There has been exponential increases in the number of weeds resistant to various herbicides commonly used to manage them, especially in field crops in the last 25 years. To better understand herbicide resistance and to minimize its development, we should examine the processes that govern it. Herbicides may classified into various families or groups based on the growth process affected by the herbicide, referred to as mode of action. Slight differences in the genetic makeup of a very small fraction of the population of a particular weed species, referred to as a biotype, may allow them to tolerate a particular herbicide group. So, when herbicides belonging to the same group are used in a given area over lengthy periods, populations of such weeds build up through selection pressure. The particular species is then referred to as an herbicide-resistant weed. The primary cause of herbicide resistance is the repeated use of the same herbicide, or herbicides, with the same mode of action.
The life cycle of the weeds play an important role in the development of resistance. Annual weeds with shorter life cycles develop resistance faster. Perennial weeds take longer time to develop resistance. More and more instances of weed biotypes are reported as evolving resistance to glyphosate. Common weeds, such as horseweed (Figure 5) or marestail (Conyza canadensis), waterhemp (Amaranthus rudis), palmer amaranth (Amaranthus palmeri), common ragweed (Ambrosia artemisiifolia), giant ragweed (Ambrosia trifida) and johnsongrass (Sorghum halepense), have been reported as resistant to glyphosate in the United States.