US growers must fight glyphosate resistance

Glyphosate tolerant crops need more than just glyphosate for weed controlFarmers in the US have been aware for some time of the threat glyphosate resistant weeds pose to their crops and livelihoods. Now, the public is becoming more aware too after recent media attention following the publication of the US National Research Council’s report: Impact of Genetically Engineered Crops on Farm Sustainability in the United States1. The report confirmed the substantial economic and environmental benefits of GM crops, but warned that care was needed to preserve the value of the technology, especially with the threat posed by glyphosate resistant weeds. Farmers growing herbicide resistant crops must ensure that a diverse range of agronomic practices are used to control weeds and must not simply rely on one herbicide mode of action. Unfortunately, for many growers the simplicity of glyphosate-based weed control is hard to resist – if they are yet to experience any problems. However, they should heed the warnings of both weed resistance experts and fellow farmers. Australian Professor Steve Powles, one of the global leaders in weed science, has warned that “… glyphosate will be driven to redundancy in large parts of North America and South America, unless growers diversify weed control now.”2 Palmer pigweed (Amaranthus palmeri) is a huge problem in southern US states. Prominent weed scientist Ford Baldwin has stated that to help prevent resistance, a burndown program using paraquat should be used instead of glyphosate3. Some farmers call the early stages of a Palmer pigweed infestation “the red tide” because the reddish colored seedlings emerge so fast between the crop rows4. Early weed removal is critical if crop yields are not to be affected. Weed species that have become resistant to glyphosate are some of the most aggressive yield-reducers. Surprisingly low densities of weed infestation can have devastating effects on crop yields if left unchecked.  

Effect of glyphosate resistant weeds on crop yields*

Weed Crop Yield reduction
Amaranthus palmeri Soybean 2 weeds/20 row feet can reduce yield by at least 23%
Amaranthus palmeri Corn 1 plant/m2 can reduce yield by 33%
Amaranthus palmeri Cotton 8 plants/m2 can reduce yield by up to 91%
Ambrosia trifida Soybean 1 plant/m2 can reduce yield by 52%
Ambrosia trifida Corn 2 plants/m2 can reduce yield by 37%
*   In the US, 10 weed species now have populations officially recorded as being resistant to glyphosate over thousands of acres in many states5. US populations of confirmed and suspected glyphosate resistant weeds on www.resistancefighter.comThe full impact of this statistic comes home when outbreaks are plotted on an interactive map which can be found at The map pinpoints where glyphosate-resistant weeds have been identified or suspected down to the county level. A Solutions Builder module on the website allows users to develop customized weed management programs for their own farm. The site also includes weed management news and a frequently updated Ask the Expert blog featuring advice and recommendations from Chuck Foresman, Syngenta manager of weed resistance management strategies. Especially worrying are the increasing number of cases of weed populations resistant to both glyphosate and herbicides with other modes of action. In Illinois, a population of waterhemp (Amaranthus tuberculatus) is known to be resistant to four modes of action of common herbicides: glyphosate, triazines (eg atrazine), ALS inhibitors (eg imazethapyr or Pursuit) and PPO inhibitors (eg lactofen or Cobra) 2. Using herbicides with different modes of action is imperative to preserve the enormous benefits of using glyphosate in weed management systems. Paraquat can be used as an alternative, much faster acting and rainfast, burndown herbicide to ensure crops get off to a clean, weed-free start. Paraquat can also be applied through hooded sprayers in between crop rows. Unlike systemic glyphosate when applied to conventional non-GM crops, paraquat’s contact-only action does not risk causing crop damage. Small amounts of spray landing on crop leaves cause only local damage and paraquat is inactivated immediately on reaching the soil. Integrated weed management


A decision support system for crop protection which focuses on long-term prevention or suppression of pest problems with minimum impact on human health, the environment, and non-target organisms. IPM takes into consideration all available pest control techniques and tactics (cultural, mechanical, biological, chemical). IPM emphasizes the growth of healthy crops for better productivity with the least possible disruption to agroecosystems.

Authoritative On-line References and Resources "The USDA's National Site for the Regional IPM Centers' Information System provides information about US commodities, pests and pest management practices, people and issues."
systems which rotate herbicide modes of action and are custom-built for specific crop situations will fight glyphosate resistance.


  1. US National Research Council’s report: Impact of Genetically Engineered Crops on Farm Sustainability in the United States
  2. Agriculture Online
  3. Delta Farm Press
  5. International Survey of Herbicide Resistant Weeds


The brand name for the leading paraquat product is Gramoxone.