Does it seem like you have more problems with giant ragweed than you did a few seasons back? If so, odds are you’re not imagining things. Farmers and industry experts report a resurgence of giant ragweed in many fields. Chad Threewits believes he can explain why many people are seeing more of these plants again.

“Growers are very focused on waterhemp, which is now also a driver weed in the eastern Corn Belt,” says Threewits, an agronomy service rep for Syngenta based in Fort Wayne, Ind. “Since it tends to germinate a bit later, many people have shifted their first weed control pass a bit later. Now, [herbicide is] not always going on early enough to catch some giant ragweed plants.”

The resurgence of giant ragweed is more obvious when fields are in soybeans vs. corn. “We have more effective residual herbicides against giant ragweed in corn compared to soybeans,” Threewits adds. “There are residuals with activity against giant ragweed in soybeans, but they are not as strong as residual herbicides that we can apply in corn.”

For corn, Threewits suggests applying a Group 27, HPPD-inhibitor preemergence herbicide for residual control of giant ragweed. Key products in the Syngenta lineup are Acuron and Storen. Other companies offer products as well.

Then, you can come back with a postemergence program and knock out any remaining giant ragweed plants, he notes.

Giant ragweed and soybeans

Several residual soybean herbicides have activity against giant ragweed, but control usually doesn’t match what you can achieve in corn in high-weed-pressure situations. An ALS inhibitor will work if ragweed in your field isn’t ALS-resistant. Syngenta offers Tendovo as a soil-applied residual herbicide at planting for soybeans, Threewits says.

The real issue is that there isn’t a good residual option to apply with post applications in soybeans, he notes.

“The big key here is to scout regularly,” Threewits says. “Most post herbicide programs have trouble controlling giant ragweed when plants get taller than 4 inches.” For comparison, that’s about the size of a soda can.

Herbicides with dicamba over dicamba-tolerant soybeans are an option if you have a dicamba product available. Registration was lifted by U.S. EPA earlier this year, but growers are allowed to apply “remaining stocks.” Pay attention to cutoff dates for applying dicamba.

If you have Enlist soybeans, an Enlist herbicide or glufosinate is an option. However, following the weed height restriction on the label is crucial, Threewits says. Allowing ragweed to get too big can cut yields. When you’re trying to time a post application to control ragweed, frequent scouting is important.

“The bigger problem comes in if you have non-GMO soybeans,” he says. “We have considerable acreage in places where growers are raising Plenish soybeans. Those varieties only contain a glyphosate-tolerant GMO weed-control trait. That is a challenge when ragweed plants are resistant to glyphosate.”

Options are basically Flexstar or Cobra, he says. Expect to see some burn after applying these products. Scouting is important with these products as well.