Evaluations of Palmer amaranth Susceptibility to Dicamba and Glufosinate.

Dicamba and glufosinate have been effective herbicides used for postemergence weed control in XtendFlex^® technology. The occurrence of Palmer amaranth (Amaranthus palmeri S. Wats.) escapes in fields that rely on these tools demonstrate the importance of monitoring the emergence and spread of herbicide resistance. For that purpose, Palmer amaranth accessions were collected from fields in regions where auxins (dicamba or 2,4-D) and glufosinate are heavily relied upon for weed control. Collections occurred in the fall of 2018, 2019, and 2020 from fields located in Mississippi, Missouri, Nebraska, Illinois, Louisiana, Tennessee, and Arkansas. Every state did not sample for Palmer amaranth each year. Herbicide treatments equivalent to a 0.5X or 1X rate of dicamba at 0.25 and 0.5 lb ae/ac or glufosinate at 0.26 and 0.53 lb ai/ac were applied to greenhouse-grown plants at the 5- to 6-leaf stage. Some accessions could not be thoroughly evaluated due to limited seed availability or lack of seed viability. For all samples reported, at least 100 plants were assessed. Thus far, the screening has evaluated 193 and 206 accessions to dicamba and 210 and 215 accessions to glufosinate at a 0.5 and 1X rate of each herbicide, respectively. Evaluations of the accessions showed a high variability of plant responses to the herbicide treatments. Dicamba and glufosinate applied at a 0.5X rate resulted in less than 60% mortality to 22 and 31 accessions, respectively. Nevertheless, a full labeled (1X) rate of dicamba and glufosinate provide at least 80% mortality of 193 and 194 Palmer amaranth accessions, respectively. Future experiments will be conducted to determine the mechanism for reduced susceptibility to these herbicides as well as alternative herbicides that can be used to control these troublesome accessions. [ABSTRACT FROM AUTHOR]