October 7, 2005

Specialist: Harvested fields springing back to life with weeds

WEST LAFAYETTE, Ind. - Crop fields brown and barren following harvest are suddenly blossoming with colorful hues, and that means weeds are moving in, according to a Purdue University expert.

"As I drive around the state, I notice many fields that have been harvested have a lot of green material that has sprouted up within the last week to 10 days," said Extension weed specialist Bill Johnson. "This is the optimal time to be considering fall-applied herbicides, specifically for fields that will remain in no-till production next year."

A warmer-than-usual early fall hastened the arrival of weeds in many parts of Indiana, Johnson said. Farmers should act quickly to keep that unwanted vegetation at bay.

"Some of the most problematic weeds that we need to consider managing in the fall are dandelion, chickweed, cressleaf groundsel or butterweed, henbit, purple deadnettle, Canada thistle and such," Johnson said. "Dandelion, pokeweed and Canada thistle are perennial weeds and, historically, we've always had better luck managing perennial weeds with fall applications versus spring or midsummer applications.

"For dandelion in particular, the optimal tactics will include waiting until we've had about one killing frost and then applying glyphosate or 2,4-D, or a mixture of the two. Typically, a glyphosate plus 2,4-D treatment is very effective on dandelions and would work well in fields that would be planted to either corn or soybeans next year. For fields going into wheat this fall, use straight glyphosate. Don't use 2,4-D."

Glyphosate is the active ingredient in Roundup, a popular herbicide. 2,4-D is a herbicide that has been on the market for nearly 60 years.

For common pokeweed, fall herbicide applications should be made before the first killing frost, Johnson said.

"If you have a field that has both pokeweed and dandelion, the timing of the application can cause you to pull your hair out," he said. "You'll need to decide which weed is worse. Most likely you'll want to make the herbicide application before a frost in order to maximize activity on both of those weeds."

For winter annual weeds - chickweed, henbit, purple deadnettle and cressleaf groundsel - herbicide applications can be made at any time, Johnson said.

In some cases producers also should consider applying a residual herbicide with their burndown product.

"Glyphosate plus 2,4-D will give you foliar activity on most of your annual and perennial weeds but will not give you any residual activity," Johnson said.

"There are two programs that will give you some residual activity in the soil. Those would be Valor plus 2,4-D and Sencor plus 2,4-D. They will have very good activity on most of your annual broadleaf weeds but will not have a great deal of activity on perennial weeds. The application rate for Valor should be 2 to 3 ounces of product per acre. The Sencor rate should be at least 8 ounces of product to provide any meaningful activity."

Other fall-applied herbicide issues farmers should keep in mind include:

• Soil temperatures - "If you wait until soil temperatures are 50 degrees or less at a 2-inch depth, that is usually the best time for application for most winter annual and perennial weeds in Indiana," Johnson said.

• Air temperatures - "If air temperatures are relatively cool, you might not get good activity with foliar-applied herbicides unless you use a contact product, such as Gramoxone or Gramoxone with Sencor," Johnson said.

• Fields being planted to corn in 2006 - Besides the glyphosate plus 2,4-D combination, Simizine plus 2,4-D and Basis plus 2,4-D have been effective in controlling winter annual and grassy weed species, respectively, in Purdue research plots.

• Fields being planted to soybeans in 2006 - Purdue research has shown that using Canopy EX plus 2,4-D or Canopy XL plus 2,4-D provides consistent control of winter annual weeds, dandelions and some annual bluegrass.

"Scepter-, Valor- and Gangster-based programs also are going out on a lot of acres and can be effective in managing most winter annuals when combined with 2,4-D or glyphosate," Johnson said. "All of these herbicides would provide some residual control of spring-emerging winter annuals and some early spring-emerging summer annuals."

For more information about fall-applied herbicides, read "Fall Applied Herbicides for Soybean, Corn, and Wheat" by Johnson and fellow Purdue weed scientist Glenn Nice. The article can be downloaded.

Writer: Steve Leer, (765) 494-8415, sleer@purdue.edu

Source: Bill Johnson, (765) 494-4656, wgjohnso@purdue.edu

Ag Communications: (765) 494-2722;
Beth Forbes, forbes@purdue.edu
Agriculture News Page

Related Web site:
Purdue Weed Science

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