Rate at Which Herbicide Resistance Appears in Weed Populations

The intensity of the selection pressure (for example, how effective is the chosen herbicide) can be measured by the germination rate, rate of seedling emergence and/or the survival of seedlings that become flowering adults.

Intensity of the selection pressure is influenced by the herbicide itself, weed biology, and other crop management choices.

When a herbicide is applied, it exerts selective pressure on a weed species in a field by killing all S-biotype plants and not injuring the R-biotype plants. Intensity of selection pressure can be increased:

1. If the herbicide acts on a single mechanism/site of action (View the mechanism/site of action animation). For example, ALS-inhibiting herbicides act on a specific single enzyme in the plant. In a S-biotype plant, the herbicide binds to the enzyme and does not allow it to function. In a R-biotype plant, the structure of this enzyme has changed so that a herbicide can no longer bind and the enzyme functions normally (Figure 5).
2. If the herbicide is highly effective (>95% control), the R-biotype will be able to flourish without competition since few, if any, S-biotypes survived.
3. If the herbicide provides soil activity for a long period of time (persistent), it can control weed species that typically emerge later in the season and thus, continue to remove S-biotypes and allow for R-biotypes to emerge and survive.

Characteristics of the weed species and its population structure will influence the rate at which herbicide resistance appears in a given population.

1. The frequency of mutations conferring herbicide resistance that occurs within the plant population depends on several genetic factors. These include gene number, inheritance mechanisms, gene type, and nature of the gene modification. Little is known about the mutation rate and frequency of R-genes in field populations of weed species. It appears that there are some rare individuals in which a naturally occurring mutation has occurred that confers resistance even before herbicide use and selection has occurred. Weed species with a diverse genetic background may have 1 R-biotype in every 1 million or 1 billion seeds.
2. Weeds have the ability to adapt and multiply in diverse environments. For example, waterhemp plants look similar to each other, but some plants germinate earlier in the spring while others produce more seed, flower early, or perhaps prefer more moisture. When a farmer grows corn in an irrigated field for several years, he is probably selecting for those waterhemp plants that grow best under irrigation. Similarly, if a farmer uses the same herbicide mode of action for several years in a row, he is selecting for waterhemp plants that are not controlled by that herbicide.
3. Weeds have the capability of producing large numbers of seeds. If an R-biotype survives and reproduces, many new R-biotype seeds are returned to the seedbank for next year’s population. The weed seedbank is a collection of old and new seed, both R- and S-biotypes. This can act as a buffer, a means to slow the rate at which the population will shift to predominantly R-biotype weed populations. This is especially true for weed species with long-lived seeds in the soil.
4. Ecological fitness of a given biotype  indicates that it will leave a greater proportion of its genes in the future gene pool of the population. The most fit plant will leave the greatest number of offspring. Differences in ecological fitness between R- and S-biotypes will influence the rate at which herbicide resistance appears, as well as development of resistant populations when not treated with the herbicide. Triazine R-biotypes often have reduced fitness and thus, may slow down the development of resistance and allow reversion of populations to susceptible levels if herbicide use patterns are changed, however in many cases, fitness is not different in R-biotypes compared with S-biotypes.