By Larry Steckel
Regrowth of Palmer Amaranth after 56 ounces of FlexStar GT
We have lost several soybean fields to glyphosate-resistant (GR) Palmer amaranth over the last several days. These fields had been sprayed when the Palmer amaranth averaged about 2 inches to 4 inches in height with Flexstar GT or Prefix plus glyphosate. I have seen similar results with our research at Jackson on GR Palmer amaranth with many of the fomesafen (Flexstar GT, Prefix, Reflex, Flexstar, Dawn, Rhythm, etc.) based products. I have been surprised that the fomesafen based products at the pint rate have been so inconsistent controlling Palmer amaranth at this height this spring. In past years we typically see 90% plus control on Palmer at this height. The three week period of hot and dry conditions may have hardened off the Palmer to the extent that the fomesafen products, particularly at the pint Flexstar equivalent rate, are not performing well.
The growers with these problems are, in most cases, wanting to spray another PPO herbicide (Flexstar, Cobra, Ultra Blazer) on these Palmer amaranth that are now re-growing from the first Flexstar treatment. Over the last several years, I have applied numerous PPO herbicide tankmix "rescue" treatments on Palmer amaranth that had grown back after a Flexstar product had been applied. What I found is there is no rescue treatment that will provide control. If you do not control Palmer amaranth with the first PPO herbicide application you essentially live with the Palmer in the field until first frost. The best option in many cases is to spray paraquat (Gramoxone Inteon) over the whole field and replant. If you elect to replant, consider using a Liberty Link soybean variety on these heavily infested fields as Ignite has been controlling 4 inches to 6 inches Palmer and, with a sequential application, even larger Palmer. Of course you can replant to a Roundup Ready variety, but be sure to apply the next PPO herbicide before the next Palmer flush is 2 inches tall. Regardless of the soybean that is replanted, please keep in mind that all the fomesafen used on the first soybean planting needs to be figured into the total used for the year (maximum 1.6 pints per acre of Flexstar or 0.375 pounds active ingredient per acre per year.)
There is no substitute for eyes on the soybean field this spring. Fields need to be scouted to determine if pre applied herbicides become activated, and once active, when the herbicide breaks. Then if at all possible, be sure to apply the fomesafen based product on the Palmer before it is 2 inches tall. It is better to error spraying too early than too late. Finally, in light of this inconsistent control at a pint I would suggest that on Palmer 3 inches to 5 inches tall use no less than 1.6 pints of Flexstar or fomesafen quivelent product. Remember, you really have only one shot to control Palmer post emergence.
Steckel is a University of Tennessee Extension weed scientist.