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THE IF-IT'S-NOT-broke-don't-fix it philosophy is a good approach to some aspects of life. But current concerns with glyphosate use indicate that this philosophy is a bad strategy for weed control in soybeans.

Of the 60 million soybean acres planted annually, about 48 million are sprayed with this reliable, cost-effective herbicide, which touts a proven 25-year-plus track record. That means, for most growers, whatever the weed problem is, glyphosate is the answer. And that, experts say, is a recipe for disaster.

“If you do the same thing every year, you're going to get burned sooner or later,” says Reid Smeda, University of Missouri weed specialist.

Chinks in the armor

For some growers, later has arrived, with small chinks beginning to appear in glyphosate's armor of control.

Three years ago, several isolated cases of resistance were reported in biotypes of marestail and ryegrass, both winter annuals. Now the University of Missouri has confirmed a single case of resistance in common ragweed, a summer annual weed found throughout the state.

Smeda says a central-Missouri grower first noticed the potential weed-control break in 2003. The grower plants only Roundup Ready soybeans and uses only glyphosate for weed control. Glyphosate was eliminating weeds over the majority of his 1,300 acres of no-till, continuous soybeans, but one 20-acre patch appeared to hold ragweed escapes. The grower turned to Smeda and Monsanto for help.

“I thought it was a simple misapplication problem at first,” Smeda recalls. But after making firsthand, in-field glyphosate applications with various rates, some much higher than recommended by the label, he realized, “Hey, we've got something going on here.”

In 2004, Smeda dug ragweed plants and transplanted them into a University of Missouri greenhouse. He again applied a variety of glyphosate rates to the suspect ragweed. Smeda says, “We found it took four to 10 times the labeled amount of glyphosate to kill the plants.”

Fortunately, Smeda and Monsanto worked together to develop a practical tankmix solution that the grower used to eliminate the resistant ragweed last season. However, Smeda says the grower may continue to encounter a resistance problem for several years, due to ragweed seed presence in the soil. For that reason, Smeda says, “Once a grower has the problem, a significant change in weed management practices will be warranted.”

Similar scenarios

Many university weed-control specialists say scenarios similar to the Missouri one will eventually play out on other farms, if soybean growers continue to rely heavily on glyphosate as their single source for weed control.

For example, Delta Farm Press, a sister publication to Farm Industry News, reported last fall that a “highly suspicious” patch of common ragweed on a farm in north-central Arkansas appeared unaffected by glyphosate.

“When we took the plants to the greenhouse, we found a lot of diversity in their ability to withstand glyphosate applications,” says Bob Scott, Arkansas Extension weed specialist. “Although we have not done all the studies needed to confirm the level or mechanisms of resistance, we do feel that this population is resistant to glyphosate.”


The good news is that weed resistance problems don't have to occur, if farmers take precautions now to safeguard glyphosate's efficacy. “Soybean growers can include other chemistries in their weed-control program and/or use a crop rotation with corn or some other crop,” Smeda says. “Don't wait until some type of weed resistance occurs to take action.” He adds that, with some planning, growers can stay within a $7 to $12/acre price range for their control measures, not including application costs.

Smeda outlines several options for growers' consideration. “You can go with a tankmix this spring that includes an ALS product such as FirstRate,” he says.

A second option is glyphosate tankmixed with a postemergence “burner” type herbicide such as Reflex or Cobra.

A third option is to consider a preemergence herbicide such as Command or Sencor.

Monsanto says FlexStar or Phoenix herbicides can be tank mixed with glyphosate for postemergence applications in soybeans. Valor or Scepter can be added to a soil-applied program.

“I pretty much recommend that growers in no-till production systems use tank mixtures, other herbicides and tillage in combination with glyphosate to help prevent resistance,” Scott says. “This is my recommendation, even if they are currently getting good control with just glyphosate. This is especially true if they are using reduced tillage and currently have to burn down common ragweed or horseweed.”

For corn, Monsanto says that popular herbicides such as triazines and growth regulators can be added to an integrated herbicide program.

Other options probably are available to growers, which can be determined with help from custom applicators or county extension agronomists. The important thing is to look now for other active ingredients, beyond glyphosate, that can control weeds. Don't wait until there is a problem, which can minimize control options.

Fall program

Looking ahead to the 2006 growing season, soybean growers may want to consider using a fall-applied program to prevent or minimize spring weed emergence, Smeda says. Options include products such as Authority, Spartan or Valor. Smeda recommends that growers apply these herbicides with either 2,4-D or paraquat when temperatures are lower than 50∞F but above freezing. With this approach, the active ingredients sit immobile in the soil until spring. When the soil starts to warm and plant growth begins, the products become activated and kill weed growth, including growers' number-one broadleaf problem, waterhemp. Smeda says that a fall control program, however, is not as effective on big-seeded, late-growing weeds such as sunflower and cocklebur. “Fall-applied herbicides are not the panacea for common ragweed, but they offer some suppression,” Smeda adds.

With regard to glyphosate use, weed-control experts stress the importance of responsible use of the chemistry with regard to application timing and labeled usage rates.

Greg Elmore, Monsanto soybean technical manager, says, “Use the right rate at the right time on the right size of weed.”

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