Figure 1. A possible progression of selection for resistant weed biotypes when a single herbicide or site of action is used continuously or without adding a herbicide with a different site of action to the tankmixture. Initially, good control would be observed providing application factors were optimal for herbicide activity (A). After several applications, a single plant may survive, grow and reproduce seed (B). That seed would germinate the following year and as a result, more plants would not be controlled the following year (C). As selection pressure continues, one would begin noticing a reduction in herbicide performance when the resistant population in the field approaches approximately 30% of the weed population (D). Providing the same selection pressure is applied to the field, the resistant weed population will continue to increase until nearly 100% of the population is resistant (E).