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Paraquat, an Older Herbicide, Making a Comeback in the Age of Resistant Weeds (07/23/20)

In the 1960’s, paraquat was registered as a broad-spectrum, non-residual, contact herbicide.

In the 1960’s, paraquat was registered as a broad-spectrum, non-residual, contact herbicide. It has herbicidal active against most plant species and it was therefore targeted for use in a variety of weed control programs. Over the years, as other pesticides came to market, paraquat use diminished, especially with the introduction of glyphosate and later glyphosate tolerant crops. 

Paraquat found a niche as a stubble treatment in wheat fallow areas, as a burn down preplant, and as desiccant of weeds and crop just prior to harvest. Today it is being used more widely on Prevent Plant acres, as a burn down especially prior to soybeans, and as a harvest aide, particularly in legume crops. NDSU is evaluating it as a directed spray with shields to control weeds in between sugarbeet rows. The cost per acre is relatively low and, in North Dakota alone, 18 different brands of paraquat are registered:  https://tinyurl.com/NDDA-Paraquat

Many farmers and applicators are at least familiar with the product. However, there are numerous others that have never used paraquat. Below are some critical points to consider, if you intend to use this herbicide:

It is a Restricted Use Pesticide because of its acute toxicity, so users will need to get certified and because of recent risk mitigation measures taken by EPA, direct supervision is NOT allowed for Private Applicators.

  • Users will need to also complete and record their participation in an additional, targeted, registrant sponsored, training on paraquat. The training is available on-line at no cost to the participant. Most people can complete the course in about an hour. You can access this training here: https://tinyurl.com/EPA-ParaquatTraining
  • Closed systems are required for most uses. This includes specialized adapters for 2.5 gallon jugs. You can see a video clip of the device which was developed by Syngenta for their Gramoxone 3.0 SL product here: https://youtu.be/duP0ThvnEAM
  • One of the major concerns EPA has raised concerning continued use is mitigating exposure to applicators and to people who are not familiar or trained regarding the acute toxicity of the chemical. Bare skin and eyes can easily be damaged by paraquat and it can pose significant injury if aerosolized drops are inhaled. Because paraquat is acutely toxic in small doses when ingested, it has been used in SE Asia as a suicide poison. In this country, fatalities have occurred when people have drank tiny quantities of paraquat because they mistakenly thought it was a soft drink in an illegally packaged beverage container.
  • Significant Personal Protective Equipment is required for applicators as well as for mixers and loaders. In addition to standard clothing, glove requirements are more intensive. Options include: chemical-resistant gloves made of barrier laminate, butyl rubber ≥ 14 mils, nitrile rubber ≥ 14 mils , neoprene rubber ≥ 14 mils, natural rubber ≥ 14 mils, polyethylene, polyvinyl chloride (PVC) ≥ 14 mils or Viton® ≥14 mils /glove requirements. Applicators will also need to use respiratory protection and safety glasses that include splash guards. Mixers and loaders must add a full face shield and a chemical resistant apron.
  • Finally, keep paraquat out of the hands of people that are not properly trained to use it. That means keeping your pesticide secure at all times! Unfortunately accidental ingestion of this product has killed people and resulted in more regulations for even responsible users. Future high profile incidents can led to more regulations and or a ban on use.

 

The good news about paraquat it is an economical herbicide that works very well when used as intended. Next are some suggestions to avoid pitfalls:

 

  • The proliferation of many generic formulations means that there are both 3 pounds per gallon of active ingredients (a.i.) and 2.0 pounds per gallon a.i. being sold. This may include both formulations being sold by the same dealer. Carefully read the ingredient statements on the label to avoid wasting product or not getting the control you were hoping for.
  • Don’t spare the water when applying paraquat. As with most contact herbicides, coverage is critical. Paraquat is no exception. 5 gallons minimum by air and 10 gallons minimum by ground are required, but experienced users find better results near 10 for air and 20 for ground. 
  • Use a quality, non-ionic surfactant to further disperse the chemical on to the leaf to maximize coverage.
  • DO NOT use nozzles that will generate relatively large spray drops and poor coverage. This would include, but not be limited to, low drift nozzles designed to apply dicamba over the top in soybeans. Generally, medium droplet sizes are preferred and can be obtained by using extended range flat flan type nozzles. You can read more about nozzle selection in NDSU’s Selecting Spray Nozzles to Reduce Particle Drift (AE1246, July 2017). The link to this publication can be found here: https://tinyurl.com/NDSU-Nozzle
  • Unfortunately, using nozzles that produce medium sized spray drops means greater potential for particle drift. Virtually all plant species are sensitive to paraquat injury. Paraquat is not systemic, but it will cause significant and easily identifiable damage to exposed foliage.
  • When used in a pre-harvest situation, make certain paraquat is registered for the crop. Most notably, the herbicide is NOT registered for use in small grains. It is widely registered in many other crops. (Note, there are a variety of labeled rates for different crops, so do not overlook this!) To minimize the potential for illegal residues showing up, use the right rate, be mindful of applying sooner than the label directs, and also ensure that the pre-harvest interval is respected before under-cutting, swathing, or combining the crop.
  • Finally, read the label and consult the NDSU Weed Control Guide for best management practices. https://tinyurl.com/NDSU-2020-WeedGuide

 

Andrew A. Thostenson

Pesticide Program Specialist

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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