Herbicides are grouped according to family (e.g., triazines) and target site of action or mechanism of action (e.g., acetolactate synthase, or ALS, inhibitors). Herbicides within a family have similar chemical structures and typically the same site or mechanism of action (MOA). Knowing the chemical family and MOA group to which an herbicide belongs and knowing which other herbicides have the same MOA are critical for creating a plan to prevent or delay development of herbicide resistance (see Summary of Herbicide Mechanism of Action).

To help ensure standardization, the EPA has requested that manufacturers include a pesticide’s MOA group number in a standard format on the label. Outside of the U.S., letters are used in place of group numbers (see Herbicide Resistance Action Committee website below). Herbicides with the same mechanisms of action are assigned the same group number. When a premix label displays the group number(s), the user can easily determine the mechanisms of action included in the premix.

If a label does not contain a MOA group number, you should find it in this table from the North Carolina Agricultural Chemicals Manual by referencing the herbicide brand or common name.

 

In addition to considering MOA group numbers in the selection of herbicides, review all resistance management recommendations printed on the herbicide label. This may include information on the best management practices for a particular product, target species of most concern and the maximum number of consecutive applications that should be made before rotating to products containing herbicides with different group numbers.