In light of the University of Missouri’s announcement* of a confirmed case of glyphosate-resistant waterhemp, Monsanto is recommending farmers use a pre-emergence soil residual herbicide in their Roundup Ready soybean system if they have dense stands of  common broadleaf weeds such as waterhemp. A residual herbicide product such as INTRRO or Valor applied prior to soybean emergence helps farmers eliminate early season weed competition and adds another herbicide mode of action into the cropping system.

“Farmers who are growing no-till Roundup Ready soybeans year after year or any farmer who has a dense stand of tough-to-control weeds such as waterhemp should use a residual herbicide prior to planting,” says Dr. Jennifer Ralston, Monsanto Roundup Technical Manager. “A continuous, no-till Roundup Ready soybean production system where only glyphosate is used increases the pressure on glyphosate as the sole tool for weed control.”

That was the situation in this particular case of glyphosate resistance. Over the past year, Monsanto has closely followed and worked with University of Missouri Extension Weed Scientist Dr. Kevin Bradley on his investigation into a suspected case of glyphosate-resistant waterhemp on two fields in northwest Missouri. Both fields have been in continuous Roundup Ready soybean production since 1996. Initial results released in September 2005 showed a lack of control at labeled rates in greenhouse trials. Field and heritability studies confirmed resistance this summer.

The waterhemp population is also resistant to ALS herbicides, further reducing herbicide choices for tank mix recommendations. For the few growers that have glyphosate-resistant waterhemp, Monsanto provides the following recommendations:

• In Roundup Ready^® soybeans – Use a pre-emergence residual product at planting such as INTRRO^® or Valor^®. Tank mix Roundup WeatherMAX with Cobra or Flexstar if post-emergence waterhemp control is needed.

• In Roundup Ready Corn 2 – Use a pre-emergence residual such as Harness Xtra or Degree Xtra at planting. Tank mix Roundup WeatherMAX with 2,4-D or dicamba (Clarity, Banvel  or Distinct) if post-emergence waterhemp control is needed.

 “We’ve planted corn on this field to see if crop rotation can make a difference,” says Dr. Bradley. “This field hasn’t received anything but glyphosate in a long time, so the addition of atrazine with other acetanilide herbicides is controlling the waterhemp nicely.”

Monsanto also recommends growers consider the benefits of a corn-soybean rotation, including the ability to use different herbicides and alternative cultural practices and the disruption of disease and insect pressure.

“Growers who want to use the Roundup Ready system should consider rotating between Roundup Ready soybeans and Roundup Ready Corn 2,” says Dr. Ralston. “The addition of atrazine and herbicides such as Harness or Degree Xtra give farmers other tools to diversify their weed control.”

Monsanto recommends the following set of general weed management practices to help growers minimize the risk of developing resistant weeds:  

• Start with a clean field by either utilizing a burndown herbicide or tillage and control weeds early.

• Use Roundup Ready technology as the foundation of your weed management program.

• Add other herbicides (e.g. a selective and/or a residual herbicide) or cultural practices (e.g. tillage or crop rotation) where appropriate as part of the Roundup Ready cropping system.

• Use the right herbicide rate at the right time.

• Control weeds throughout the season and reduce the weed seed bank.

• Report any incidence of repeated non-performance on a particular weed to your local Monsanto representative, retailer, or county extension agent.

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* Missouri confirms world's ninth glyphosate-resistant weed