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Research Highlights
Herbicide-Resistant Common Ragweed Control: Managing Large Weeds (Part 2 of 2)

Herbicide-resistant common ragweed in a soybean field Photo: Sarah Hirsh

By Laura Temple

Maryland farmers often report challenges controlling common ragweed, leading to significant soybean yield loss, according to Kurt Vollmer, University of Maryland Extension specialist focusing on weed management at the Wye Research and Education Center. 

“In Maryland, glyphosate-resistant common ragweed is widespread,” he says. “However, we’ve also found populations of the weed that are resistant to ALS-inhibitors like FirstRate® or Raptor®. Even more concerning, a couple of those populations also demonstrate resistance to PPO-inhibiting herbicides like Reflex®.”

The Maryland Soybean Board checkoff funded on-farm research to identify effective control solutions for heavy common ragweed pressure with resistance to one or more herbicides.

Most common ragweed emerges by early May, and part 1 of this series focuses on limiting early season emergence. However, common ragweed can escape early management or emerge later, as time depletes residual herbicide control. When weather or other factors delay post-emergence herbicide applications, weeds continue to grow and can become too large for post-emergence herbicide to effectively control. This is the focus of Vollmer’s research and part 2 in this series. 

“Most area farmers are no-till, which limits post-emergence control options to herbicides,” Vollmer says. “To manage common ragweed with resistance to three different herbicides, farmers need to consider soybean varieties with stacked trait packages, like the Enlist® system.”

The Enlist system allows growers to apply glufosinate, commonly known as Liberty®, Enlist One®, the 2,4-D approved for use on Enlist crops, as well as glyphosate, without harming the crop. Alternatively, growers can use the Xtendflex® system, which also allows for the application of glufosinate, glyphosate or dicamba products registered for over-the-top use in soybeans.

“We recommend growers use more than one effective herbicide mode of action to control herbicide-resistant common ragweed, and to spray weeds when they are less than 3-inches tall,” Vollmer says.

Figure 1. Applications of fomesafen (FOM) and glufosinate (GLU) alone provided less control of 6” to 12” common ragweed compared to sequential and multiple herbicide applications. Sequential applications were applied 15 days apart based on conditions, and this data was gathered three weeks after the second application. Source: Kurt Vollmer

However, when it isn’t possible to follow those recommendations, his 2021 research offers additional options as rescue treatments for hard-to-control common ragweed.

Synergy of Multiple Herbicides or Applications

Vollmer’s research included two trials, both focused on options available with the Enlist trait package for soybeans.

“Combining multiple effective post-emergence herbicides or multiple applications can provide synergy to control common ragweed that exceeds 6-inches,” he continues. “These trials evaluated the effectiveness of tank mixes and sequential herbicide applications on different sizes of common ragweed.”

Figure. 2. Three weeks after application, 2,4-D, glufosinate (GLU) or any combination of herbicides controlled 6” to 12” common ragweed reasonably well. However, if the weeds reached 14” to 18” tall, 2,4-D with either glufosinate or glyphosate provided the most control. Source: Kurt Vollmer

In the first trial, Vollmer evaluated combinations of 2,4-D, glufosinate and fomesafen, which is a PPO-inhibitor, on common ragweed measuring 6 to 12 inches tall. The trial included each herbicide alone, in sequential applications and in various tank mixes.

“When spraying weeds between 6 and 12 inches tall, we were able to get over 90 percent control with at least two applications or by tank mixing herbicides,” he explains. 

A second trial in the same field compared how 2,4-D, glufosinate and glyphosate performed on common ragweed at two growth stages, 6 to 12 inches and 14 to 18 inches. The results show that as weeds grow, the effectiveness of rescue treatments diminish.

“The combination of 2,4-D and glufosinate appeared to be the best option to control taller weeds, as it allowed for the use of two effective modes of action,” Vollmer says. “But that synergism only worked so well as weeds got larger, because control decreased.” 

These results reinforce the importance of spraying weeds when they are smaller, as recommended by Vollmer and other agronomists. 

“Although rescue treatments can help, growers should use caution because they cannot apply a lethal dose of herbicide, which can lead to the development of additional herbicide resistance,” he says. “This research demonstrates that tank-mixes of multiple herbicide groups or sequential applications are the best option to control large common ragweed.”

This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.