Wallaces Farmer

Postemergence weed control programs need a good preplant or preemergence herbicide to go with them.

Rod Swoboda 1, Editor, Wallaces Farmer

April 14, 2015

5 Min Read

Studies have shown yield losses in U.S. crops due to uncontrolled weeds exceed billions of dollars annually. Especially now, with lower grain prices, farmers can't afford to lose yield to weeds and grasses in their fields.

First and foremost, don't ignore weed resistance problems, advises Mike Owen, an Iowa State University Extension weed scientist. Since the introduction of glyphosate-tolerant corn and soybeans, glyphosate has been an efficient and valuable tool for controlling emerged weeds. In recent years, however, growers have experienced many challenges when using glyphosate alone.


Those include yield reduction due to weed competition early in the season, inability to make perfectly-timed glyphosate applications, weed escapes resulting in the need for multiple glyphosate applications and the development of weeds resistant to glyphosate. Growers who use two or three applications of glyphosate are finding it is no longer feasible without additional expense and increased time.

Glyphosate resistant weeds are an increasing problem
Weed resistance to glyphosate herbicide is becoming more of a problem. Waterhemp, giant ragweed and marestail are problem weeds showing resistance in Iowa. But it isn't just glyphosate resistance that's causing complications.

"We're starting to see some cross-resistance in isolated pockets of the state," says Dean Grossnickle, a Syngenta agronomy service rep based in Iowa. "Growers are seeing waterhemp that's resistant to ALS herbicides as well as glyphosate, in their fields. Weeds are a moving target. To combat these issues, you need to be sure to use products with multiple, effective modes of action and overlapping residual control."

Good reasons to apply a preplant or preemergence herbicide
The old saying, "Start clean, stay clean," should be on the forefront of every grower's mind at the beginning of the planting season, says Grossnickle. Starting clean can be done with a tillage pass or a burndown herbicide application, but for additional weed control a preplant incorporated or a preemergence herbicide may be used.


"Applying a preplant or preemergence herbicide is important," he says. "That first flush of weeds can be very damaging to yield. If you let those weeds grow, even the small weeds will rob sunlight, water and nutrients from a crop. Any competition costs you yield and money."

Sometimes Mother Nature sends unexpected weather patterns and growers are unable to apply a postemergence herbicide in a timely manner. "It's critical to use a preemergence herbicide because often we aren't able to get a post treatment applied at the right time and the weeds are silently robbing yield," notes Grossnickle. "The field may be clean at the end of the season, as a result of you spraying a postemergence treatment, but weeds have already taken money out of your pocket."

Use a postemergence herbicide with multiple modes of action
A planned two-pass herbicide program of a pre- followed by a post-herbicide with overlapping residual control has shown to be very effective in reducing weed competition and preserving yield.

Growers should be using a postemergence herbicide with multiple modes of action and longer-lasting residual control to manage weed resistance, says Owen. Multiple modes of action save growers time, increase yield and maximize profits by allowing for greater application flexibility, removing yield robbing weed competition, preventing escapes and eliminating the need for extra passes.

Don't let weeds make deposits in your weed seed bank
"It's crucial to avoid depositing seeds in the weed seed bank," says Grossnickle. While a preemergence herbicide may have controlled those weeds upfront, weed escapes later in the season can contribute heavily to new seed entering the weed seed bank. "The weed seed bank is the number of seeds present in the soil. Postemergence weeds that pop up later in the season can contribute to the weed seed bank," he notes.

When those seeds produce weeds the next year, they will compete with corn or soybeans and have potential to decrease yield. By reducing the number of weed seeds in the soil this season by using a postemergence herbicide, you have more options in the future. It's important to remember this is a whole systems approach, says Owen. Your weed control this year translates into weed control next year.


Take a look at new post-herbicide options recently introduced
There are solutions. Some of the new herbicides that have come on the market the past few years offer multiple modes of action. "Designed for glyphosate-tolerant corn, Halex GT corn herbicide is glyphosate with a residual herbicide combined and it provides a convenient alternative to other postemergence, straight glyphosate corn herbicide programs," says Grossnickle.

"By combining three modes of action, Halex GT makes weed resistance one less thing you have to worry about," he adds. "We recommend that you always add atrazine to Halex GT when possible, for an additional mode of action and synergistic weed control. This combination can be applied up to 12-inch corn. If atrazine can't be used, then add a dicamba product."

The label on Halex GT says it provides burndown control of emerged weeds, control of glyphosate resistant weeds, and residual control of grasses and broadleaf weeds including ragweed, waterhemp, lambsquarters, pigweeds and foxtails.

"Weed management can make or break your yield," Grossnickle observes. "A postemergence Halex application will clean up weeds that have escaped and protect the corn up until and through canopy. The synergy you get from the multiple modes of action makes this a very effective program." He adds, "When choosing postemergence herbicide products consider using a post-herbicide application that will save you time and money by providing more effective weed control in just one pass."

About the Author(s)

Rod Swoboda 1

Editor, Wallaces Farmer

Rod, who has been a member of the editorial staff of Wallaces Farmer magazine since 1976, was appointed editor of the magazine in April 2003. He is widely recognized around the state, especially for his articles on crop production and soil conservation topics, and has won several writing awards, in addition to honors from farm, commodity and conservation organizations.

"As only the tenth person to hold the position of Wallaces Farmer editor in the past 100 years, I take seriously my responsibility to provide readers with timely articles useful to them in their farming operations," Rod says.

Raised on a farm that is still owned and operated by his family, Rod enjoys writing and interviewing farmers and others involved in agriculture, as well as planning and editing the magazine. You can also find Rod at other Farm Progress Company activities where he has responsibilities associated with the magazine, including hosting the Farm Progress Show, Farm Progress Hay Expo and the Iowa Master Farmer program.

A University of Illinois grad with a Bachelors of Science degree in agriculture (ag journalism major), Rod joined Wallaces Farmer after working several years in Washington D.C. as a writer for Farm Business Incorporated.

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