Herbicides: What to Mix and What Not to Mix

Tacy Callies Herbicides, Tip of the Week

Add water to the herbicide sprayer tank before adding the products for tank-mixing.
(Photo by Robert Riefer, UF/IFAS)

By Ramdas Kanissery

Tank-mixing different herbicides with multiple selectivity and modes of action will broaden the range of weeds controlled in a single application. Although it is an effective and popular strategy, when components (herbicide products and spray additives) in the tank mixes are not compatible with each other, the resulting combination can produce unintended and unsuccessful results.

Every so often, herbicides don’t bring out the best in each other when combined in a single application. Herbicide “antagonism” is a phenomenon wherein different herbicides in a mixture result in reduced weed control performance compared to the individual herbicides applied alone. Although such effects depend on the targeted weed’s type and growth stage, spray mixes containing antagonistic products are generally less potent and ineffective for weed management.

A common example of this type of interaction is when one herbicide product interferes with the weed uptake and movement of another product. For instance, when a contact herbicide such as glufosinate or paraquat is mixed with a systemic herbicide like glyphosate, the contact herbicide quickly burns the weed foliage. It potentially restricts the weed’s ability to absorb an effective dose of the systemic herbicide, glyphosate, thereby reducing its efficacy.

Spray additives, for example, surfactants, are materials used to improve herbicidal efficacy or enhance its applicability and are often one of the crucial components of a tank-mix. Unfavorable interactions can occur between the herbicide product and the incompatible surfactants in the spray tank. For instance, paraquat will react with products containing anionic surfactants and result in precipitation or ‘curdling’ and clog the screens and spray nozzles. Hence, only non-ionic surfactants or crop oil concentrate must be mixed while using paraquat in the tank-mix.

A clogged sprayer filter screen is the result of incompatibility issues between the tank-mix components. (Photo by Robert Riefer, UF/IFAS)

It is not just a matter of which products can be safely and effectively tank mixed, but also what order different formulations should be added to the tank. Mixing products in the correct sequence can help eliminate many challenges associated with tank-mixing. To learn about the correct mixing order of herbicides, see EDIS publication AE246 and a previous Citrus Industry article on this topic.

Several apps and websites are available to assist growers with the proper tank-mixing order of crop protection products, including herbicides. Precision Laboratories is an online app with several agrochemical products in its database, including herbicides and other spray additives. When the various products planned in the tank-mix are selected, an output of the correct and compatible mixing order will be provided by this app.

Check the herbicide label:
Pay attention to the label for a recommendation on effective tank-mix partners (e.g., compatibility with other herbicides, suggested surfactants, etc.) and specific mixing restrictions.

Do a jar test: If the label does not provide any tank-mixing recommendations or the planned mix is not well known, consider doing a jar test before proceeding with the tank-mixing. Check out this Citrus Industry article for more information on conducting a jar test.

Use the right water volume: Mixing problems are more likely to happen when products are added to insufficient water volumes in the spray tank.

The simpler, the better: Plan for a simple tank mix. The more products added to the tank, the more chance for incompatibility issues in the tank.

By Ramdas Kanissery is a an assistant professor at the University of Florida Institute of Food and Agricultural Sciences Southwest Florida Research and Education Center in Immokalee.