Can brand or generic herbicides be tank-mixed?

by Rodrigo Werle (UW-Madison Extension Cropping Systems Weed Scientist)
and Glenn Nice (UW-Madison Pesticide Applicator Training Program Manager)

With the onset of the spray season and concerns regarding waterhemp and other troublesome weed species, farmers and consultants are further diversifying and including herbicides with soil residual activity in their weed control programs. This past week we received several questions regarding herbicide tank mixtures. One common question was whether brand and/or generic s-metolachlor/metolachlor-, atrazine- and/or mesotrione-based products could be tank-mixed. The Federal Insecticide, Fungicide and Rodenticide Act (FIFRA) allows for the tank mixing of any pesticides or pesticides with fertilizer when NOT prohibited on the label. This means that, as long as it is not prohibited on the label, you can mix various registered labeled herbicides for weed control in a particular crop. However, there are a few things you should be aware of:

  • follow the proper order for tank mixing (see Purdue Extension Publication: “Avoid Tank Mixing Errors”).
  • some pesticides are not compatible (they can be chemically or physically incompatible). Thus, a jar test should be conducted prior to mixing and spraying to assure the products are indeed compatible. Remember, you are responsible if the mix “goes bad” in the tank.
  • follow the most restrictive tank mix instructions for the products involved.
  • keep in mind that use rates vary by product and formulation; don’t assume that rates are the same for all products with the same active ingredient.
  • many labels, for example the new dicamba products registered for use in dicamba-resistant soybeans (Xtend), have statements such as: use approved tank mix products as directed by the label or website. This statement prohibits all tank mixes except for the ones the manufacturer has approved and specifies in their labels or webpages.

While making your own pre-mix may reduce costs, it may increase the chances for error (e.g., the likelihood of accurately picking and measuring one product off of the shelf is higher than when multiple tank mix partners are involved) thus increasing the potential for crop injury, carryover concerns and/or lack of appropriate weed control. Commercially available pre-mix herbicides have been extensively studied by the chemical companies with the objective to enhance weed control while minimizing crop injury.

When developing a custom pre-mix or selecting a commercially available pre-mix herbicide, it’s important that farmers and crop consultants understand their target weed species and soil types, and have a solid crop rotation plan in mind. Knowing the weed species present in a particular field will allow for selecting effective active ingredient(s) to optimize weed control while minimizing the spray of unnecessary active ingredients. Understanding that soils (texture, pH and OM) vary across and within fields will allow for proper herbicide rate selection, optimizing weed control while minimizing chances for crop injury and/or carryover into subsequent crops.

For assistance with herbicide selection:

Always read, understand and follow the pesticide label.

Thanks to Dr. Jed Colquhoun (UW-Madison Professor and Extension Weed Scientist) for valuable feedback towards this article.