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Research Highlights
Herbicide-Resistant Common Ragweed Control: Limiting Emergence (Part 1 of 2)

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

By Laura Temple

Maryland farmers have reported substantial soybean yield losses caused by extreme common ragweed pressure and challenges controlling those weeds, according to Sarah Hirsh, University of Maryland Extension agent. 

“We’ve confirmed populations of common ragweed on the eastern shore and southern Maryland with three-way resistance to glyphosate, ALS-inhibitors like FirstRate® or Raptor®, and PPO-inhibitors like Reflex® or Valor®,” she says. “This combination of herbicide resistance leaves soybean farmers with limited herbicide options that tend to be more expensive.”

The Maryland Soybean Board checkoff funded research led by Hirsh helps to identify more effective solutions to manage common ragweed that is resistant to one or more herbicide modes of action. Her team conducted on-farm research in fields with heavy, herbicide-resistant common ragweed pressure. 

Based on observed patterns, 95 percent of common ragweed will likely emerge by early May, so Hirsh focused on early-season management. This is the focus of part 1 in this series. However, the weed may emerge later or escape early-season management. Part 2 of this series explores post-emergence control options for large, harder-to-control, herbicide-resistant common ragweed.

Even without residual herbicides, these photos, taken on July 2, 2019, show how allowing the cover crop to grow longer suppressed common ragweed emergence. Photos: Sarah Hirsh

“Early-season management of common ragweed depends on reducing prevalence before or at planting,” Hirsh explains. “I focused on tools commonly available to Maryland farmers: cover crops and pre-plant or at-plant herbicide applications.”

Most Maryland farmers use cover crops, and wheat is one of the most common choices, according to Hirsh. She says farmers often save seed from their winter wheat to plant as a way to minimize costs. Over three seasons, she looked at the impact of different cover crop burndown timings on common ragweed populations, as well as evaluated the use of a residual herbicide with the burndown.

Adjusting Cover Crop Management for Weed Control

In 2019 and 2020, Hirsh studied three different cover crop burndown timings of the wheat that the cooperating farmers chose to plant. The 2021 trial focused on just two burndown timings, but it also compared using a wheat cover crop to no cover crop. Throughout the study, glyphosate and 2,4-D were used for pre-plant burndown, while at-planting burndown applications consisted of glyphosate plus glufosinate. All residual treatments were linuron plus metribuzin (Linex® plus Dimetric®).

“The most encouraging treatment was planting green, or planting soybeans before or as the cover crop was terminated, and including a residual herbicide with the burndown,” she reports. 

Over the course of the study, she observed that common ragweed populations consistently tended to be lower following cover crops. When cover crops grew longer and produced more biomass, fewer weeds emerged.

Figure 1. Common ragweed populations based on cover crop burndown timing, 2019-2020. Average common ragweed populations 30 days after planting in 2019 and 2020 on-farm trials. The combination of cover crop (CC) burndown plus residual herbicide at planting resulted in the lowest weed count with the fewest pre-emergence herbicide applications. Different letters indicate statistically significant differences in results. Source: Sarah Hirsh, University of Maryland

“We don’t need the highest amount of cover crop biomass in the world to combat weeds,” Hirsh says. “When planting green, the wheat grew to between 3 and 4 feet before it was terminated, and the biomass appeared to be adequate to suppress weeds. At the same time, delaying cover crop burndown did not impact yield.”

Common ragweed populations were also consistently lower following application of a residual herbicide at planting, based on Hirsh’s observations. She found that if a residual was used at planting, a pre-plant residual application offered no benefit. 

“Most farmers in Maryland incorporate cover crops into their systems,” Hirsh says. “Based on this research, they can improve common ragweed control by just managing those cover crops a little differently.”

Figure 2. Common ragweed populations with and without cover crops and residuals, 2021. Common ragweed populations through early July in 2021 on-farm trials. Again, the combination of cover crop (CC) burndown (BD) plus residual herbicide at planting kept common ragweed populations consistently low throughout the observation period, regardless of burndown timing. Source: Sarah Hirsh

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.