An Update on Glyphosate-Resistant Weeds in Tennessee

Three glyphosate-resistant (GR) weeds are becoming an increasing problem for cotton growers in Tennessee.  GR horseweed is now an issue on all cotton production acres in Tennessee.  This weed initially caused a reduction in conservation tillage practices but now growers have adopted burndown strategies to control this weed.  Giant ragweed is becoming a more prevalent problem in cotton fields in Tennessee.  Historically, giant ragweed was mostly confined to field edges but in recent years has progressively spread into cotton fields.  In just the last two years growers reported that glyphosate was not providing the same level of giant ragweed control as before.  Field as well as greenhouse research has shown that giant ragweed in our cotton fields has more tolerance to glyphosate than giant ragweed in fence rows.  We have been extensively investigating glyphosate-resistant Palmer amaranth reports across much of West Tennessee.  We have identified GR Palmer amaranth in 10 counties.  Most of these cases are spots of GR Palmer ranging from the size of a car to several acres to just scattered plants.  A couple fields in Shelby and Tipton counties have GR Palmer infestation similar to what has been reported in Georgia.  On-going greenhouse screens have confirmed resistance in a number of these fields.