14.13 Weed Population Genetic Diversity

It is common for different populations or biotypes of a weed species to differ in herbicide susceptibility. There is a great deal of genetic diversity within certain weed species. Some of this diversity relates to herbicide susceptibility. An extreme case of differential response within a species is illustrated by resistance wherein one biotype may be susceptible and another totally unaffected by the same herbicide (Figure 16). There are many examples of differential herbicide susceptibility within a species that do not involve resistance.

Even though plants with this genetic make up (biotypes) are not herbicide resistant, some will be more completely controlled than others. Using the same herbicide repeatedly on a weed population containing biotypes of different susceptibility will result in the population shifting over time to the more tolerant biotype(s), making the herbicide less effective in a weed management plan.