Poor performance of dicamba on Palmer amaranth was again the theme of the last ten days. There have been numerous reports typically with attached pictures of large patches of Palmer amaranth recovering from dicamba application/s (Picture 1).
Follow-up Liberty applications have been applied to many of these pigweed escapes in the last couple of weeks. I visited a few of these fields recently. Control 12 to 16 days after the Liberty application on the dicamba pigweed escapes ranged from 70 to 90% (Picture 2). The size of the pigweed when the dicamba was applied and how close the follow-up Liberty application occurred appears to be the determining factors on the level of control.
The good news is that timely dicamba application to 2” Palmer amaranth followed by a Liberty application no later than 10 days later appears to provide control in the range of 85 to 90% (Escapes in Picture 2). In most cases that is typically good enough to mitigate serous yield loss this year. However, those populations will likely go to seed and be the population next year that will need to be managed.
Fields visited with late dicamba applications are another matter as one can expect little control this year and most probably yield loss (Picture 3).
There have been a few reports of Palmer amaranth escaping Enlist as well. This is not a surprise as our research to date has shown that Palmer amaranth that has evolved resistance to dicamba will often be resistant to 2,4-D as well. Fortunately, Enlist One can be mixed with Liberty an in our research that mixture has provided robust control of Palmer amaranth that has shown resistance to the auxin herbicides.
Some have indicated they plan to prioritize deferred maintenance on their hooded sprayers and use them to help control escaped pigweeds. Following are the better hooded or post direct mixtures for pigweed control.
Hooded or Post-Direct Application
Liberty 32 ozs + Direx 32 oz/A or Caparol 32 oz/A or Valor 1.5 ozs/A or Zidua 3.2 ozs or Dual Magnum 16 ozs – Cotton must be at least 6” tall.
Advantages: It will control horseweed up to 10” in height. Good control of many broadleaf weeds including Palmer amaranth and morningglory species. Liberty has a lot of surfactant already present so additional surfactant is not needed. Most cotton now has good Liberty tolerance and sloppy post direct is not as damaging when tankmixed with Zidua or Dual Magnum.
Considerations: In order to protect from resistance developing, consider using a paraquat product if Liberty (glufosinate) has already been applied over the top.
Hooded only Applications
Paraquat (Gramoxone Inteon 32 oz/A) + Direx 16 oz/A or Caparol 32 oz/A or Valor 1.5 ozs/A- Cotton must be at least 15” tall and have a woody stem.
Advantages: This is the most consistent mixture to control large Palmer (>12”)
Considerations: Needs to be applied carefully! Use 0.25% COC for best results on large Palmer (>6”). As always with paraquat, be mindful of mix order and water pH.