We have visited fields where growers have found that Palmer amaranth that escaped an auxin herbicide (Engenia, XtendiMax, Enlist One) application also escaped follow-up glufosinate (Liberty) application (Pictures 1 and 2). Auxin herbicide-resistant Palmer amaranth was documented in Tennessee last year. As such, it was not a surprise to find dicamba or 2,4-D fail to control pigweed. Glufosinate-resistant Palmer amaranth was documented in Arkansas last year. We have not found glufosinate-resistant Palmer amaranth on the Tennessee side of the river. However, the reports from these fields has us concerned the glufosinate-resistant biotype has indeed crossed the river. Part of the reason for the concern is that we had research fairly close to these suspect fields that showed similar poor Palmer control with dicamba or 2,4-D followed by glufosinate.
Inconsistent control of Palmer amaranth with glufosinate is not that uncommon. It can be caused by a host of environmental reasons including applications applied near sundown, cool weather right around application or rain within 4 hours of spraying. Therefore, follow up greenhouse research will be needed this fall to examine if the failed Palmer control was due to the environment or to glufosinate-resistance.
In the meantime, the reported fields where both an auxin herbicide and glufosinate failed to control Palmer amaranth are all in cotton fields to date. A good approach in those fields now is to get the hoods out and apply paraquat with a residual and remove these escapes ASAP!