Key Role of Residual Herbicides Is Finding of Long-Term Weed-Resistance Study

ST. LOUIS, Mo., (June 14, 2007) – Use of residual herbicides in Roundup Ready® cropping systems not only greatly improves control of tough weeds such as lambsquarters, pigweed and wild buckwheat, but also reduces the potential for development of weed resistance.

These are two clear findings from an extensive, decade-long weed-resistance study conducted at multiple irrigated and non-irrigated sites across four states: Nebraska, Colorado, Kansas and Wyoming. This research encompassed a continual rotation of Roundup Ready crop technologies in corn, soybeans, sugarbeets and spring wheat*.

The purpose of this 10-year, four-state study was to create an environment of continuous Roundup Ready crops to determine if, when and how weed resistance to glyphosate might develop over time. After nine years of continuous use of full rates of Roundup agricultural herbicides, there was a shift in weed population to common lambsquarters. However, where a residual herbicide was used at planting in conjunction with Roundup herbicides applied over the top in Roundup Ready® Corn, lambsquarters was again controlled, according to Dr. Robert G. Wilson, a weed scientist at the University of Nebraska and an academic principle in the study.

When the study was initiated, Roundup Ready Corn was used until Roundup Ready Corn 2 was developed, then the latter technology was used. The study looked at different rotation combinations of the four Roundup Ready crops. It also examined the benefits offered by residual herbicides and the effect of using reduced application rates of glyphosate applied over the top in the development of glyphosate weed resistance.. The research carefully studied herbicide-treated weeds to determine whether tolerance or resistance developed as a result of multiple glyphosate applications across the four crops for an extended period of time.

“The use of a preemergence herbicides provided better overall weed control in the corn, soybeans and sugarbeet crops. A preemergence herbicide was not used in the spring wheat. The use of these residual preemergence herbicides definitely had an impact on hard-to-control weed species such as lambsquarters and pigweed,” says Dr. Wilson. “We were able to control these tough weeds with up to 80 or 90 percent efficiency when a preemergence herbicide was used, and the weeds were easier to control with glyphosate.”

Special attention in the study was directed at Roundup Ready Corn 2 in the later years of the trials that compared herbicide programs with and without the use of a residual herbicide, followed with either over-the-top applications of Roundup agricultural herbicides or use of other conventional herbicides applied either pre-emergence or post-emergence. The residual corn herbicides used included Harness® brands and Degree XTRA®. The soybean preemergence herbicides used were Spartan® and Prowl®, and Nortron® was used in sugar beets.

“In corn, we’ve seen lambsquarters cut yields by 40 percent or more,” Wilson explains. “When you have a weed that might be hard to kill with glyphosate, you need to pick a residual herbicide that has that weed on the label and then use a full rate of that residual herbicide according to soil-type recommendations.”

Among the many benefits of using a residual herbicide in a Roundup Ready Corn 2 System is that different herbicide modes-of-action are used and crucial, early-season weed control is provided so corn can get off to a vigorous stand, the Nebraska researcher notes. Multiple modes of action do reduce the potential for weed resistance to develop, he says.

“We’ve also established that you get better weed control with glyphosate when a preemergence herbicide is applied,” Wilson continues. “After the residual treatment, the weeds are smaller and more susceptible to control with glyphosate, and there are fewer weeds to contend with. You can also get better, more effective coverage with your over-the-top herbicide sprays when weeds are small.”

Wilson adds that preemergence residual herbicides are economical to use and that most farmers can probably afford them. “Once the farmer has to contend with a weed shift to hard-to-control weeds, he gets a good return on his herbicide investment when residual herbicides are used,” the researcher says.

*The Roundup Ready® wheat used in this study was for experimental purposes only. Roundup Ready wheat has not been commercialized.

Always read and follow pesticide label directions. Roundup Ready® crops contain genes that confer tolerance to glyphosate, the active ingredient in Roundup® agricultural herbicides. Roundup agricultural herbicides will kill crops that are not glyphosate tolerant. Harness® and Degree® brands are restricted-use pesticides and not registered in all states. The distribution, sale or use of an unregistered pesticide is in violation of federal and state law and is strictly prohibited. Harness®, Degree®, Degree XTRA®, Roundup® and Roundup Ready® are registered trademarks of Monsanto Technology LLC. All other trademarks are the property of their respective owners. ©2007 Monsanto Company.

Residual herbicide test plot at Panhandle Research and Extension Center, NE

Photo Caption: The benefit of using a residual herbicide is clearly evident in this test plot at the Panhandle Research and Extension Center in Scottsbluff, Nebraska, which is part of a long-term weed resistance study. Degree Xtra® residual herbicide was applied on the rows on the right, resulting in effective weed control. In comparison, there is a dense population of common lambsquarters in the rows on the left, which received no preemergence herbicide treatment.