Managing Herbicide-Resistant Palmer Amaranth in Mississippi Soybean

Information in the Feb. 18 2016 edition of AgFax Weed Solutions confirms that a significant portion of the Palmer pigweed population in the Midsouth is resistant to PPO herbicides, and the magnitude of this resistance is likely to increase.  This adds another layer of resistance to herbicides in this weed, and further complicates management systems for its control.

PPO resistance means that herbicides such as Flexstar/Reflex, Sharpen, and Valor (click here for all herbicides in the PPO inhibitor group 14), which have been applied for pigweed control for several years, have likely been overused to the extent that Palmer has developed resistance to the herbicides in this MOA group.

In response to this additional resistance in Palmer pigweed, Mississippi specialists (Drs. Jason Bond, Trent Irby, and Dan Reynolds) have compiled a soybean weed control guide to reflect the potential for PPO-resistant pigweed. Click here for that guide.

According to Dr. Larry Steckel, UT Associate Professor and Row Crop Weed Specialist who is quoted in the above AgFax article, Liberty (and compatible glufosinate products) is the last herbicide that will control Palmer amaranth post-emergence in soybeans where PPO resistance is a factor. His recommendations for using Liberty herbicide in LibertyLink soybeans are provided.  Briefly, they are:

  • Coverage with Liberty or equivalent glufosinate products is critical, and applying in at least 15 gallons of water is recommended.

  • The time of day that glufosinate is applied directly influences pigweed control.

  • In soybeans, consider mixing a PPO herbicide with glufosinate to achieve more consistent control of Palmer pigweed.

  • Utilize cultural weed control practices such as cover crops.

  • Rotate soybeans with corn or grain sorghum so that other herbicide chemistry is used.


Be sure to read the entire article in the above AgFax Weed Solutions edition to learn the details of how to manage Palmer amaranth in the face of this new development.

Composed by Larry G. Heatherly, Feb. 2016, larryheatherly@bellsouth.net