Herbicide resistance develops in a field because of a process known as selection pressure. Herbicide resistant weed biotypes already exist in any given population of weeds but usually are not present in large numbers. The resistant population becomes dominant only after the repeated use of a single herbicide or herbicides with the same mode of action removes the susceptible population and allows the resistant population to survive and produce seed.

Herbicide Resistant Weeds Selection Pressure<BR>Source: J.L. Gunsolus.  Herbicide Resistant Weeds.  1998. <BR>North Central Region Extension Publication 468.  <BR>