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Palmer Amaranth--Pigweed

Continued reliance on the same mode of action for weed management for years has led to the development of herbicide-resistant weeds throughout the United States.

Palmer amaranth (also known as Palmer pigweed)* is one of the most difficult weeds to control because it has developed resistance for multiple herbicide modes of actions. These include ALS inhibitors (Group 2), Photosystem II inhibitors (Group 5), EPSP synthase inhibitor (Group 9), PPO inhibitors (Group 14), and HPPD inhibitors (Group 27). By 2009, the Southern Weed Science Society annual weed survey listed pigweed as the most troublesome weed in cotton for nine states in the South, including Alabama. In 2008, the Alabama Cooperative Extension System and the United States Department of Agriculture Agricultural Research Service confirmed glyphosate-resistant pigweed (figs. 1 and 2) in southeast Alabama. Not only is pigweed a strong competitor against crops but it is also a prolific seed producer. A single female pigweed plant can produce up to 460,000 seeds when competing with dryland cotton, according to a University of Georgia study. The seeds are small (1 to 2 mm) and can be easily transported from field to field on farm equipment even after thorough cleaning. Large pigweed can also interfere with harvest by clogging equipment and increasing harvest time, costing producers time and money. Clean fields at planting, a good residual program, and use of multiple modes of actions are critical for optimal pigweed control.

* Pigweed throughout this article refers to Palmer amaranth.

Palmer Amaranth Pigweed

Figure 1. Henry County glyphosate-resistant pigweed.

Palmer Amaranth Pigweed

Figure 2. Large pigweed in Baldwin County survived two passes of disking before planting and growing back in late April.

Pigweed resistance has not been surveyed in Alabama since 2008. To test pigweed herbicide resistance, mature seed heads were collected from 58 cotton, peanut, and soybean fields in 2015 and 2016 on multiple female plants (all populations collected are Palmer amaranth except two from Baldwin that are spiny amaranth). The seeds from each field location were planted in a greenhouse in 20 pots with a 50/50 sand and organic potting soil mix. Once pigweeds reached a height of 1 to 2 inches, they were treated with commonly used POST herbicides at 2X or 4X the highest label rate with 0.25 percent v/v of NIS. The plants were sprayed in a spray chamber at 20 GPA. At 14 to 18 days after treatment, plant counts were done to determine mortality rates of each pigweed population at each herbicide rate. For instance, if ten pigweeds germinated in one pot and only five were totally dead, the mortality rate is 50 percent even though the other five plants did show some injury symptoms.

Survey Results

The following tables show the mortality rates of pigweed for each herbicide rate at each location and the average mortality rate of all the fields collected. Only one location’s pigweed (location 2) in 2016 was not resistant to glyphosate while pigweeds from all 57 other locations showed glyphosate resistance at different levels (fig. 3). Pigweeds at location 5 were still sensitive to Envoke, which is an ALS inhibitor (sulfonylurea family). However, this population has evolved resistance to Staple and Cadre, which are also ALS inhibitors (pyrimidinylthio-benzoate and imidazolinone family). This is consistent with previous reports that pigweed showing resistance to one ALS-inhibitor family may be controlled with another family of ALS-inhibitor herbicides. The 2015 pigweeds collected from two locations showed signs of PPO resistance, so these populations were planted and tested again with more PPO inhibitor herbicides applied at 2X and 4X rates. One population (location 11) showed PPO resistance (fig. 4). Overall, Cobra 32 oz/A and Liberty at 64 oz/A (fig. 5) provided the best results with 100 and 92 percent, respectively in 2015 as well as 95 percent for both treatments in 2016. Glyphosate and ALS-inhibitors are mostly ineffective on pigweeds at 2X and 4X over the label rates.

Figure 3. Glyphosate-resistant pigweed populations at 14 days after a treatment of Roundup Powermax 64 oz/A.

Figure 4. Possible PPO-resistant population collected from a Limestone County field (3 cups on the left) at 14 days after a treatment of Reflex 96 oz/A. The right two rows of pigweed came from a sensitive population and have been completely killed by the same Reflex application.

Figure 4. Possible PPO-resistant population collected from a Limestone County field (3 cups on the left) at 14 days after a treatment of Reflex 96 oz/A. The right two rows of pigweed came from a sensitive population and have been completely killed by the same Reflex application.

Figure 5. A population collected from a Baldwin County field in 2016 at 14 days after being treated with Liberty 64 oz/A

Figure 5. A population collected from a Baldwin County field in 2016 at 14 days after being treated with Liberty 64 oz/A


Recommendations for Growers with Resistant Pigweed Problems

  • Consider using Liberty + Dual Magnum/Warrant/Outlook in POST applications instead of Roundup + Staple/Envoke in cotton.
  • Avoid Cadre, Classic, Pursuit, and Strongarm in peanut due to ALS-inhibitor resistance. Apply Ultra blazer, Cobra, Gramoxone, 2,4-DB, and Storm to control small pigweeds (less than 6 inches) instead.
  • Residual herbicides are absolutely indispensable for resistant pigweed management! Use Valor, Reflex, Direx, Cotoran, Caparol, Prowl H2O, and other dinitroaniline herbicides to provide residual control of pigweed in cotton. Apply Valor, Prowl H2O, Sonalan, Warrant, Zidua, Outlook, or Dual Magnum in peanut as residual pigweed control options.
  • In areas with heavy pigweed infestation, layby or POST-directed application is absolutely needed to prevent late-season pigweed growth. Use Gramoxone in combination with residual herbicides such as Direx, Valor, Dual Magnum, Cotoran, MSMA, Zidua, etc.
  • PPO resistance (group 14) is a real threat! Do not use PPO herbicides and Liberty more than twice a year. Always include other modes of action with PPO herbicides and Liberty to slow down weed resistance evolution.
  • Dicamba and 2,4-D are new tools to control resistant pigweeds when they are smaller than 5 inches. But please use extreme caution when using them due to drift and off-target injury concern.
  • Alabama field trials in 2017 have shown that Xtendimax + Roundup Powermax or Enlist Duo followed by Liberty + Dual Magnum four days later provided significant growth stunting and suppression of large pigweeds at 12 to 18 inches tall. Research continues and results will soon be available to the public.


2015 Pigweed Mortality Rates

LocationCountyRoundup 64 0z/ARoundup 128 oz/AStaple LX 8 oz/AStaple LX 16 oz/ACadre 8 oz/ACadre 16 oz/ACobra 32 oz/ALiberty 64 oz/A
AVERAGE MORTALITY RATE4%22%3%10%8%12%100%92%


2015 Pigweed Mortality Rates for PPO Suspected Resistant Populations (2nd test)

LocationCountyCobra 32 oz/ACobra 64 oz/AGoal 48 oz/AGoal 96 oz/AReflex 96 oz/A


2016 Pigweed Mortality Rates

LocationCountyRoundup 64 oz/AEnvoke 0.3 oz/AStaple LX 8 oz/ACadre 8 oz/ACobra 32 oz/ALiberty 64 oz/A
Average Mortality Rate 38%24%23%25%95%95%

*Mortality rates were determined by counting the total number of pigweed seedlings killed by herbicide treatments divided by total number of pigweed germinated across all pots for each location.

Download a copy of ANR-2386 Palmer Amaranth (Pigweed) Resistance in Alabama. 

Use pesticides only according to the directions on the label. Follow all directions, precautions, and restrictions that are listed. Do not use pesticides on plants that are not listed on the label.

The pesticide rates in this publication are recommended only if the product(s) is registered with the Environmental Protection Agency and the Alabama Department of Agriculture and Industries. If a registration is changed or canceled, the rate listed here is no longer recommended. Read and follow all directions on the label.

Trade names are used only to give specific information. The Alabama Cooperative Extension System does not endorse or guarantee any product and does not recommend one product instead of another that might be similar.


Featured image by Howard F. Schwartz, Colorado State University, Bugwood.org

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