Wallaces Farmer

How to rob a (weed seed) bank

Stopping seed is a key in halting waterhemp.

Gil Gullickson, editor of Wallaces Farmer

June 21, 2024

3 Min Read
hand of man bending down in cover crop
COVER UP: Cover crops, such as this cereal rye, can reduce waterhemp density without impacting soybean emergence, growth, canopy development and grain yields.Holly Spangler

If you’re worried about dicamba leaving the soybean herbicide scene, remember this.

Waterhemp has a weakness. Its seeds quickly die. Leveraging this flaw can help farmers manage this pugnacious pigweed.

A 1997 to 2000 trial conducted at the University of Illinois by graduate students and USDA Agricultural Research Service scientists studied seed viability of a 1996 field with dense waterhemp populations.

“The waterhemp was so thick that you literally could not see it was a corn crop,” says Aaron Hager, U of I Extension weeds specialist.

The research team measured seed viability from 1997 to 2000 in the top 6 centimeters of soil (2.36 inches) after waterhemp shed its seed in 1996. Seed viability levels in each year were as follows:

  • 1997: 39%

  • 1998: 28%

  • 1999: 10%

  • 2000: .004%

“If you limit seed production, waterhemp numbers will plummet,” Hager says.

Non-chemical control

Herbicides, of course, can halt seed production by killing weeds. But if dicamba is nixed as a herbicide option, more pressure is placed on remaining herbicides to control weeds. Ultimately, remaining herbicides used repeatedly can select for more herbicide-resistant weeds.

Enter non-chemical strategies.

“They can preserve our herbicides and make them last longer,” says Meaghan Anderson, Iowa State University Extension field agronomist. Tactics include:

Tillage. “This will not work for everyone,” Anderson says. For farmers who have a cultivator, rotatory hoe or other tillage tool, though, it may be a practical control option.

“It can work on just parts of a field,” she says. “This is another year where I’m hearing about much larger weed populations along field edges. You could just make a pass around the outside of the field.”

Cover crops. 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.

Narrow rows. By canopying sooner, narrow rows squelch sunlight that weeds need to grow and set seed. In 2019 and 2020, Ramawatar Yadav, an ISU weed scientist, compared treatments for soybeans planted in 15- and 30-inch spacings. They examined waterhemp control the prior year in corn:

* Poor waterhemp control (30%)

* Good waterhemp control (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.

Harvest Weed Seed Control

Harvest Weed Seed Control systems developed in Australia destroy weed seeds before they germinate. Some HWSC practices funnel harvest residue containing weed seed into narrow windrows at harvest. Farmers can then burn it, spray it or leave it to rot.

Another practice uses seed impact mills that fit on the back of combines.

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

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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