Wallaces Farmer

Overcoming resistance starts with weed seeds

Successful crop production starts with the seed. So why not do the same with weed management?

Gil Gullickson, editor of Wallaces Farmer

June 10, 2024

3 Min Read
Seedlings emerge weed-free under a mat of cereal rye cover crop
COVER CROP WEED SUPPRESSION: Soybean seedlings emerge weed-free within a thick mat of cereal rye cover crop residue. Gil Gullickson

Herbicides have been, are and will continue to be part of weed management. Due to herbicide resistance issues, though, they need help. Stopping weeds right at the seed stage is one way to do it.

“We need to do everything we can to keep these plants from producing seed, because that’s the way resistance will spread and perpetuate,” says Meaghan Anderson, Iowa State University Extension agronomist.

Multiple practices can help farmers battle weeds.

Enlist help of cover crops

Cover crops can smother weeds before they form seeds.

“If you can generate enough biomass, you can get suppression,” says Michael Flessner, a Virginia Tech weed scientist who works with the Grow group, a public-led resource for farmers on weed resistance issues.

However, the degree of weed suppression depends upon location, Flessner says. Biomass content that squelches weeds tends to rise under warm winter temperatures.

“If you’re up near the Canadian border, it’s probably too cold,” he says.  “If you’re closer to the Rockies, it might be too dry. But if you’re in an area that can grow a big cover crop, we’ve seen tremendous success.”

ISU trials in 2020 at Boone showed that a cereal rye cover crop reduced waterhemp density more than 30% and waterhemp seed production by 65% when compared to no cover crops.

It also didn’t negatively impact soybean emergence, growth, canopy development and grain yields when researchers terminated cereal at soybean planting.

“It’s a challenge knocking down waterhemp and keeping it down,” says Nick Christenson, who farms in Cumberland. “We have tried some cover corps, and they have helped tremendously in trying to control it.”

Till or not to till

In the agronomic world, tillage to control weeds can be a tough decision to make, particularly on no-till acres.

Tilling can shatter soil structure and break soil channels that aid water infiltration. On the other hand, Palmer amaranth and waterhemp that get out of hand can make it difficult to establish a crop.

Remedial tillage leverages the short lifespan — three to five years — that pigweeds such as waterhemp have.

“If we can bury them beyond the depth in which they germinate, we can get rid of a lot of those seeds that will become weeds,” Flessner says. Nixing these seeds can help farmers get the populations under control as they once again no-till, he adds.

Control weeds at harvest

Harvest weed seed control systems developed in Australia destroy weed seeds before they germinate. One such system uses seed impact mills that fit on the back of combines.

“We’ve seen 97% to 98% of weed seeds killed in soybeans,” Flessner says. Meanwhile, a trial in 2020 by ISU weed researchers exceeded a 90% kill rate.

There’s a catch, though. These units require more horsepower to run, so a Class 8 combine or larger is needed.

“You’ll also burn more diesel in addition to the capital costs to purchase one,” Flessner says.

Not all weed seeds pass through the combine either. ISU research found between 30% and 33% of waterhemp seed bounced off the combine’s header.

Like any control measure, though, weeds may adapt.

“We’ve seen this time and time again,” Flessner says. “Weeds may flower and set seed earlier. We may see a shift to more prostrate weeds that lie below the cutter bar.”

Still, harvest weed seed control systems  can play a role in an integrated weed management program by reducing weed seed numbers and taking the heat off herbicides.

“It’s highly effective at killing weed seeds,” Flessner says.

Throw some shade

Narrow rows that squelch weed growth also prevents them from forming seeds. In 2019 and 2020, ISU weed scientist Ramawatar Yadav compared treatments for soybeans planted in 15- and 30-inch spacings. They examined waterhemp control the prior year in corn:

  • poor control at 30%

  • good control at 90%

  • zero weed seed produced the prior year

All treatments had the same herbicide program in the subsequent soybean year. Nine weeks after planting, waterhemp densities were at least 13% lower in narrow-row spacings. Meanwhile, aboveground waterhemp biomass was reduced more than 40% in 15-inch rows compared to 30-inch rows.

About the Author(s)

Gil Gullickson

editor of Wallaces Farmer, Farm Progress

Gil Gullickson grew up on a farm that he now owns near Langford, S.D., and graduated with an agronomy degree from South Dakota State University. Earlier in his career, he spent 13 years as a Farm Progress editor, covering Minnesota and the Dakotas.

Gullickson is a widely respected and decorated ag journalist, earning the Agricultural Communicators Network writing award for Writer of the Year three times, and winning Story of the Year four times. He is a past winner of the International Federation of Agricultural Journalists’ Food and Agriculture Organization Award for Food Security. He has served as president of both ACN and the North American Agricultural Journalists.

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