Farmers applied glyphosate herbicide on nearly one in five corn acres in 2003 — and that trend is heading upward, according to USDA. In fact, farmers throughout the top 18 corn-growing states have posted a 14% upward trend in glyphosate use since 1998. Despite glyphosate's increasing popularity, most weed scientists recommend using more than just one management practice to control weeds.

Glyphosate, more commonly known as Roundup, is used as a post-emergence weed control product in tandem with biotech, herbicide-tolerant corn hybrids and soybean varieties.

Although Roundup is popular among farmers because it controls a wide spectrum of grass and broadleaf weeds, other post-emergence products for both biotech and non-biotech corn hybrids are also increasing in popularity.

“I see Roundup, Liberty and Callisto gaining ground and possibly replacing some of the traditional, high-rate atrazine products that we use here in Indiana,” says Bill Johnson, Purdue University Extension weed scientist. “I don't think we'll quit using atrazine, but the environmental pressures on it are continuing and will likely increase.”

Despite the water-quality concerns related to its use, atrazine remains the most popular corn herbicide in the nation. According to USDA statistics, during 1998-2003, farmers applied atrazine on roughly seven out of every 10 acres of corn in the top 18 corn-growing states.

Currently in Indiana, “about half the corn acres are getting a planned post-emergence treatment after a pre-emergence treatment,” says Johnson. “However, I wouldn't be surprised if every acre has some post-emergence treatment on it within the next five to 10 years.”

More farmers are using both pre-and post-emergence herbicides to stop weeds like giant ragweed, which germinate over long periods of time and are difficult to thwart with just one weed-control system, Johnson explains. He says waterhemp, horseweed (marestail) and lambsquarters are other problem weeds that typically require more than one weed control program to knock out.

A one-two weed-control punch is fairly common in other top corn-growing states. For example, in Minnesota “at least 50% of the acres” are applied with a pre-emergence product, followed by a post-emergence herbicide, says Jeff Gunsolus, University of Minnesota Extension weed scientist.

“Most farmers are using a pre-emergence herbicide to take out the initial flush of weeds,” explains Gunsolus. “This helps to reduce the stress of getting into the field in time to apply Roundup or Liberty later.”

Controlling weeds before they reach 4 in. tall is crucial to protect corn yields, Gunsolus says. He adds that a pre-emergence herbicide application will provide both early season control and also buy farmers additional time to follow up with a post-emergence application.

“If you put a pre-emergence herbicide down first, that gives you an extra seven to 10 days to apply your post-emergence product,” says Gunsolus. “Plus, I think people who rely on multiple passes of glyphosate will become frustrated by less consistent weed control over a wide range of weed species and environmental conditions, which will result in lower economic returns.”

Bob Hartzler, Iowa State University Extension weed management specialist, agrees. “A few people in Iowa have tried to go with a total post-emergence program in corn, but they've had mixed results,” he says. “Farmers here recognize the need for early season weed control.”

Hartzler says most Iowa farmers use a one-half to two-thirds reduced-rate pre-emergence herbicide, followed by a post-emergence product. “If a broad-spectrum post-emergence herbicide is part of a planned program, it's usually not necessary to apply a full rate of the pre-emergence herbicide to adequately protect yields,” he says.

Farmers who fail to control weeds before they grow too tall will experience both early season yield loss and inconsistent control, Hartzler warns.

“The biggest thing is the timeliness of application — not putting things off so that the size of weeds will hurt yields,” he says. “No one wants to come back a second time to spray, but it's hard to get it all done with just one shot.”

In areas where corn is used locally and export restrictions on biotech corn are less of a concern, Roundup Ready corn acres have expanded, says Hartzler. “In the northwest corner of Iowa, Roundup Ready corn is planted on 50% or more of the acres,” he says. “Also in these drier areas, pre-emergence herbicides tend to be less consistent and a total post-emergence program makes a better fit.”

Bob Narem, a crop consultant from Twin Brooks, SD, estimates that farmers in the eastern part of his state use glyphosate on about 20% of the corn acres. However, in parts of Central South Dakota, farmers are applying glyphosate on more than half their corn acres, he reports.

Narem cautions that where glyphosate is used on a continual basis, lambsquarters, buckwheat and marestail are likely to develop into problem weeds. “For the sake of time and simplicity, the bigger farms want to keep their corn and soybean herbicide programs the same,” he says. “But using a pre-emergence corn herbicide ahead of a post lessens the chance of weed resistance.”

Narem adds that starting out with a pre-emergence product provides a greater window of time in which farmers can apply post-emergence products to control late-emerging weeds.

However, in parts of Central Illinois, where most corn is bound for European export markets, farmers have yet to grow much Roundup Ready corn, says John Woerner, an independent crop consultant from Sullivan, IL. “The grain elevators haven't bought it because they can't separate it from conventional corn.”

Woerner says the most popular herbicide in his area lately has been Harness Extra. “Harness Extra doesn't require as much moisture to activate as some other herbicides,” he says. “That helps to reduce risk with early season weeds.”

Gunsolus says pre-emergence products such as Harness, Surpass and Dual are widely used in Minnesota to protect corn yields and to reduce concerns over the timing of post-emergence applications. He adds that atrazine is still a common tankmix for postemergence broadleaf weed control in corn, and Callisto is also catching on as a post-emergence broadleaf corn herbicide.

According to USDA, acetochlor (Harness, Surpass) remains the second most popular corn herbicide, behind atrazine, in the top 18 corn-growing states, with applications on about a quarter of the corn acres in those states during 2003. Glyphosate ranked third in 2003, tied with S-metolachlor (Bicep, Dual), while mesotrione (Callisto) ranked fifth.

Purdue's Johnson advises farmers who want help in making decisions about various weed control products and tactics to review printed weed control guides available from the Agricultural Extension Service or to meet with crop consultants or local Extension educators. Johnson also recommends trying a new computer software program, called WeedSOFT, developed by the University of Nebraska.

The WeedSOFT program has been field-tested and validated in a number of North Central states, including Illinois, Indiana, Kansas, Michigan, Missouri, Wisconsin and Nebraska. Farmers who want to find out more about WeedSOFT can log on to: http://weedsoft.unl.edu.

A number of universities also have Web pages specifically devoted to weed management. In fact, a group of Extension weed scientists have developed a poster showing weeds that are likely to become new threats to corn growers. You can find this poster and other weed control information by logging on to your state's University Agricultural Extension Web site or calling your local Extension educator.

Glyphosate-Resistance Prevention Tips

Twelve weed scientists from eight Midwestern states recently agreed on three recommendations to help prevent weeds from developing resistance to glyphosate herbicides.

Weed scientists have long warned farmers about the potential for glyphosate-resistant weeds to develop if glyphosate is used repeatedly without also interspersing cultivation or a different herbicide mode of action.

However, despite repeated warnings, glyphosate-resistant horseweed (marestail) has now been reported in 12 states. In addition, common ragweed was reported for the first time this year to be glyphosate resistant in a Missouri soybean field. This field had been planted to Roundup Ready soybeans since 1996.

In December 2004, University weed scientists from Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, Ohio and Wisconsin announced new guidelines to reduce the risk of other weeds from developing resistance to glyphosate. Their recommendations include:

  • Alternating glyphosate use with other herbicide modes of action between years.

  • Incorporating appropriate integrated weed management practices such as soil-applied herbicides and cultivation with the use of glyphosate.

  • Tankmixing glyphosate in no-till systems with another mode of action like 2,4-D in burndown treatments.