Syngenta identifies key approach to protect yields from resistant giant ragweed

Farm Forum

GREENSBORO, N.C. – As corn and soybean growers continue the fight against weed resistance, Syngenta provides key insight into how to manage giant ragweed, a large-seeded broadleaf weed that truly lives up to its name.

Giant ragweed can reach up to 20 feet high, pilfering water, nutrients and sunlight from surrounding corn and soybean plants as it grows. Like other large-seeded broadleaf weeds, it is hard to manage as its seeds germinate deep within the soil profile, shielded from herbicide applications. According to Purdue University, giant ragweed can produce more than 5,000 seeds per plant and is often 1 to 5 feet taller than the crop with which it is competing.

Many growers fight to manage giant ragweed in their fields as it has shown resistance to multiple modes of action (MOAs). The Take Action Organization reports giant ragweed resistance to ALS inhibitors first occurred in Indiana, Illinois, Ohio and Iowa in the late 1990s and early 2000s. Furthermore, glyphosate-resistant giant ragweed was first confirmed in the Eastern Soybean Belt and has now been confirmed in 11 states across the Midwest and Southern U.S.

“Giant ragweed is the worst broadleaf weed problem in all of our corn/soybean cropping systems in Indiana,” said Bill Johnson, professor of weed science at Purdue University. “The fact that it can emerge from soil depths up to 5 inches and that glyphosate (Group 9) and ALS (Group 2) resistance in this weed is common makes control very challenging with our current herbicide arsenal.”

According to Joe Wuerffel, Ph.D., research and development scientist at Syngenta, giant ragweed historically has been one of the first weeds to emerge out of the ground and germinate in early March for 30 to 40 days. “But recently, this difficult-to-control weed has extended its germination period to 60 to 90 days in the growing season, resulting in the need for residual herbicides and multiple MOAs,” said Wuerffel.

Growers must account for early-emerging weeds, like giant ragweed and lambsquarters, and late-emerging weeds, like waterhemp and Palmer amaranth. To protect yields and manage resistant giant ragweed season-long, Dane Bowers, herbicide technical product lead at Syngenta, recommends the following practices:

• Start clean: Know if giant ragweed is present in the spring, then apply an effective burndown herbicide, or use tillage, to set up a clean field for planting. When growers use tillage to prepare the seedbed, make sure that equipment is set to ensure that emerged weeds are removed and not just injured.

• Don’t wait: Make timely, full-rate applications before weeds are 3 to 4 inches tall.

• Use residuals: Apply multiple, effective MOAs with residual activity on giant ragweed.

• Monitor season-long: A second post-emergence application may be required.

“In this area, we see a lot of giant ragweed,” said Matt Rausch, a grower in Winamac, Indiana. “We’ve had problems where weeds have gotten out of hand late in the season, and after they harden off, they become very hard to kill. It is better to get them under control when they are small, at the beginning of the season.”

For early-season giant ragweed management, growers can apply Acuron or Acuron Flexi corn herbicides from Syngenta. Acuron has four active ingredients and three effective MOAs, while Acuron Flexi has three active ingredients and two effective MOAs. Both herbicides contain the active ingredient bicyclopyrone, which provides improved control of large-seeded broadleaf weeds like giant ragweed.

For soybeans, a pre-emergence herbicide applied prior to the weed emerging is recommended. A post-emergence application of Flexstar GT 3.5 herbicide, which delivers two different MOAs, can help control glyphosate- and ALS-resistant giant ragweed.

Decades of research and development have put Syngenta at the forefront of introducing herbicides with multiple, effective MOAs and extended residual control to help fight resistance. The Syngenta Resistance Fighter® program provides education, local recommendations, and a comprehensive herbicide portfolio to help growers and retailers effectively manage resistant weeds in their area.

Learn more about how to identify and manage giant ragweed in this video with Wuerffel.

For more information about how to fight weed resistance, visit Join the conversation online – connect with us at