Palmer Amaranth (Pigweed) Resistance in Alabama

A G R I C U LT U R E

► 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 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

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Figure 1. Henry County glyphosate-resistant pigweed.

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

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

* Pigweed mentioned in this document refers to Palmer amaranth.

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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 ResultsThe 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 a 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

2015 Pigweed Mortality Rates

Location County Roundup 64 oz/A

Roundup 128 oz/A

Staple LX 8 oz/A

Staple LX 16 oz/A

Cadre 8 oz/A

Cadre 16 oz/A

Cobra 32 oz/A

Liberty 64 oz/A

1 Autauga 0% 25% 0% 0% 0% 0% 100% 100%2 Autauga 14% 40% 0% 15% 0% 0% 100% 100%3 Chilton 0% 0% 2% 0% 0% 0% 100% 100%4 Dallas 0% 20% 0% 21% 0% 0% 100% 100%5 Elmore 0% 40% 17% 0% 25% 17% 100% 100%6 Henry 0% 14% 0% 9% 0% 10% 100% 100%7 Henry 0% 33% 0% 10% 0% 21% 100% 100%8 Henry 0% 27% 0% 10% 20% 18% 100% 100%9 Henry 0% 15% 7% 5% 5% 10% 100% 100%10 Henry 5% 17% 0% 7% 0% 5% 100% 100%11 Limestone 4% 25% 0% 0% 6% 7% 3% 100%12 Madison 17% 7% 0% 15% 37% 50% 100% 100%13 Russell 7% 47% 7% 18% 0% 17% 100% 100%14 Shelby 0% 5% 5% 5% 6% 8% 100% 100%15 Shelby 15% 8% 0% 23% 0% 13% 100% 100%16 Talladega 0% 44% 0% 13% 40% 17% 100% 100%17 Talladega 13% 20% 0% 12% 0% 13% 100% 100%18 Talladega 5% 8% 7% 12% 6% 3% 50% 100%

Average Mortality Rate 4% 22% 3% 10% 8% 12% 100% 92%

(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 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.

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

Location CountyCobra 32 oz/A

Cobra64 oz/A

Goal48 oz/A

Goal 96 oz/A

Reflex96 oz/A

11 Limestone 3% 7% 50% 90% 27%18 Talladega 50% 100% 100% 100% 100%

2016 Pigweed Mortality Rates

Location CountyRoundup 64 oz/A

Envoke 0.3 oz/A

Staple LX 8 oz/A

Cadre 8 oz/A

Cobra 32 oz/A

Liberty 64 oz/A

1 Baldwin 60% 25% 45% 11% 100% 100%2 Baldwin 100% 67% 0% 100% 100% 100%3 Baldwin 40% 22% 13% 12% 95% 89%4 Baldwin 31% 38% 25% 25% 100% 100%5 Baldwin 0% 100% 14% 53% 89% 100%6 Baldwin 52% 29% 31% 23% 83% 100%7 Baldwin 38% 0% 25% 23% 100% 86%8 Baldwin 19% 32% 35% 31% 100% 100%9 Baldwin 33% 25% 17% 38% 100% 97%10 Baldwin 20% 42% 7% 22% 82% 100%11 Barbour 46% 40% 27% 17% 92% 100%12 Elmore 38% 39% 45% 12% 93% 81%13 Escambia 44% 27% 34% 23% 100% 100%14 Escambia 58% 33% 8% 28% 100% 100%15 Geneva 40% 13% 47% 30% 97% 100%16 Geneva 36% 8% 14% 24% 50% 85%17 Geneva 41% 40% 24% 22% 100% 100%18 Geneva 28% 45% 9% 22% 100% 100%19 Geneva 50% 26% 23% 28% 80% 100%20 Henry 39% 11% 30% 28% 100% 100%21 Henry 50% 0% 32% 21% 100% 87%22 Henry 77% 8% 7% 27% 82% 100%23 Henry 55% 29% 50% 25% 75% 83%24 Henry 36% 15% 29% 15% 100% 100%25 Henry 23% 13% 21% 22% 100% 100%26 Henry 18% 15% 8% 16% 100% 100%27 Henry 29% 13% 11% 17% 100% 100%28 Henry 44% 16% 16% 25% 100% 100%29 Henry 18% 14% 10% 22% 95% 87%30 Henry 28% 29% 36% 29% 100% 90%31 Henry 28% 6% 14% 28% 100% 86%32 Henry 20% 6% 12% 25% 100% 100%33 Henry 18% 15% 16% 18% 100% 86%34 Houston 25% 21% 35% 23% 100% 90%35 Houston 62% 28% 24% 18% 100% 92%

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2016 Pigweed Mortality Rates36 Houston 75% 20% 29% 21% 100% 96%37 Houston 13% 16% 13% 18% 91% 100%38 Limestone 38% 9% 36% 6% 100% 75%39 Macon 29% 19% 16% 14% 100% 97%

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.

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 inch) 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 t all. Research continues and results will soon be available to the public.

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

Steve Li, Extension Specialist, Assistant Professor, Crop, Soil, and Environmental Sciences, Auburn University

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.

For more information, contact your county Extension office. Visit www.aces.edu/directory.

The Alabama Cooperative Extension System (Alabama A&M University and Auburn University) is an equal opportunity educator and employer. Everyone is welcome!

New August 2017, ANR-2386

© 2017 by the Alabama Cooperative Extension System. All rights reserved.

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