Our Products

Print This Page

Weed Resistance

PDF Download

Weed Resistance
(.pdf 281K)

Allegations/Claims

  • Weed resistance occurs only with a glyphosate based herbicide.
    Weed resistance has been found with most of today's classes of herbicides. In fact, relative to other herbicide classes, glyphosate is one of the most commonly used herbicides in agriculture today even though the number of weed species found to be resistant to glyphosate is relatively small.

Why are herbicides used?

  • Herbicides are chemical substances used to stop the growth of weeds in order to prevent weeds from negatively competing with crops or other desirable plant species, or they are used to entirely prevent plant growth in a particular area, e.g. sidewalks or roadsides. Herbicides are the primary tools used to control weeds in modern agriculture. "Through their effective use in combination with other weed control practices, herbicides are today, and will remain, an integral part of food production."

Sources: Herbicide Resistance Action Committee International Survey of Herbicide Resistant Weeds

What is weed resistance? – Key Points

  • When a single herbicide or class of herbicides are used repeatedly without the concurrent use of other weed control methods, the emergence of weed populations that are resistant to herbicides is possible.

  • This repeated use of herbicides with the same mode of action (how an herbicide affects a weed) will expose weed populations to selection pressure that may lead to an increase in the number of surviving, resistant individuals in the population. The resistant weed population may increase to the degree that commercially acceptable weed control cannot be achieved via the application of only that herbicide. Herbicides with a different mode of action and/or the use of other methods for controlling the resistant weed may become necessary.

More information can be found at the International Survey of Herbicide Resistant Weeds. www.weedscience.org

Glyphosate and Glyphosate Resistance

  • Glyphosate is the active ingredient in Roundup® herbicide. Roundup Ready® crops are modified to contain a transgene that confers resistance to glyphosate so that weeds can be controlled by safely spraying Roundup agricultural herbicide over the crop.

  • Glyphosate is one of the most common herbicides used in agriculture today and and several weeds have developed resistance to glyphosate.

  • Glyphosate resistant weeds have been found in areas where glyphosate has been used for weed control in orchards, roadsides and where Roundup Ready® crops are grown.

  • As glyphosate resistant weed biotypes have been identified, they have been effectively managed with other herbicides and/or cultural practices when applied as recommended.

Weed Resistance Management

Monsanto sponsors extensive research, performed by or with academic scientists, extension specialists and crop consultants. These partnerships provide an understanding of weed control systems, weed biology and cases of weed resistance when they occur. This information is published in scientific journals and placed in educational materials that are disseminated to growers and retailers. Product stewardship is considered to be a fundamental component of customer service and responsible business practices.

For a weed to be confirmed as resistant to an herbicide and listed on the International Survey of Herbicide Resistant Weeds, it must meet the following criteria:

  • Fulfillment of the Weed Science Society of America definition of resistance and the survey's definition of an herbicide-resistant weed.

  • Data confirmation using acceptable scientific protocols

  • The resistance must be heritable (passed on to the next generation)

  • Demonstration of practical field impact

  • Identification as a problem weed, not the result of deliberate or artificial selection.

Occurrence of Weed Resistance

Worldwide, the weed species confirmed to be resistant to glyphosate have been found in conventional and non conventional crops and in areas where Roundup Ready® crops are grown.

The species found in the US are:
Palmer amaranth, Common ragweed, Marestail, Fleabane, Waterhemp, Giant ragweed, Italian Ryegrass, Rigid Ryegrass and Johnsongrass

The species found outside the US are:
Rigid Ryegrass, Italian Ryegrass, Fleabane, Horseweed, Goosegrass, Buckhorn Plaintain, Johnsongrass, Wild Poinsettia, Ragweed Parthenium, Liverseedgrass, and Echinochloa colona (known as barnyard grass in Australia)

For information on specific locations see International Survey of Herbicide Resistant Weeds. www.weedscience.org

Glyphosate Stewardship
Monsanto takes product stewardship and claims of resistance to glyphosate very seriously. We work directly with growers to address incidents of alleged resistance. Our glyphosate stewardship program includes research, education and management recommendations. The education is based on these five points:

  1. Start with clean fields and control weeds early.

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

  3. Add other herbicides and cultural practices where appropriate as part of the Roundup Ready System.

  4. Use the right herbicide rate at the right time.

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

The following Monsanto individuals are responsible for global stewardship in their respective regions:

Rob Nixon/Boyd Carey US
Jorgelina Carcova/Pablo Grosso LAS
Eduardo Perez Pico/Oscar Heredia LAN
Luciano Fonseca Brazil
Chris Anderson Canada
Richard Garnett/Marie-Pierre Plancke EU/Africa
Santanu Dasgupta India
David Penna Asia Pacific
John Soteres Global Scientific Affairs

Resources

2007 Monsanto Technology Use Guide includes weed resistance management information as well as specific application guidelines by crop. http://www.weedresistancemanagement.com/cropSpecific.html

International Survey of Herbicide Resistant Weeds. It's important to have one definitive, authoritative source for information on confirmed cases of herbicide-resistant weeds. That source is the International Survey of Herbicide Resistant Weeds at www.weedscience.com. The survey is a collaborative effort among weed scientists in more than 80 countries, whose aim is to maintain scientific accuracy in the reporting of herbicide resistant weeds globally. The effort is supported and funded across the industry chain by the Herbicide Resistance Action Committee, the North American Herbicide Resistance Action Committee, and the Weed Science Society of America.

Monsanto's educational weed resistance management website for its farmer customers which provides comprehensive and timely information on weed control strategies for Roundup Ready cropping systems. Available online are "kits" that contain reference materials and factsheets detailing practical management tactics for hard-to-control weeds. www.weedresistancemanagement.com

The Herbicide Resistance Action Committee (HRAC) facilitates the effective management of herbicide resistance by fostering global understanding, cooperation and communication between industry, government, distribution channels, grower groups and farmers. The aims of HRAC have the general purpose of supporting a cooperative approach to the management of herbicide resistance.www.plantprotection.org/HRAC

Online Weed Resistance Management Learning Centers is a computer-based training module to educate growers and agriculture industry professionals on how to reduce the potential development of resistant weeds specifically in corn.

National Corn Growers Association
American Soybean Association
National Cotton Council
National Assoc of Wheat Growers
US Canola Association

Weed Resistance Risk Management Tool provides a simple, web based tool for customers that will help them gauge their risk of developing weed resistance to glyphosate and identify specific practices based on their responses, that can be implemented to reduce that risk. www.weedtool.com

References

Broster, J., Pratley, J. 2006. A Decade of Monitoring Herbicide Resistance In Lolium Rigidum In Australia. Australian Journal of Experimental Agriculture. 46(9): 1151-1160.

Culpepper, A., Grey, T., Vencill, W., Kichler, J., Webster, T., Brown, S., York, A., Davis, J., Hanna, W. 2006. Glyphosate-resistant Palmer Amaranth (amaranthus Palmeri) Confirmed in Georgia. Weed Science. 54(4): 620-626.

Green, J., Hazel, C., Forney, D., Pugh, L. 2008. New Multiple-Herbicide Crop Resistance and Formulation Technology to Augment the Utility of Glyphosate. Pest Management Science. 64: 332-339.

Green, J. 2009. Evolution of Glyphosate-Resistant Crop Technology. Weed Science. 57(1): 108-117.

Gustafson, D. I. 2008. Sustainable Use of Glyphosate in North American Cropping Systems. Pest Management Science 64: 409-416.

Halfhill, M., Good, L., Basu, C., Burris, J., Main, C., Mueller, T., Stewart Jr., C. 2006. Transformation and Segregation of GFP Fluorescence and Glyphosate Resistance in Horseweed (Conyza canadensis) Hybrids. Plant Cell Reports. DOI: 10.1007/s00299-006-0219-1: 9 pages.

Hanson, B., Shrestha, A., Shaner, D. 2009. Distribution of Glyphosate-resistant Horseweed (conyza Canadensis) and Relationship to Cropping Systems in the Central Valley of California. Weed Science. 57(1): 48-53.

Powles, S., Preston, C. 2006. Evolved Glyphosate Resistance in Plants: Biochemical and Genetic Basis of Resistance. Weed Technology. 20: 282-289.

Preston, C., Belles, D., Westra, P., Nissen, S., Ward, S. 2009. Inheritance of Resistance to the Auxinic Herbicide Dicamba in Kochia (kochia Scoparia). Weed Science. 57(1): 43-47.

Sammons, R.D., Heering, D.C., Dinicola, N., Glick, H., and Elmore, G.A. 2007. Sustainability and Stewardship of Glyphosate and Glyphosate-Resistant Crops. Weed Technology 21:347-354.

Shaner, Dale. 2009. Role of Translocation as a Mechanism of Resistance to Glyphosate. Weed Science. 57(1): 118-123.

Wakelin, A., Preston, C. 2006. Inheritance of Glyphosate Resistance in Several Populations of Rigid Ryegrass (Lolium rigidum) from Australia. Weed Science. 54: 212-219.