Triazine-resistant biotypes or strains of common lambsquarters, smooth pigweed, common ragweed, and common groundsel have been confirmed in New York State. Populations of these weeds were originally controlled with one or more of the triazine herbicides ( (*atrazine and *†Princep) at normal use rates; however, shifts to weed populations dominated by the triazine-resistant strains have occurred in many locales. These triazine-resistant strains are not controlled with extremely high triazine use rates. In addition, there is cross-resistance among the different triazine herbicides used in corn, soybeans (Sencor), and alfalfa (Sencor and Velpar).
This situation has prompted refinements in the control guidelines for annual broadleaf weeds in field corn. Triazine herbicides have played, and will continue to play, an important role in field corn weed control programs; however, effective control programs for these triazine-resistant strains will involve the use of crop rotation and cultivation along with herbicide rotation and/or use of herbicide combinations that include herbicides with different sites of action (how they affect weeds). These practices will also delay development of weed populations that are resistant to triazine and other herbicide groups.
Rotating herbicides with different sites of action and the use of tank mixes or sequential applications that involve herbicides with different sites of action are key elements in herbicide resistance management plans. To do this most effectively, everyone involved in decisions about weed management must have site of action classification for herbicides readily available. The Weed Science Society of America (WSSA) has approved a numbering system to classify herbicides by their site of action (Mallory-Smith, C.A. and Retzinger, E.J. 2003. Revised classification of herbicides by site of action for weed resistance management strategies. Weed Technol. 17:605-619). In this system, a group number is given to all herbicides with the same site of action. To further efforts in management of existing herbicide-resistant weed populations and to delay or avoid development of new herbicide-resistant weed populations, these “GROUP NUMBERS” are included in the “Chemical weed control tables” in each crop section of this guide. Since herbicide resistance management is most effective when practiced across all crops in rotation, “GROUP NUMBERS” for all herbicides in the Cornell Guide For Integrated Field Crop Management can be found in Table 8.2.1. Mode of action/site of action and chemical families for site of action groups can be found in Table 8.2.2.