Are all weeds and herbicides vulnerable to resistance?

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Some herbicide mode of action groups are more prone to develop herbicide resistance from frequent use. Weeds emerged with resistance to Group 2 herbicides in the early- to mid-1980s, only a few years after their introduction (Figure 22). Since that time, more than 170 weed species have been identified with Group 2 resistance. The second most problematic herbicide mode of action is the Group 5 herbicides, although much of this resistance developed back in the 1970s and 1980s with atrazine use in corn (Figure 22). Glyphosate, a Group 9 herbicide, became the third most problematic mode of action, but only after the widespread and frequent use of glyphosate in Roundup Ready crops starting in the late 1990s accelerated resistance. The ACCase inhibitors or Group 1 herbicides are in fourth place, based on the number of resistant weed species, followed by the Group 4 synthetic auxins (Figure 22).

Figure 22. This chart shows the number of resistant species for several herbicide modes of action. These are the top five herbicide MOA’s based on the number of resistant weed species. (Chart credit: The International Herbicide-Resistant Weed Database, accessed online, Nov. 2, 2023)

Some weedy plants are also more susceptible to resistance evolution (Figure 23). Within the grass family (Poaceae), rigid and Italian ryegrass, barnyardgrass, annual bluegrass, wild oats and goosegrass all have multiple resistance (Figure 24). Palmer amaranth, waterhemp, smooth and redroot pigweed, horseweed, and both common and giant ragweed all have biotypes resistant to multiple modes of action. Several weeds have evolved both target-site resistance and non-target-site resistance. The problem species discussed most often in both the scientific and ag communities include rigid ryegrass in Australia and Palmer amaranth and waterhemp in the U.S.

Figure 23. This graph shows the number of herbicide-resistant weed species by plant family. (Chart credit: The International Herbicide-Resistant Weed Database, accessed online, Nov. 2, 2023)

Figure 24. This graph displays the top weed species resistant to multiple herbicide modes of action. (Chart credit: The International Herbicide-Resistant Weed Database, accessed online, Nov. 2, 2023)

In general, herbicides are at higher risk for resistance if they target a single site of action, and if the weed species have a higher frequency of the resistant biotype in the population (e.g. one in one million vs. one in 10 million individuals carry the resistance trait) or if the target field has a greater population size (e.g. 100 seeds vs 1,000 seeds per square foot in the soil seed bank). Resistance evolution takes longer for some herbicides and weeds, especially if the herbicide targets multiple sites of action, is used in combination with other herbicides, and is used with other effective non-chemical weed control options.

Weeds with diverse genetic backgrounds are more prone to resistance evolution. The annual ryegrasses and some pigweeds both have diverse genetic backgrounds. Two of the most important characteristics for evolution of resistance are reproductive capability and how seeds disperse and spread into new areas. Some of the most problematic weeds produce thousands of seeds, the seeds are wind dispersed and/or the plants require outcrossing for successful reproduction. For example, waterhemp and Palmer amaranth can produce over 100,000 seeds per plant, have separate male and female plants (dioecious), and resistance moves with wind-blown pollen. The herbicide mode of action and application frequency, targeted weed species and genetic potential for resistance, and the diversity of weed management tactics all influence the potential for resistance evolution.