This is shaping up to be the “Year of the Pigweed”.  That is how one consultant described our situation which, unfortunately, for many fields is right on the mark. Frequent flooding and hard rains last week followed by more of the same this week make it impossible to spray Palmer amaranth as timely as needed. 

This lack of time is made even worse by the fact that fields continuously saturated for a string of days rapidly degrades most pre applied herbicides.  A good example is fomesafen.  In saturated field situations the residual fomsafen provides rapidly degraded to the point that it typically last just a few days.  Other herbicides vary on how long they last under saturated soil conditions but all of them are compromised to some degree.

Unfortunately, it will be several days at best before the soil dries enough before a sprayer can cross many of these fields.  All fields will have large Palmer amaranth before they can be sprayed. Fields that had a large seed bank of Palmer amaranth could easily be at the point of no return if we cannot get them sprayed quickly.

Lining up a plane for aerial application would be a good option. The PPO inhibiting herbicides (Flexstar, Cobra and Ultra Blazer) would be the go to products on Roundup Ready soybean and all have aerial labels.  Of these three herbicides fomesafen (Flexstar, Flexstar GT, Rhythm) products have had the greatest success controlling border line to large 3 to 4”pigweed. Utilized at its highest use rate plus 1% MSO, in our research, has shown to have the best chance to control large Palmer amaranth.  These products are followed closely by either Cobra or Ultra Blazer.  The Cobra or Ultra Blazer options should be used if a fomesafen product was already utilized on the field.  Repeated applications of fomesafen will likely cause injury to rotated grain sorghum or corn next year. Moreover, be aware that the plant back to corn or grain sorghum after a fomesafen application is 10 months.

In my mind, with ground or air, the best Palmer “rescue” option in Roundup Ready soybean would be the highest rate of Flexstar GT plus 1% MSO followed about 7 to 10 days later with either Ultra Blazer or Cobra at their highest rates.  In our research last year with these treatments we got about 65% control of pigweeds in that 6” range. This is far from great but it is much better than the typical strikeout we get with large Palmer with just one application of a PPO herbicide.

The picture is a little better in Liberty Link soybeans or glufosinate tolerant cotton as large Palmer can be burned back with an application of 29 oz/A of Liberty.  Be prepared to come back with another application of Liberty about 7 to 10 days after application to finish off the survivors. This type of sequential program typically provides >90% control of Palmer even in that 6 to 8” range.

In some cases depending upon conditions a 3^rd Liberty application may be needed. Season use limit of Liberty over cotton is 87 ozs/A (3 x 29ozs).  This recommendation of 3 applications is made with respect to LL cotton. Be aware that 3 applications of glufosinate over cotton with moderate tolerance has often resulted in yield loss in our research. Moreover, be cautious applying Liberty to already stressed cotton that is only moderately glufosinate tolerant.

In Roundup Ready cotton a hooded application of Gramoxone Inteon with crop oil plus a residual of your choice will work fairly well cleaning out the row middles.  The grower will have to live with the Palmer in the row or chop it out.