Continued reliance on the same mode of action for weed management for years has led to the development of herbicide resistant weeds throughout the US. Pigweed is one of the most difficult weeds to control because it has developed resistance for multiple modes of actions of herbicides including 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 9 states in the south including Alabama. The Alabama Cooperative Extension and USDA-ARS confirmed glyphosate-resistant Palmer amaranth in east-central Alabama in 2008.
Not only is pigweed a strong competitor against crops, it is also a prolific producer; a single pigweed plant can produce up to 100,000 seeds in a crop setting. The seeds are small (1-2mm) and can be easily transported from field to field on farm equipment even with thorough cleaning. Large pigweed can also interfere with harvest by clogging up equipment and increasing harvest time, costing producers time and money.
Clean fields at planting, A good residual program, and using multiple modes of actions are critical for optimal pigweed control.
Pigweed resistance has been not surveyed in Alabama since 2008. To test pigweed herbicide resistance, mature pigweed seed heads were collected from 58 cotton, peanut, and soybean fields in 2015-16 on multiple female plants (all populations collected are Palmer amaranth except for two from Baldwin are spiny amaranth).
The seeds from each field location were planted in a greenhouse in 20 pots with a 50/50 sand and potting soil mix. Once pigweeds reached a height of 1-2 inches, they were treated with commonly used POST herbicides at 2X or 4X of the highest label rate with 0.25% v/v of NIS.
The plants were sprayed in a spray chamber at 20 GPA. At 14-18 days after treatment, plant counts were done to determine mortality rates of each pigweed population at each herbicide rate. For instance, if 10 pigweeds germinated in one pot and only 5 were totally dead, the mortality rate is 50% even though the other 5 plants did show some injury symptoms.
The table in the PDF below shows the mortality rates of pigweed for each herbicide rate at each location, and the overall mortality rate of all the fields collected. Only one location’s pigweed (Location 2) in 2016 was not resistant to glyphosate while all 57 other locations showed glyphosate resistance at different levels.
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 report 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 at 2X and 4X rates. One population (location 11) showed PPO resistance (Picture 4).
Overall, Cobra 32 oz/A and Liberty at 64 oz/A (Picture 5) provided the best results with 100% and 92% respectively in 2015 as well as 95% for both treatments in 2016. Glyphosate and ALS-inhibitors are mostly ineffective on pigweeds at 2X and 4X over the label rates.
For further details about the results of this study, please download the document Pigweed Resistance.
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, Classis, Pursuit and Strongarm in peanut due to ALS-inhibitor resistance. Apply Ultra blazer, Cobra, Gramoxone, 2,4-DB and Storm to control small pigweeds instead.
- Use Valor, Reflex, Direx, Cotoran, Caparol, Prowl H2O and other yellow herbicides to provide residual control of pigweed in cotton. Apply Valor, Prowl H2O, Sonalan, Warrant, Zidua 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.
- Dual Magnum provide good residual pigweed control but using it on sandy soil could easily cause cotton seedling injury if rainfall occurs after planting and during seedling emergence. Past research data has shown peanut tolerance to Dual Magnum applied PRE, but several Alabama peanut farmers have reported peanut injury and J-rooting after applying Valor and Dual Magnum behind the planter with heavy rainfalls occurred during seed germination. More research will be conducted in future to evaluate peanut variety tolerance to Valor, Dual Magnum and other soil herbicides under simulation of heavy rainfall during seed germination and cracking.