With the prevalence of glyphosate-resistant waterhemp across most of our region, we have very few herbicide options for management in our soybeans.  The PPO-inhibiting (group 14) herbicides are what we have been relying on for the last several years in glyphosate-tolerant soybeans to obtain adequate control of this hard to manage pigweed species.  Soon, these options will be even more limited. 

Fomesafen-containing herbicides (Flexstar®, Flexstar GT®, Prefix®, etc.) are popular PPO-inhibiting options for managing waterhemp.  However, the use of these herbicides will need to be discontinued as the calendar turns over to July.  There is a need for caution when using these herbicides after July 1st, as fomesafen carryover into corn can occur the following season.  Fomesafen carryover is characterized by veinal chlorosis on corn leaves that is evident by the V3 growth stage (Image 1).The Flexstar label states a 10-month rotational restriction when planting corn the following season.  This means that an application occurring on July 1st of this year will not allow for corn planting until May 1st of next year.  

With fomesafen-containing herbicides slowly becoming a less viable option with each passing day, what options remain?  The most common solutions would be a product that contains lactofen (Cobra®).  Cobra® will be one of our main options for managing our resistant pigweeds as we move forward this season.  This will be more suitable late season as there is less concern about potential herbicide carryover.  

Some may not have a positive reaction to Cobra® applications due to soybean injury after application.  However, with the warmth that occurs with later season applications, the soybeans should recover rapidly.  Additionally, the use of crop oil concentrate (FS COC Supreme™) will be needed to obtain optimal control of waterhemp.  The addition of COC to the mix does not mean that water conditioners are not needed.  The water conditioners will still be required for optimum control if glyphosate is used.

Overall, be conscious about the risks when applying fomesafen late season.  Also, just because we may need to switch to a different PPO-inhibiting herbicide does not mean all of the same rules we have been following do not apply.  We must still spray our waterhemp when it is small, use the correct nozzles, and be aware of GPA to get the best results.  Good luck with the remaining postemergence applications!