Introduction

The introduction of synthetic herbicides revolutionized weed control. However, the repeated use of herbicides led to the evolution of herbicide-resistant weed populations. Herbicide resistant weeds are one of the major challenges in weed management. The selection of resistant weeds reduces control options and may increase control costs. In many cases, the most effective and economical herbicide is lost because it is no longer effective due to resistant weeds. The more effective a herbicide the greater the selection pressure; and therefore, the greater the probability that only resistant biotypes will survive the treatment. (Read more about the selection of herbicide resistant weeds)

Herbicides kill plants by interrupting the normal growth of the plant. Herbicides usually bind to a molecular target site, often a protein, in the plant. Once the herbicide binds, biochemical or physiological processes in the plant are inhibited and the plant dies. Herbicide-resistant plants possess at least one mechanism that allows them to survive herbicide treatment.

Herbicide resistance is defined as the inherited ability of a biotype to survive and reproduce following exposure to a dose of a herbicide that is normally lethal to the wild type. Herbicide resistance is a response to selection by a herbicide. Since it is an inherited ability, there must have been a genetic change responsible for the resistance mechanism. Most often resistance is controlled by a single, dominant or semi-dominant gene.

Herbicide-resistant biotypes may be present in a weed population in very small numbers. The repeated use of one herbicide or herbicides with the same site of action allows these resistant plants to survive and reproduce. The number of resistant plants increases until the herbicide is no longer effective. There is no evidence that the herbicide causes the mutations that lead to resistance.  

The first herbicide resistant weed Senecio vulgaris  L., was identified in 1968 in a nursery in Washington State. To date, 261 different herbicide-resistant biotypes have been identified. In some instances, the same species has been identified with resistance to different herbicides in different locations (Figure 1). Resistance has occurred to most herbicide chemical families. More information about the herbicide-resistant species can be found at www.weedscience.com.