Researchers with the University of Tennessee Institute of Agriculture have confirmed the discovery of glyphosate-resistant johnsongrass in Lauderdale and Fayette counties in west Tennessee. Studies showed that johnsongrass from fields in these counties survived glyphosate at 2-3 times the labeled rate. Greenhouse screenings were conducted at the West Tennessee AgResearch and Education Center.

Glyphosate-resistant johnsongrass has been confirmed in Arkansas and Mississippi since 2008. While there have been reports of glyphosate misses on johnsongrass in Tennessee, this is the first time researchers have been unable to control suspected populations in a greenhouse setting. Johnsongrass is the sixth confirmed glyphosate-resistant weed species in Tennessee.

Glyphosate is an herbicide that is widely used to control johnsongrass in Tennessee’s major agronomic crops: corn, cotton and soybeans. Research has shown that uncontrolled johnsongrass can significantly reduce crop yields, costing farmers money. However, Larry Steckel, University of Tennessee Extension weed specialist, says there are some viable herbicide options for controlling this weed. He recommends clethodim for cotton and soybeans and nicosulfuron for corn. Another option is glufosinate; however, Steckel says it will require sequential applications.

“With each of these other herbicides, in contrast to glyphosate, applications will need to be made to smaller weeds to maximize control,” Steckel stresses. “Control will be maximized if applications to johnsongrass plants are less than 12 inches in height.”

Steckel recommends a fall herbicide application in fields where johnsongrass has been problematic. He warns that not controlling johnsongrass rhizomes in the fall could limit the crops a producer will be able to plant in the spring. Also, in the spring, incorporating pre-emergence herbicides, like Dual or Zidua-based products, in weed control systems will help producers manage johnsongrass seedlings, while ensuring the continued effectiveness of current post-emergence herbicide options, Steckel says.