Weed-control experts acknowledge that glyphosate resistance concerns exist and are warning soybean growers to act now to protect the integrity of this chemistry.

Nearly 80% of the U.S. soybean acreage, most of it in Roundup Ready soybeans, is treated with a glyphosate-based product each year. And in many cases, those acres are treated more than once with glyphosate, compounding resistance concerns.

Extension weed specialists repeatedly give two basic recommendations that can help soybean growers stave off or prevent glyphosate resistance: Use full rates of glyphosate when you use it, and include other herbicide chemistries in your weed-control program.

For full rates of glyphosate, that means spending about $5 to $12/acre for the product alone, says Bryan Young, weed specialist at Southern Illinois University, Carbondale. The cost spread has to do with variations in usage rates and the difference in cost between those glyphosate products that are sold with no services attached and those that are sold by companies offering full control guarantees.

To minimize the potential for resistance, Young and other weed specialists say growers must also employ other chemistries. The cost for including these products in a control portfolio will range from $5 to $15/acre, depending upon options used.

Sound steep? David Heering, Roundup technical manager for Monsanto, says the addition of other chemistries is usually an unnecessary expense in soybean acreage. “If I were in growers' shoes, I would look for effective, cost-efficient weed control, which is what Roundup provides,” he says. “If it's a dead weed, it won't produce seed.” Heering adds that the company has worked with glyphosate for 28 years with few resistance concerns and that in southern states many growers have used Roundup Ready soybeans continuously for eight years without any problems.

Fast-moving marestail

The geographical extent of possible glyphosate resistance and the number of weed species involved are somewhat murky. Both are still being defined. However, glyphosate resistance seems to occur primarily in weed populations of horseweed (marestail) or waterhemp. The geographical extent of resistance in these weeds is more difficult to define.

Chuck Foresman offers some perspective. He says his company, Syngenta, first identified glyphosate-resistant marestail in parts of western Tennessee two years ago. “There's probably close to 500,000 acres with some resistant marestail there now, and we've also seen it in Kentucky, Ohio and Indiana,” he says. “We expect to see it spread to Illinois, Arkansas and Missouri.” Populations of resistant marestail also have been identified in the eastern Corn Belt states of Delaware, Maryland, New Jersey and Virginia.

Until recently, marestail wasn't an economic concern for soybean growers, relative to other weed problems, notes Bill Johnson, extension weed specialist at Purdue University. Soybean growers simply controlled marestail through tillage or burn-down treatments. Johnson says its increasing significance is due to several factors, with a primary one being early planting. “When the planter disrupts the growing weed, it typically goes into a stagnant state and doesn't grow rapidly, making it difficult to kill with a herbicide that relies on being drawn into the weed through its root system,” Johnson says.

Other reasons for poor control of marestail are that the weed is a prolific seed producer, with tens of thousands of seeds per plant, and its small, lightweight seeds are easily carried to neighboring fields by the wind. Johnson adds that he has documented resistant populations of marestail in soybean fields in the southern one-third of Indiana, where some marestail plants survived even four times the labeled rate of glyphosate.

Waterhemp worries

Waterhemp is notorious for its ability to withstand tough control measures. Experts say this tenacity has enabled it to form pockets of resistant weeds within the total waterhemp population.

In Iowa, Extension Weed Specialist Mike Owen says he has seen individual, resistant waterhemp plants scattered in a soybean field alongside waterhemp plants that didn't exhibit resistance. His research at Iowa State University revealed plants that were able to survive even 3½ times the labeled rate of glyphosate. He says research is under way to try to help weed specialists better understand the weed.

“We don't know the mechanism of action — why those plants are resistant — though we hope to know by this spring,” he says, referring to current university research. “We also don't know the frequency of those resistant plants within the total species population.”

Because waterhemp is not a self-pollinating weed, Owen anticipates that resistant waterhemp plants will spread slowly, unlike marestail. Even so, he says, based on how widely and repeatedly glyphosate is used, “farmers need to realize that, from our perspective, the evolution of resistance to glyphosate is inevitable.”

However, Monsanto's Heering says waterhemp does not exhibit resistance but rather what he calls “differential tolerance.” “What we're seeing is that as growers delay their applications, they aren't increasing their Roundup rates to correspond to weed size, which is permitting waterhemp to escape control,” he says.

In Illinois, extension specialists are not prepared to say that they see waterhemp resistance yet within the state. “We haven't confirmed it, but we are concerned it could be present,” says Christy Sprague, extension weed specialist, University of Illinois. “We do know there is considerable ALS, PPO [protox inhibiting herbicides] and triazine resistance problems in our fields.”

Young of Southern Illinois agrees and says that in those areas where waterhemp is resistant to the ALS and PPO products, glyphosate is the only chemistry that offers postemergence control of that weed. He says that if those same waterhemp plants with resistance to ALS and PPO products also then become resistant to glyphosate, no chemical measures will be available to control them after emergence. “It won't matter whose product you use, it won't work, period,” he says.

Young and other weed specialists understand that for soybean growers to justify the expense of using other chemistries, along with glyphosate, they need good economic incentives. One is to better address early-season weed problems. Soil-applied herbicides help eliminate early weed flushes and prevent early-season yield losses, and he references this past year as an example. “In a short-season, dry year like 2002, we had some yield losses from early-season weed competition that impacted profitability,” he says. “A soil-applied herbicide would have paid for itself in some fields.” Young also is concerned about growers who plant Roundup Ready corn and therefore don't rotate out of the glyphosate technology at all, compounding potential resistance concerns.

No new options

From anyone's viewpoint, one of the main reasons to protect the glyphosate chemistry is this: There simply are no new chemistries to replace it.

“The companies aren't looking for new products,” Sprague says. “It's just not happening.”

Foresman says companies must have blockbuster products that provide a financial incentive to justify the research expense, and the payback just isn't there based upon the overall economics in agriculture. The result: “We're going to see fewer and fewer new modes of action introduced to bail growers out of weed resistance problems like we have in the past,” he says. “Since 1994, there have only been two new active ingredients introduced: Liberty Herbicide [glufosinate] in 1994 and the HPPDs [Balance in 1998, and Callisto in 2002].”

Sprague adds, “If glyphosate resistance becomes widespread, growers may face going back to wider rows, more cultivation and the use of older chemistries.”

“All this raises the specter on how best to use the glyphosate technology to sustain it,” Foresman says. “We need to preserve it.”

How? One way is to choose your glyphosate wisely.

“The issue is how concentrated the product is with the active ingredient,” Sprague says. “From product to product, the glyphosate formulations may vary widely or just slightly. We tell soybean growers, ‘Read the label so you know what's in the product, and the appropriate rates for the weed type and size you're trying to control. Also be sure to determine whether you need a surfactant.’”

Guidelines for use

“What we're doing is asking farmers to think strategically about their herbicide program and to think long term, because once you get glyphosate resistance in your fields it doesn't go away,” Foresman says. He adds that in a Syngenta-sponsored survey conducted by the American Farm Managers Association of 50 farm managers and rural appraisers, 53% said uncontrolled weeds can reduce the rental value of farmland by up to 17%. In addition, they said that glyphosate resistance is their number-one weed-resistance concern.

Use full rates

Reduced rates of glyphosate may not deliver the weed-control punch you need and may permit tolerant weed species to gain a foothold in your fields, Foresman explains. You cannot afford to allow weeds to escape and reproduce. “You can't allow tolerant weed species to build up and dominate the seed bank in your fields,” he says. “That will cause you problems down the road.”

Heering encourages growers to make sure that they match the right rates of glyphosate to the size of weed they're trying to control. “That's really critical as the weed increases in height,” he says.

Rotate chemistries

To preserve the integrity of glyphosate, you must include other chemistries for weed control. Foresman tells growers to use no more than two applications of glyphosate within one field over a two-year period. In those fields where you need more than one in-season herbicide application, use an alternative to glyphosate for the burn-down treatment.

In Illinois, Sprague encourages soybean growers to use a different soil-applied herbicide. “Don't overlook older broadleaf chemistries, such as metribuzin [Lexone or Sencor], or older grass herbicides such as metolachl or pendimethalin [Dual or Prowl],” she says.

Rotate Roundup Ready crops with conventional crops

For many growers, this option may not work. But if it can, take advantage of it, Foresman advises.

Heering counters that rotating out of the technology is unnecessary. He says that both commercial experience and long-term university research demonstrate that growers can successfully grow Roundup Ready crops in rotation with each other.

Marestail control options

To alleviate marestail, Bill Johnson says you must first know what is in your fields. “You've got to know what's out there,” he says. “Don't plant into existing marestail. If it's tall, you're asking for problems.” He provides the following recommendations for control.

If glyphosate-resistant marestail is present, apply 2,4-D or Amplify/FirstRate with glyphosate. Johnson says this approach will provide some residual weed control. He cautions that soybean growers need to follow the specific planting intervals recommended following 2,4-D use. Heering agrees and says if there is resistant marestail present, then a burn-down treatment using 2,4-D makes sense.

If you need a quick burn-down, go to a Gramoxone-based burn-down program with 2,4-D. “If you think you have glyphosate resistance and/or ALS resistance, this is the way to go,” Johnson says.

Johnson hopes soybean growers take steps now to preserve the glyphosate technology, much the same way he observed growers' efforts to retain atrazine. “With atrazine we do a good job of using it as a foundation product and then plugging other products in around it,” he says. “If we don't do the same thing with glyphosate, we're going to run this technology into the ground and wish that we hadn't.”