Make Your Weeds Queasy

July 26, 2013 10:00 PM
Make Your Weeds Queasy

Targeting seedbanks limits spread, protects yields

Farmers who have left weeds unattended in a field know that what happens next is akin to leaving two rabbits alone in a hutch. The problem is, the devastating consequences for a field are anything but cute and fuzzy.

"The ground turns purple" with weeds, says Larry Steckel, University of Tennessee Extension weed scientist. "You can actually see it driving by."

Weed seeds are remarkably resilient and a bane for farmers. Traditionally, research indicates that species such as pigweed and waterhemp can remain viable beneath the soil surface for four or five years, explains Kevin Bradley, University of Missouri Extension weed scientist. Weeds such as morningglory are viable for decades.

Those weed seeds not only live a long time, but they are also prolific. One waterhemp plant can produce 250,000 seeds on average, while Palmer pigweed routinely produces 500,000.

Use all available tools. A good plan is essential for managing weed seeds. Bob Kochendofer, who farms 1,100 acres of corn and soybeans near Jonesville, Mich., uses a pull-type Hardi sprayer with 90' booms to spray for chickweeds and other weeds in the fall. He then disks all of his soybean ground and pulls a packer over it at the same time in the spring. In the fall, he chops cornstalks to keep weed seeds in line.

"I figure you tear it up, and what weeds are stronger, you’ve got them tilled under again," Kochendofer says. He uses a post-emergence herbicide in the spring once the soybeans are up.

"There’s not one tool that’s going to cure all my problems," says Ryan Britt, who farms 5,000 acres of corn, soybeans, wheat, pasture, hay and cattle with his family near Salisbury, Mo. With no-till, he manages weeds using herbicides, including residuals, with unique modes of action.

Experts say farmers should sandbag weed seedbanks using tillage, natural predators, herbicide schedules with overlapping residuals, cover crops and even hand-weeding. As resistant weeds expand their footprint, a single approach is hazardous.

Deep tillage works by pushing weed seeds away from their comfort zone. Pigweeds germinate less than a ½" below the soil surface, while giant ragweed and cocklebur germinate between 2" and 3" deep, explains Aaron Hager, University of Illinois Extension weed scientist.

Machinery can bury seed between 10" and 12" into the soil profile, limiting their access to conditions that make germination easy, Bradley adds. Fields where deep tillage has been used are no-tilled the following year to avoid churning seeds back up.

At the same time, growers should realize that deep tillage might only delay the weed-seed problem, says Adam Davis, ecologist, USDA–Agricultural Research Service Global Change and Photosynthesis Research Unit. His research shows that seeds kept within 2" of the soil surface disappear more quickly than first thought. Common lambsquarters and velvet­leaf—once believed to have half-lives of 15 years—can decline by 50% in approximately two years. The same is true for waterhemp.

"You could actually master [fields] in seven years, tops," Davis says.

Find the trouble spots. Rather than using machinery to bury the problem, Davis says, existing equipment can remove weed seeds for good. Weed patches can be mapped at harvest so combines don’t cut through them. Afterward, they can be burned down or chopped with an aftermarket combine product that damages weed seeds, reducing their viability. Birds, worms and microbes can remove up to 95% of shed weed seeds, so farmers should wait at least four weeks after harvest before deep-tilling.

To complement machinery, farmers in southern Illinois and southern Iowa apply overlapping residual herbicides. A burndown with two or three active ingredients can be applied before corn or soybean planting, followed by a second application after emergence, Bradley says. About five or six chemicals are labeled for that use pattern.

Many Southern growers also perform post-harvest weed control, spraying a burndown herbicide plus the residual promoxin. On average, they spend about $20 per acre on post-harvest herbicides, Steckel notes.

Herbicide costs continue to rise, from roughly $30 per acre back when Roundup controlled Palmer amaranth to more than $60 per acre today, he says. Hand-weeding in the South happens on 50% of cotton fields and 15% of soybean fields, pushing costs to as much as $100 per acre.

As farm operator for the newly launched Chariton County, Mo., Soil Health Research and Demonstration Project, Britt also has an interest in cover crops. For example, cereal rye is toxic to waterhemp, Britt says, and planting into cover crops prevents sunlight from penetrating into the weeds below. He recently planted cover crop mixes on seven fields to learn how those plants affect weed seedbanks and more on no-till land. Over time, data will be compared to that from tilled land with no cover crops.

Ultimately, farmers should purge weed seeds with a big-picture outlook.

"I’m almost more concerned about soil health than I am resistant weeds," Britt notes.

You can e-mail Nate Birt at

For more weed seedbank management tips from the experts, including 12 best practices for resistant weeds, visit You can also view a graphic about the cost of weed seeds by Monsanto.

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