How does herbicide resistance happen?
Herbicide-resistant individuals initially may be present in a weed population at exceptionally low frequency. These “biotypes” differ from the majority population because they have mechanism(s) that allows them to survive exposure to the herbicide. Maybe only one individual out of 10 million plants carries the resistant trait. Often the frequency or rate at which this mutation occurs within a weed species is much lower than this. But, with repeated application of the same herbicide or herbicide group over time, without the use of other effective weed control strategies, the frequency of the resistant individuals or biotype within a population increases (Figure 3). Although the herbicide is still controlling susceptible individuals, the resistant biotype survives and produces seed. Using multiple effective modes of action (MOA), rotating modes of action and using non-chemical control tactics can help reduce selection for herbicide-resistant weeds.
Figure 3. After repeated use of the same herbicide group for multiple seasons, a resistant biotype may be selected that survives the application. As illustrated, the resistant individual (a) survives the herbicide and can (b) mature and produce seed. If the herbicide program continues, (c) more individuals survive the herbicide, they mature and produce seed and eventually, (d) the dominant biotype in the field is resistant. (Adapted by William Curran and Lourdes Rubione from University of Minnesota’s herbicide-resistance management webpage).
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Depending on the weed species and the growing conditions, plants can produce anywhere from a few hundred seeds to more than 100,000 seeds per plant for species like pigweeds. The soil in a typical agricultural field contains hundreds to several thousand seeds per square foot. One thousand seeds per square foot equates to over 43 million seeds per acre. At initial low frequency, resistant plants can be hard to detect as they evolve and survive in a field. Figure 4 shows how resistant Palmer amaranth quickly spread through a field within three growing seasons when it was not effectively managed.
In addition to herbicides, several factors can reduce weed survival, such as environmental conditions, plant pathogens, and predation. Even when herbicides are 90% or more effective, some weed escapes are expected. Once the percentage of a resistant weed species population reaches about 25%, it becomes evident from the escaped weeds that something has changed. Once resistance appears, it can spread quickly and unfortunately, a gradual decline in performance over time is rarely observed. Rather, unless the field is being scouted regularly, the shift to a dominant herbicide-resistant weed biotype may be missed until poor weed control becomes very visible.
Herbicide resistance can evolve quickly over time with frequent use of the same herbicide mode of action and without use of other effective weed control strategies. Resistance from some herbicide groups was evident after three to five years of repeated use, but with other modes of action, it may take seven to 10 (or more) years before pervasive resistance becomes evident. The herbicide mode of action and application frequency, specific weeds and their genetic potential for resistance, and the diversity of weed management tactics practiced on the farm all influence this timeline.
We can observe a similar shift in species/biotypes with other agricultural practices and even in other managed ecosystems. For example, management practices can shift a weed or plant population away from some species/biotypes and towards others, when other effective weed control tactics are not used.
Examples of weed species’ shifts from non-chemical weed management:
- Frequent mowing in turfgrass or perennial forage crops selects for short-statured annuals and perennials that tolerate mowing.
- Continuous no-till practices select for small-seeded weed species that can successfully germinate and emerge near the soil surface as well as perennials that survive better without soil disturbance. Large-seeded broadleaves that prefer some seed burial to germinate are often reduced in long-term no-till fields.
- Rotating to fall-seeded winter annual crops like winter wheat and other winter cereals reduces the frequency of summer annual weeds and selects for more fall-emerging species.
Figure 4. These images illustrate how herbicide-resistant Palmer amaranth rapidly escapes control over time, from an escaped plant here and there early in the infestation (1st year), to a more visible spread of herbicide-resistant Palmer amaranth (2nd year), to domination of the field by the herbicide-resistant biotype (3rd year). (Photo credits: Art Bradley and Alan York, NC State University)
