What are cross and multiple resistance?

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Herbicide resistance within or across chemical families with the same mode of action is called cross resistance. For example, cross resistance is common for Group 2 herbicides. Even though there are five different herbicide families within the Group 2 herbicides, they are all ALS inhibitors, so resistance to one herbicide may eliminate the effectiveness of them all (Figure 14).

Figure 14. Cross resistance occurs within or across chemical families with the same mode of action. Pursuit (imazethapyr) and Classic (chlorimuron) belong to different chemical families within the Group 2 herbicides. (Photo credit: Claudio Rubione, GROW; graphic credit: Lourdes Rubione)

Resistance across different modes of action is called multiple resistance. Multiple resistance can occur when resistance evolves after repeated use of a herbicide group followed by the repeated application of other herbicide groups. It seldom occurs simultaneously, although this is less certain with metabolic resistance. The evolution of non-target site resistance complicates both cross and multiple resistance since there is evidence that these can evolve without exposure to specific herbicide groups. This will be discussed more in Section 8.

Here’s an example of how multiple resistance evolves: Let’s say Group 2 and Group 5 herbicides are applied annually for several years for weed control in a field in both corn and soybean rotations. Waterhemp control worsens as the waterhemp population first evolves Group 2 and then Group 5 herbicide resistance. In this scenario, the grower then adopts Roundup Ready corn and soybean technology and begins using glyphosate, a Group 9 herbicide, every year in both crops. After multiple years of using glyphosate and the Group 2 and 5 herbicides, the waterhemp population has evolved multiple resistance to three different herbicide groups (2, 5, and 9) (Figure 15).

Figure 15. Multiple resistance is resistance across different modes of action. When a weed is resistant to a Group 2 herbicide, Pursuit (imazethapyr), a Group 5 herbicide, Atrazine, and a Group 9 herbicide, Roundup (glyphosate), it has multiple resistance to three different modes of action. (Photo credit: Claudio Rubione, GROW; graphic credit: Lourdes Rubione)

Some weedy plants are also more susceptible to resistance evolution. Within the grass family, rigid and Italian ryegrass, barnyardgrass, annual bluegrass, wild oats and goosegrass all have multiple herbicide resistance. But in some other grass species such as annual foxtails (Setaria sp.), multiple resistance is not common. Likewise, Palmer amaranth, waterhemp, smooth and redroot pigweed, horseweed, common and giant ragweed and kochia all have biotypes resistant to multiple modes of action, while other dicot weeds such as eastern black nightshade and Pennsylvania smartweed have not evolved multiple resistance. Several weeds have also evolved both target-site and non-target site resistance. This is discussed more in Section 8.