early spring, quickly generating seeds and adding to the weed seed bank. The rosette stage is the best time for herbicide application and the best way to keep populations under control.

Best Practices for Resistant Weeds

Better weed management begins with understanding what happened this season, then identifying a control program that begins as soon as the combine leaves the field.

Multiple modes of action are the best management tool against the spread of resistant weeds. Ideally, a fall burndown herbicide should be used with a preemergence or at-plant residual herbicide application for best control. Northern Illinois DuPont Crop Protection Technical Sales Agronomist Chris Streit says, “Layering preemergence and postemergence residual herbicides and rotating crops from year to year are good management practices for managing hard-to-control weeds.”

Stretch Input Dollars

DuPont offers growers added financial benefits with the TruChoice® Early Pay Multiplier program. The program lets growers multiply their crop protection dollars up front, then make final crop protection decisions when they purchase products from their DuPont retailers.

Contact your local DuPont agronomist or retailer to learn more about the TruChoice® Early Pay Multiplier program and customized weed-control solutions that will help deliver peak performance from every acre.

TruChoice® programs terms and conditions apply.

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