Authors: Jay Ferrell (Associate Professor, Weed Science, University of Florida) and Mace Bauer (Columbia County Agriculture Extension Agent)

Many crop producers discovered compatibility issues last season when paraquat (Gramoxone SL and others) was added to the tank. While the issue was not “common,” it did happen often enough to create the necessity for awareness of the issues and potential solutions.

What happened?
The incompatibility observed was in the form of a “brown slime” which formed in the tank. This material coated the inside of tanks and caused screens and nozzles to clog. Removing this slime from the screens proved to be time consuming and created a great deal of aggravation.

When did it happen?
The compatibility issues were most common when paraquat was mixed with fomesafen (Reflex, Dawn, others), but other combinations were also problematic. After extensive testing, it is believed the main issue is with paraquat interacting with adjuvants. In particular, compatibility issues occurred when there was a low water volume in the spray tank with paraquat plus incompatible adjuvants dumped in together. Of course, the addition of fomesafen increased the likelihood of problems.

Which surfactants are incompatible?
Dozens of surfactants have been tested. Every major manufacturer has products which cause incompatibility and those that work well. Unfortunately, it is common for surfactant manufactures to tweak their blends each year. Therefore, one product which did not have issues last year may cause incompatibility this year. Consult with your ag chemical dealer to determine which products are working well, but it is recommended jar test is performed to ensure incompatibility is not an issue for the specific tank mix. Directions for the jar test are below.

When is incompatibility most likely?
Incompatibility occurs when concentrated paraquat and concentrated surfactants come in contact. Therefore, adding both products together at low water volumes (either in an injection tank or in a sprayer with only a few gallons of water) is when problems are most commonly seen. Adding fomesafen to this mix increases the likelihood of incompatibility. These issues are always worse when the sprayer is set to apply 10 gallons per acre (GPA) or less. The key is having enough water to dilute the surfactant and the paraquat. Increasing spray volume to 15 GPA will dilute the paraquat and surfactant to a greater extent, reducing the likelihood of incompatibility.

When is incompatibility most commonly avoided?
Avoid many of these problems by following a simple mixing order. 1. Fill the spray tank half full of water and add the surfactant while agitating. This gives the surfactant time to dilute in a greater water volume. 2. Add other herbicides, starting with dry materials and followed by liquids. 3. Add the paraquat.

Will this mixing order solve all the problems?
No, but it will definitely help. Some surfactants will react with paraquat regardless of when they are added. Others will only be problematic if added after the paraquat. So adding the surfactants first and using plenty of water will often avoid problems. The jar test is critical to ensure the choice of products and mixing order will not lead to problems.

Is a compatibility agent a must if spraying paraquat?
No, some surfactants never interact with paraquat. However, some do and can be reversed with a good compatibility agent. Choose to use one of these compatible products. It is essential to add the compatibility agent BEFORE adding the surfactant and BEFORE any herbicides.

Points to remember

1. The more water mixed in, the less likely incompatibility will occur.
2. Add the surfactants to the tank first and allow them to mix thoroughly in the water.
3. Add the paraquat last.
4. If incompatibility is a concern, perform a jar test to confirm no problems will arise. For an excellent step-by-step guide on the jar test, see: UGA Gramoxone Compatibility Test
5. If incompatibility occurs in the spray tank, commercial degreasers, such as those available at an auto parts store, have commonly worked to clean tanks. NEVER enter a spray tank and use proper protective equipment at all times when cleaning hoses and nozzles.

Download and print the Syngenta fact sheet for Tank Mixing Instructions for Paraquat to share with employees: GMX Tankmix guidelines.

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