Weed of the month: Giant ragweed

  • Giant ragweedCommon name: giant ragweed, great ragweed, horseweed, Texas great ragweed, buffalo weed, bitterweed, bloodweed
  • Scientific name: Ambrosia trifida
  • Giant ragweed is a native annual and is prevalent throughout the United States and much of Canada.
  • In soybeans, one giant ragweed plant can produce as many as 5,100 seeds per square yard. In corn, the number of seed produced per square yard can top 3,500.1
  • It emerges as temperatures warm in early spring, continuing to emerge into late July.
  • Because it emerges early in the season and grows very rapidly, giant ragweed competes with corn and soybeans for water, nutrients and light for the entire growing season.
  • Even relatively low populations of this competitive weed can cut soybean yield by 50 percent and can reduce corn yield by 13 percent.1
  • Research has shown the presence of stem-boring or stalk-boring insects in giant ragweed plants may reduce the effectiveness of glyphosate treatments as a control method.1

Fast facts from Jason Bond, Extension weed scientist, Mississippi State University, Dow AgroSciences

To correctly identify giant ragweed when scouting, look for the following plant-distinguishing features:

  • The seedling’s cotyledons are spoon-shaped and are relatively large, reaching up to 1⅝ inch in width and 1¾ inches in length.
  • Leaves generally have three distinct lobes but can have as many as five lobes.
  • Giant ragweed blooms from July to October in most areas of the country.
  • In corn and soybean fields, giant ragweed often grows anywhere from 1 foot to 5 feet taller than the competing crop. 1
  • Giant ragweed can reach up to 17 feet in height, creating a large, dense canopy that can quickly shade out area crops.

Resistance statistics*

  • Herbicide-resistant giant ragweed has been documented throughout much of the U.S. corn and soybean production area.
  • According to TakeActionOnWeeds.com, resistance to glyphosate (Group 9) has been confirmed in 11 Midwestern and Southern states.
  • Since the late 1990s, giant ragweed has shown resistance to ALS inhibitors (Group 2).
  • Resistance to both ALS-inhibitors (Group 2) and glyphosate (Group 9) has been documented in giant ragweed populations in several Midwestern states.

*Resistance confirmation does not necessarily include all weeds and may vary among different areas of each state.

Giant ragweed control/management tips:

Bond says:

  • Start the crop year with a weed-free environment by applying a preplant burndown followed by a preemergence herbicide with residual activity to suppress later-emerging populations.
  • Use multiple postemergence herbicides with multiple modes of action, basing the number and frequency of applications on the size of the weeds and the population of giant ragweed present.

General tips to manage herbicide-resistant weeds

Growers in the Midwest and Midsouth face some of the most difficult-to-control weed species. Taking the following steps during the season can help manage weed resistance issues:

  • Develop an integrated weed management plan that delivers multiple modes of action throughout the season. With resistance increasing, the Enlist weed control system allows growers to use multiple postemergence modes of action, including glufosinate, glyphosate and a new 2,4-D in soybeans and FOPs, glyphosate and a new 2,4-D in corn.
  • Use full rates of herbicides during applications. Do not use partial rates or trim back for any reason, including cost.
  • Spray when weeds are small. Although it can be challenging because of weather and other factors, this is the ideal application timing.
  • Scout fields regularly to identify weeds when they are small and easy to control.

Dow AgroSciences weed control solutions:

SureStart® II herbicide
Resicore® herbicide
Keystone® NXT herbicide
Keystone® LA NXT herbicide 

FulTime® NXT herbicide
Surpass® NXT herbicide 
Durango® DMA® herbicide
Duramax® herbicide
Enlist Duo® herbicide, as part of the Enlist weed control system

Sonic® herbicide
Surveil® herbicide
Durango® DMA® herbicide
Duramax® herbicide
Enlist Duo® herbicide, as part of the Enlist weed control system

Additional information:

More information can be found online at:  Management of Herbicide-Resistant Giant Ragweed — Take Action on Weeds

1Glyphosate, Weeds, and Crops Group, Purdue Extension Education. 2007. Biology and Management of Giant Ragweed. http://weedscience.missouri.edu/publications/gwc-12.pdf
2Take Action on Weeds. 2014. Management of Herbicide-Resistant Giant Ragweed. http://takeactiononweeds.com/wp-content/uploads/FactSheet_GiantRagweed.pdf

®™Trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow. FulTime NXT, Keystone LA NXT and Keystone NXT are federally Restricted Use Pesticides. Enlist Duo is not registered for sale or use in all states or counties. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your area. Duramax, Durango DMA, Enlist Duo, FulTime NXT, Keystone LA NXT, Keystone NXT, Resicore, Sonic, SureStart II, Surpass NXT and Surveil are not registered for sale or use in all states. FulTime NXT, Keystone LA NXT, Keystone NXT, Resicore, SureStart II and Surpass NXT are not available for sale, distribution or use in Nassau and Suffolk counties in the state of New York. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Always read and follow label directions. ©2017 Dow AgroSciences LLC