Research HighlightsHerbicide Resistance and Antagonism Reduce Grass Control
By Laura Temple
Ten to 15 years ago, marestail was the top weed problem in Tennessee soybean fields. As the winter annual developed resistance to glyphosate, it became a year-round challenge. The advent of other herbicide-tolerant technologies in soybeans allowed the use of effective herbicides, and it is no longer a problem.
But other weeds have become more noticeable. In the past few years, Mid-South farmers have reported higher populations of barnyardgrass species, especially junglerice, in soybeans and other crops.
The Tennessee Promotion Board invests soy checkoff dollars to support University of Tennessee Extension research on herbicide-resistant weeds, and the national soy checkoff also provides funding. Dr. Larry Steckel, row crop weed management professor with the Department of Plant Sciences, University of Tennessee Institute of Agriculture, oversees this research and education.
“We conducted a series of studies to try to determine why junglerice and related barnyardgrass species have become major weeds,” Steckel says. “Reports of problems with these grasses came primarily from fields using dicamba-resistant seed technology.”
His team’s research started with a survey of weed escapes in dicamba-tolerant crops from more than 100 fields in 2018 and 2019. Grasses actually escaped more often than Palmer amaranth, which was identified in half the fields surveyed. Junglerice was the most common weed present, identified in more than 70 percent of the fields. Barnyardgrass escaped in 40 percent of the fields. Other noted grass escapes included fall panicum and goosegrass.
“Following that survey, greenhouse and additional research found herbicide antagonism is the main reason herbicide applications missed grasses,” Steckel explains. “While glyphosate resistance had developed in about 13 percent of the escaped junglerice, we learned that dicamba actually limits the translocation of glyphosate in these grasses.”
While glyphosate and dicamba tank mixes provide needed broadleaf weed control in these fields, he says other factors of dicamba applications add to the poor grass control.
“Dicamba needs to be applied with nozzles and drift reduction agents that create large droplets,” he says. “Those droplets roll off grasses relatively easily, further reducing control.”
According to Steckel, his team’s research found that sequential applications of dicamba and glyphosate provide much more effective junglerice control than tank mixes. He advises starting with a strong pre-emergence herbicide program that includes grass control. For post-emergence weed control, he recommends applying glyphosate seven to 10 days after dicamba for much better grass control in soybeans and other dicamba-tolerant crops.
He also notes that cover crops can play a role in grass weed control, but that more research is needed in this area.
Video: Doctorate student Clay Perkins summarizes the research on herbicides resistance and antagonism in junglerice.
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.