Herbicide resistance refers to the inherited ability of a biotype of a weed to survive a herbicide application to which the original population was susceptible. A biotype is a group of plants within a species that has biological traits (such as resistance to a particular herbicide) not common to the population as a whole.

Herbicide resistance is potentially a very serious problem facing all growers. Worldwide, more than 100 biotypes of weeds are known to be resistant to one or more commonly used herbicides. In North Carolina, we currently have a biotype of goosegrass resistant to dinitroaniline herbicides (Prowl, Sonalan, and Treflan), a biotype of cocklebur resistant to MSMA and DSMA, and a biotype of annual ryegrass resistant to Hoelon.

Until recently, there was little concern about the development of herbicide resistance in North Carolina. Although we have three species with biotypes resistant to certain herbicides, the occurrence of these biotypes was easily explained by growing crops in a monoculture. Growers who were rotating crops had little need to worry about resistance. The situation, however, has changed in recent years because of the development and widespread use of several herbicides having the same mechanism of action. Mechanism of action refers to the specific process through which a herbicide kills a susceptible plant. Today, herbicides having the same mechanism of action can be used on several crops that may be grown in rotation. Of particular concern are those herbicides that inhibit the ALS enzyme system. Several of our most commonly used herbicides are ALS inhibitors. In addition, many of the new herbicides expected to be registered within the next five years are ALS inhibitors. As a group, ALS inhibitors have a number of characteristics that seem to make them prone to the development of plant resistance.

Herbicides are used in crop production simply because they are more effective or more economical than other means of weed control. If resistance to a particular herbicide or family of herbicides evolves, suitable alternative herbicides may not exist. For example, there is currently no alternative herbicide to control Hoelon-resistant ryegrass. Hence, herbicides should be viewed as resources to be protected. We must use herbicides in a manner that deters the development of resistance.

An understanding of how resistance evolves is essential to understanding how to avoid resistance. There are two prerequisites for herbicide resistance evolution. First, individual weeds possessing genes conferring resistance must be present in the native population. Second, selection pressure resulting from extensive use of a herbicide to which these rare individuals are resistant must be exerted on the population. Resistant individuals, if present, make up a very low percentage of the overall population. Typically, resistant individuals are present at frequencies ranging from 1 in 100,000 to 1 in 100 million. If the same herbicide or herbicides with the same mechanism of action are used continuously, the susceptible individuals are killed but the resistant individuals are unharmed and produce seed. If the selection pressure continues for several generations, the resistant biotype will ultimately make up a high percentage of the population. At that point, acceptable weed control can no longer be obtained with the particular herbicide or herbicides.

The single most important component of a management strategy to avoid evolution of herbicide resistance is the rotation of herbicides having different mechanisms of action.

Tank mixes or sequential applications of herbicides having different mechanisms of action are often touted as components of a resistance management strategy. If the components of the tank mix or sequential applications are chosen wisely, this strategy can be very helpful in delaying resistance evolution. Unfortunately, many of the requirements of tank mix or sequential applications to avoid resistance are not met with commonly used mixtures. To be most effective at preventing resistance evolution, both herbicides used sequentially or in tank mixtures should have the same spectrum of control and should have similar persistence.