What Is Herbicide Resistance?

— Written By and last updated by Patricia Burch

Herbicide resistance is often classified when a biotype (a group of specific plant species that has traits uncommon to the whole population) of a weed survives following a herbicide application in which the original weed population of that weed was susceptible.

Over the years, North Carolina farmers have been concerned with herbicide resistance. This is a very serious problem that the Agricultural Industry is facing today. There are over 100 resistant biotypes of weeds worldwide.

In the past, because of farmers using crop rotational methods, resistance was never in the forefront. Today, because of herbicides having similar mechanisms of action and the ability to use these same mechanisms of action across a broad range of crops are becoming limited. Mechanism of action is a specific process in which an herbicide kills the targeted plants.

Herbicides we are concerned about with resistance are those within the ALS enzyme makeup. The most commonly used herbicides are those of the ALS inhibitors.

Why do farmers use herbicides versus other means of weed control?

Farmers use herbicides because they are more effective and economical compared with other weed control options. It is a good practice for farms to maintain accurate records of herbicide usage in each field. This is useful down the road when making crop management decisions.

How does a farmer know if they have herbicide resistant weeds?

Most weed control failures are not the result of herbicide resistance. Farmers should eliminate the following possibilities before assuming that the species is resistant. Possible causes of failure to work include misapplication due to inadequate formulation rate, poor coverage, poor mixture, or lack of use of adjuvant, unfavorable weather conditions, poor timing of application (in particular the height of the weeds could be too large for efficient control), and lastly the emergence of weeds after the application of a short-residual herbicide.

Ultimately, if a farmer suspect’s herbicide resistance, they should contact their local county Extension Agent and chemical company representative for advice. They should also avoid spreading the weed seed to other fields and plan their weed management program accordingly to fit current and following crops while utilizing different mechanism of actions.