How Does Resistance Develop?

Proper weed control has always been a critical challenge for crop producers because weeds compete with crops for light, water and nutrients. Herbicides used in modern agricultural systems enable farmers to manage most weeds across vast acreages.

However, changes can occur in response to herbicide use and other management decisions. Changes in weed populations begin when a small number of plants within a species, called a “biotype,” have a distinct genetic makeup that allows them to tolerate a particular herbicide application. Multiple weed biotypes can exist in a single field.

As a grower continues to use a particular herbicide without any other herbicide modes of action, or doesn’t use any other cultural practices, the resistant biotype continues to survive and produce seed. Subsequent populations of the resistant biotype will continue to increase until they are the dominant weed in the field.

Weed scientists cannot predict exactly which weed species will have biotypes resistant to certain herbicides. Prediction can be difficult due to complex biology and environmental interactions.

Scientists have found that there are particular weed characteristics that can facilitate development of herbicide resistance. These include:

  • large amount of seeds produced per plant
  • high levels of germination of those seeds
  • several weed flushes per season, and
  • high frequency of resistant genes

Monsanto and university weed scientists have also identified specific common factors that are often present in areas where glyphosate resistance has developed. These factors are:

  • Limited or no crop rotation
  • Limited or no tillage practices
  • A high dependency on glyphosate alone or a limited use of other herbicides, and
  • Reduced rates of glyphosate

Confusion about what is or is not weed resistance is common. Herbicides are not known to directly cause genetic mutations in weeds that lead to resistance. However, herbicide resistant biotypes may already exist in native weed populations. When a herbicide is applied over and over again, some of these biotypes survive, mature and produce seed. If a farmer relies on only one herbicide with the same mechanism of action, again, the percentage of the resistant biotypes in the population is likely to increase. This is referred to as herbicide selection pressure.