Set your Strategy for Spring Weeds
By James D. Ritchie

The usual suspects, plus some new species, will be waiting for you this year.

The main goal in the life of a weed is to reproduce—to bloom and set seed. And some of them insist on getting an early start at it.

Winter annuals such as henbit, chickweed and downy brome are still thriving in some fields. As the sun climbs higher in the sky, spring-emerging weeds like common and giant ragweed, several grasses and even lambsquarter are beginning to appear.
 
“Know your weeds and adjust your control program accordingly,” said Lyndon Brush, MFA staff agronomist.

“You have to get on these weeds early. Each field is different, and you need to look at it that way. That complicates your weed control, but it’s part of good crop management.

“As an early pre-plant burndown, I still like a glyphosate [the active ingredient in Roundup] plus 2,4-D application, and that’s what most growers use,” he said. “But there are cases where a Gramoxone plus crop oil treatment may be an excellent program.
 
“While very early treatments, including fall-applied herbicides, can take out broadleaved weeds very effectively, they open the door for a host of grasses—cheat, downy brome, ryegrass,” Brush said. “These grasses can grow quickly, sometimes to the point that they can even interfere with corn planting—and we’re planting corn earlier all the time. You need to be aware of grass weeds and be ready to adjust your weed control to handle them.”

Kevin Bradley, University of Missouri weed scientist, agrees. “Look at the weeds you have and come up with a recipe to control them. That’s a tall order at times and can add to weed-control costs, but it’s essential if you are going to avoid worse problems later on. For example, ryegrass is tough to control and can grow rapidly to compete with the crop, especially in wheat and no-till corn. The ragweeds are causing more problems and germinate earlier in the year,” Bradley said.

Complicating weed-control chores is a growing list of plant pests that are resistant to glyphosate. Missouri now is home to at least three weeds with officially identified glyphosate resistance: tall waterhemp, common ragweed and horseweed (mare’s tail), which gives the state the dubious distinction of owning more glyphosate-resistant weeds than any other state.

“We didn’t create resistant weeds,” said Bradley. “They existed naturally. We simply select for them by using the same herbicide continuously.”

But they create complications in weed control. Glyphosate resistance means rotating to another herbicide with a different mode of action, either as a pre-emergence treatment or a different post-emergence herbicide or both.

“With most resistant weeds, we’re still able to get control, but with some pretty expensive programs in many cases,” Bradley added.

As if glyphosate resistance and more adaptable weeds weren’t enough, a whole platoon of tough weeds is trying to establish an early toehold in some fields.

“Most of these are not widespread problems, at least not yet,” said Bradley. “But where they occur, I would definitely watch them and try to keep them from becoming more of a problem than they already are. All of these weeds are hard to control and not easily eliminated.”

Take field horsetail (also called scouring rush), a perennial with a spreading rhizome system from which new plants arise. Field horsetail is unique in that it produces two types of stems and two different types of growing habits.

“It’s most common in low-lying, wet areas but has a tendency to encroach into adjacent fields,” said Bradley. “Since it produces rhizomes, any kind of shallow tillage, such as chisel-plowing or disking, spreads the rhizomes. Not much research has been done on field horsetail but enough to tell us that not many herbicides are effective against it.”
 
The same goes for Asiatic dayflower, an annual weed that is becoming more of a problem in no-till soybeans. The weed forms dense colonies that can cause big yield losses in both corn and soybeans.

“Again, few herbicides give good control of Asiatic dayflower,” said Bradley. “Iowa State research shows that in soybeans, Spartan, Firstrate and Sencor can provide acceptable control when applied as pre-emergence treatments. But applications need to be made before the weed is 6 inches tall.”

Field pansy (or “Johnny-jump-up”) is another potential bad actor that can germinate in either fall or spring.

“Field pansy is another weed that is not controlled easily by glyphosate in the spring or even by standard glyphosate plus 2,4-D burndowns,” noted Bradley. “The addition of dicamba (Banvel, Clarity) to a glyphosate burndown provides good control of field pansy.”

If you have infestations of these three newcomer weeds or have found a successful strategy for controlling them, Kevin Bradley would like to hear about them. You can e-mail him at bradleyke@missouri.edu “Controlling many of these weeds will be more expensive up front,” said Bradley. “But you can pay a little more now or a lot more later, in the form of lost crop yield.”

MU weed scientist Kevin Bradley maintains a weed identification web site. It contains images of weeds, a guide for identifying herbicide injuries and a catalog of pest management guides from the University of Missouri. To view it, visit www.plantsci.missouri.edu/weedscience and click on the tab marked "web resources."

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