Crop protection and seed industry representatives, government researchers, crop advisors, agronomists, regulatory authorities and farmers are coming to understand the grave threat herbicide resistance poses to the crop production systems that sustain human populations. Here's an overview of what herbicide resistance is, how it develops, and how it can be mitigated.

Herbicide resistance versus herbicide tolerance

Herbicide resistance is the ability of a weed biotype to survive an herbicide application, where under normal circumstances that herbicide applied at the recommended rate would kill the weed. In contrast, plant tolerance to a particular herbicide is the inherent ability of that plant species to survive and reproduce after treatment with that herbicide.

Nature of herbicide resistance

Resistance of weed biotypes to herbicides is a consequence of naturally occurring mutations and evolutionary processes. Individuals within a species that are best adapted and not susceptible to a particular practice, such as application of a specific herbicide, are selected for and will increase in the population. Mitigating or slowing the evolution of herbicide resistance relies on reducing selection pressure for resistance through application of a diversity of weed management practices.

There are two general categories of resistance mechanisms, target-site resistance and non-target-site resistance. Target-site resistance inhibits herbicide action by: a change in structure of the target protein that decreases herbicide binding to its usual site of action; an increase in target protein expression; or an increase in copies of the gene containing the target site. Non-target-site resistance includes decreased translocation of an herbicide to its site of action, increased metabolic detoxification of an herbicide, and sequestration or immobilization of an herbicide in a part of the plant so it cannot reach its site of action.

Herbicide tolerance in crops may be induced by techniques such as genetic engineering or selection of variants produced by tissue culture or mutagenesis.

Cross resistance and multiple resistance

Cross resistance occurs when a plant has one mechanism that enables plants to survive treatment with herbicides from different chemical classes or with differing modes or sites of action. Multiple resistance refers to plants that have more than one mechanism that enables them to survive treatment with herbicides with differing modes or sites of action. It is particularly challenging to control weed biotypes that are resistant to multiple herbicide modes or sites of action. Controlling weed biotypes with cross or multiple resistance is best accomplished with management practices that integrate a diversity of chemical and non-chemical measures.

Use of single herbicide mode or site of action accelerates herbicide resistance

Genes that enable weed species to resist herbicides are present in all weed populations. The frequency of occurrence of genes that impart resistance influences the rate at which resistance to herbicides in those populations evolve. It is difficult to accurately predict the rate at which resistance will increase until herbicides of a particular mode or site of action are exclusively applied over an extended period of time.

Individual weeds that resist a particular mode or site of action are able to mature and set seed. Plants arising from those seeds the next growing season will have a higher frequency of resistance to that herbicide mode or site of action. Resistance to that herbicide site of action will be selected for and will accelerate with long-term repeated application of the same herbicide or other herbicide with the same site of action. After several seasons of selection, resistant weed species biotypes will come to dominate the weed population. Strength of resistance selection pressure depends on several variables including herbicide efficacy and site of action, weed biology and ecology, longevity of the weed seed in the soil and weed management practices used.

A growing number of herbicide-resistant weed species

About 250 weed species have been confirmed to be resistant to at least one herbicide mode or site of action around the world. In a growing number of situations herbicide-resistant weeds limit crop production.

Resistance affects every sector of the agriculture industry

The growing incidents of weeds developing resistance to herbicides is a critically important issue for all stakeholders of the field crop agriculture sector including farmers, crop production advisors, researchers, pesticide product regulatory authorities and the crop protection and seed industry members. Once in a field, herbicide resistance can remain at a relatively high frequency because it declines at a slower rate than it evolves.

Herbicide resistance is one of the primary issues threatening sustainable agricultural systems globally. Herbicide resistance increases the complexity and often the cost of weed management programs. Herbicides provide effective, reliable and economical weed control for broad-acre agriculture and their utility and longevity must be preserved. The fear that farmers may end up with few or no herbicides that control weeds resistant to multiple herbicide modes or sites of action is becoming a reality.

Herbicide resistance best management practices

Increasing the diversity of chemical and non-chemical weed management practices used in crop management programs including use of multiple herbicide modes and sites of action applied together each cropping season will slow herbicide resistance development and reduce the adverse impacts of herbicide resistance on crop production systems. View our Best Management Practices to learn more.

Global Herbicide Resistance Action Committee 

The global Herbicide Resistance Action Committee (HRAC) has been established to coordinate and communicate effective, reliable, practical and economical herbicide resistance management strategies that reduce the adverse impacts of herbicide-resistant weeds on crop yield and quality. HRAC fosters co-operation among crop protection and seed industry representatives, government researchers, crop production advisors, agronomists, regulatory authorities and farmers in the implementation of strategies designed to mitigate and slow development of herbicide-resistant weeds so farmers can continue to provide the food and fiber required to sustain human populations and stability of human societies.