Manage ragweeds to improve yields

For farmers in the Upper Midwest, ragweeds present a season-long problem. While not a new issue, changes in emergence patterns and growing herbicide resistance have made these weeds a tougher foe. 

Both common and giant ragweed have been an issue in the Midwest for years, but managing these weeds has become more difficult as they adapt to overcome farmers’ efforts. 

“Here in Michigan, most of our fields have common ragweed,” says Christy Sprague, a weed Extension specialist and professor at Michigan State University. “As you move further south, for example, in Indiana and Illinois, giant ragweed becomes more pronounced. Both of these weeds are pretty widespread.”

Left unchecked, ragweeds can aggressively reduce yields. One giant ragweed in a 10-square-feet area can reduce soybean yields by 52%, while two common ragweeds in the same area can reduce soybean yields by 30%. 

Identifying Ragweeds

While ragweeds have few look-alike species, knowing how to identify giant and common ragweeds can better aid management decisions. 

Common ragweed can be identified by its pinnate leaves with deep lobes. In optimal conditions, the plant can reach 6 feet in height. 

Giant ragweed has more shallowly lobed leaves, with three to five lobes and a greater leaf area. Giant ragweed also grows faster than its counterpart and reaches heights of 8 to 10 feet. Giant ragweed is typically one of the first weeds to emerge in a field.

“We’ve seen a shift in the emergence patterns of giant ragweed,” Sprague says. 

“They still come up early, but continue to emerge in May, June, and sometimes early July.”

Where tillage or a good burndown application was previously effective to manage giant ragweed, the extended emergence patterns mean postemergence control is now more essential. 

Herbicide Resistance

Ragweeds have developed resistance to several herbicide classes, making them more difficult than ever to control. Both giant and common ragweed species have demonstrated resistance to ALS inhibitors (Group 2) and glyphosate (Group 9), and some common ragweed populations have shown resistance to PPO inhibitors (Group 14).

“To manage ragweed, it’s important to identify if you have a resistance issue,” Sprague says. “Then focus your herbicide program on how to manage around that resistance issue, starting with a good soil-applied product and then coming in with a well-timed postemergence application.” 

To determine best management practices, farmers should evaluate previous herbicide application success and scout fields routinely to understand emergence patterns and weed control failures.

For ideal control, common ragweed should be sprayed before reaching 4 inches tall, while giant ragweed should be sprayed before reaching 6 inches tall.

Herbicide resistance issues can also limit seed choices, especially when it comes to soybeans. 

“For example, if you have an ALS- and glyphosate-resistant common ragweed population, you’d really have to focus on applying either PPO inhibiting herbicides or herbicides that can be used with newer traits like Enlist E3, LibertyLink, or XtendFlex soybeans,” Sprague says. 

Other Management Options

While ragweeds can be found in both no-till and conventional till systems, tilling could exacerbate issues by bringing seeds closer to the soil surface. Common ragweed can emerge from 2 inches deep, and giant ragweed can emerge from as far as 6 inches deep.

“One thing you might see with no-till fields is if you are able to keep a field clean for a couple years, you’re not bringing new seed up to the soil surface,” Sprague says. “That could be one approach to help decrease plant populations.”

For soybean fields, planting in narrow rows can also be helpful. 

“Earlier canopy closure can be helpful in limiting the sunlight that could potentially germinate those seeds,” Sprague says.

Rotating between corn and soybeans can provide different herbicide program options, allowing farmers to switch up products. The differences in the crop canopy can also influence the competitiveness of the weeds, having an effect on control and seed production.