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Controlling seed bank key to slowing weed resistance

Tom J. Bechman weeds growing in soybean field
PRICE OF POOR CONTROL: Here is what a field can look like in September if you allow too many weeds to escape control. Each plant could add up to 250,000 seeds to the weed seed bank.
Here are tips for taking out weeds and slowing resistance.

There could be a perfect storm brewing this year. The weed population got a boost in the super-wet spring of 2019, when some fields were left fallow. Many produced weeds that went to seed, adding to the seed bank.

Along comes 2022, with higher herbicide costs and tight supplies of postemergence products. The temptation could be to skimp on rates or skip a cleanup application. Some believe the result could be disastrous.

“Illinois already has 29 resistant weeds, Michigan 25, Ohio 19 and Indiana 18,” explains Drake Copeland, technical service manager with FMC Corp. “Weeds escaping control produce seed, which adds to the seed bank. That promotes quicker development of resistant weeds.”

Copeland and Nick Hustedde, FMC technical service manager in southern Indiana, recently outlined ways you can reverse course and slow weed resistance.

Weed shift

If marestail and common lambsquarters are most prevalent and a few waterhemp plants show up, the weed spectrum can change quickly, Copeland says.

He cites one study where marestail and common lambsquarters dominated a field. Waterhemp was introduced. By year six, the weed population was 80% waterhemp, 10% marestail and 10% common lambsquarters.

The shift can occur rapidly because waterhemp and nearly all pigweed species, including Palmer amaranth, are prolific seed producers.

“One year of seeding can lead to seven years of weeding,” Hustedde says.

Why? Because most pigweed species can produce 250,000 seeds per plant growing within a crop field. Isolated plants with no competition produce up to 1 million seeds. Waterhemp tends to continue germinating later than most weeds, and late emergers produce seeds.

The only saving grace, they note, is that after four years in the soil, only about 10% of waterhemp seeds are viable, compared to morningglory with an in-soil life of 39 years and velvetleaf at 50 years. However, 10% of 250,000 from one plant is still 25,000 new plants.

Case against cutting rates

Applying herbicides at rates that kill 4-inch weeds on 2-foot weeds won’t kill them. “It’s a sublethal rate for 24-inch weeds,” Copeland says. “Those weeds recover and produce seeds. That’s a practice leading to quicker weed resistance.”

Weed scientists rate herbicides for percent control of weeds. At one time, some thought 95% control was effective.

“You can make the case today that 95% control is a failure,” Copeland says, “You’re battling prolific weeds like waterhemp, and you’re already concerned about weed resistance.”

You can aid chemical weed control with other methods, Hustedde says. Whether they fit your farm depends on several factors.

“Studies show that tillage can significantly reduce weed populations compared to no-till systems, for example,” he says. “But tillage isn’t an option on highly erodible land.”

Some people have reverted to 30-inch rows for soybeans. But that gives up a proven method for aiding weed control, Hustedde says. “Canopy closes quicker in 15-inch rows, and that helps shade out late-emerging weeds.”

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