Research HighlightsInvestigating Poor Grass Control in Dicamba-Tolerant Crop Systems
In this article, you’ll find details on:
- Poor control of goosegrass in dicamba-tolerant crops raises questions about herbicide antagonism and weed resistance in South Carolina.
- In Clemson trials comparing different spray volumes and nozzle types, glyphosate alone consistently provided better goosegrass and Texas panicum control than when tank mixed with dicamba.
Photo: Mike Marshall
By Laura Temple
Although figuring out how to control weeds in soybean fields has gotten more complicated, applying the chosen herbicides is relatively straightforward. Follow label directions to combine the chosen herbicides, adjuvants and other additives in the tank, and spray.
Then watch for results.
“In 2022, farmers called about poor goosegrass control, especially in dicamba-tolerant systems,” says Mike Marshall, assistant professor of agronomic and forage weed science at Clemson University. “We’ve been seeing more grass escapes in row crops in recent years.”
He decided to investigate because poor weed control raises concerns about herbicide antagonism and weed resistance. With South Carolina Soybean Board support, he studied goosegrass and Texas panicum control with glyphosate and dicamba in 2023.
Application Logistics
Marshall explains that dicamba applications require nozzles that create large droplets to minimize drift. In contrast, glyphosate is usually applied with nozzles that create a fine spray to maximize getting the herbicide on the weeds.
“Goosegrass has skinny leaves, and it is hard to get good coverage,” he says. “We examined if using a higher spray volume would overcome coverage issues.”
His trials compared spray volumes of 10, 15 and 20 gallons per acre and different nozzle types, as well as glyphosate alone and tank mixed with dicamba.
“Unfortunately, different spray volumes showed no difference in grass control,” Marshall reports. “Glyphosate alone consistently worked better than when applied with dicamba. However, Texas panicum didn’t show as much drop off in control as goosegrass did.”
He notes that Texas panicum has larger leaf blades that likely capture the large droplets of the tank mix better than goosegrass blades.
Additional Factors
Marshall says the results in 2023 raised more questions. For example, the cool spring in 2023 delayed the timing of grass emergence, which could have influenced herbicide effectiveness in the initial trial.
“Are we seeing true herbicide antagonism?” he asks. “Do dicamba drift reduction agents affect glyphosate? Or do environmental factors and grass growth stage impact control? Or, is herbicide resistance showing up in South Carolina? We are continuing the research to better understand what is happening.”
Glyphosate-resistant goosegrass has been confirmed in Mississippi and Tennessee. It has developed resistance to other herbicides across the South, as well. To date, the only herbicide resistance confirmed in goosegrass in South Carolina is resistance to DNA, or yellow, herbicides like Prowl and Treflan.
Marshall’s second year of trials will use a single spray volume and focus on goosegrass control. He expects a more typical spring, so he will be able to compare environmental conditions and herbicide efficacy between the two seasons.
“Because of herbicide resistance in Palmer amaranth and other weeds, farmers need weed control options like dicamba,” he says. “Tank mixing saves time and money on applications, but farmers need to know if that affects control of other weeds, so they can make more informed decisions about when and how to control weeds in specific soybean fields.”
Published: Aug 12, 2024
The materials on SRIN were funded with checkoff dollars from United Soybean Board and the North Central Soybean Research Program. To find checkoff funded research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.
