Herbicide Resistance in Waterhemp and Palmer Amaranth

As harvest leads to preparing for the following year, brushing up on what it takes to keep weeds from putting a damper on your next crop is always a good idea. Waterhemp and Palmer amaranth are two significant pests for which you should be keeping an eye out.  

Why do these weeds keep popping up in the resistance discussion?

Both waterhemp and Palmer amaranth belong to a special group of the Amarathus genus, which are dioecious -- meaning male and female flowers grow on separate plants. For successful reproduction, pollen must transfer from the male plants to female plants. Because of this, genetic diversity within these two species tends to be greater than that of most agronomic weeds. This increases the potential for evolving and spreading herbicide resistance genes and other traits that improve survival.

Confirmed herbicide resistance

These two plant species have an amazing ability to adapt and develop resistance to herbicides. To date, both have confirmed resistance to eight herbicide groups: Group 2, 3, 4, 5, 9, 14, 15 and 27. 

Waterhemp Herbicide Resistance Map

Palmer Amaranth Herbicide Resistance Map

Here is a quick summary of these herbicide groups and some examples of products they include:

  • Group 2: Acetolactate Synthase (ALS) Inhibitors
    Includes products such as Accent® Q, components of Authority® Assist and Authority® First, Beacon®, Classic®, components of Clearpath®, components of Fierce®, FirstRate®, Londax®, Newpath®, Permit®, Pursuit®, Scepter®, Staple®, Synchrony®, and components of Valor® XLT
  • Group 3: Root Growth Inhibitors
    Includes products such as Prowl® H2O and Treflan®
  • Group 4: Plant Growth Regulators
    Includes products such as 2,4-D, Banvel®, Clarity®, components of Clearpath®, Engenia®, components of Enlist Duo®, Enlist One® and Facet®
  • Group 5: Photosystem II (PSII) inhibitors
    Includes products such as atrazine, components of Authority® MTZ, components of Bicep II Magnum®, components of Boundary®, Caparol®, components of Harness® Xtra and metribuzin
  • Group 9: EPSP Synthase Inhibitors
    Includes all glyphosate products
  • Group 14: PPO Inhibitors
    Includes products such as Aim®, components of the Authority® family of herbicides, Cobra®, Envive®, components of Fierce®, Flexstar®, components of Gangster®, Reflex®, Sharpan®, Spartan® and Valor®
  • Group 15: Shoot Growth Inhibitors
    Includes products such as Dual Magnum® (s-metolachlor), Harness®, Warrant®, Degree®, Surpass® (acetochlor), Zidua® (pyroxasulfone) and Outlook® (dimethenamid-p)
  • Group 27: Pigment Synthesis (HPPD) Inhibitors
    Includes products such as Balance Flexx®, Callisto®, components of Halex® GT, Laudis®, components of Lexar® EZ and components of Realm Q®

What to consider when developing a management plan

Although waterhemp and Palmer amaranth have shown resistance to multiple herbicide groups, it is important to remember that there are few, if any, cases that have resistance to all the listed herbicide groups in this article. These herbicide groups are still important options to control the weed pests that we have in agronomic crops in the U.S.

What is important is that you rotate your chemistries and crops to most effectively deal with these difficult to control species. As always, consult with your trusted FBN agronomist to design the best herbicide plan for your operation and talk to your FBN account executive about adjuvant options to maximize weed control.

Want to know more? We'll discuss crop-specific strategies for dealing with waterhemp and Palmer amaranth when growing corn, soybeans and cotton in future posts on Emergence.

Scout your fields with greater insights

Scouting your fields can give you a good idea of what pressures you're dealing with. You can up your scouting game utilizing precision maps and data specific to your farm and fields. Download our FBN Precision Mapping Guide to find out how.

FBN precision maps can help you evaluate your farm fields

Sources: 

Map Credit:

Heap, I. The International Survey of Herbicide Resistant Weeds. Online. Internet. Friday, October 18, 2019