What is Herbicide Resistance?

Plant populations respond to herbicide applications in one of three ways:

  1. Plant populations are killed when the herbicide is applied at labeled use rates. These plants are called susceptible.
  2. Plant populations survive as they are naturally immune to a particular herbicide when applied at its labeled use rate. These plants are called tolerant. For example, atrazine will kill many annual grass weeds, however, atrazine applied to grain sorghum or corn causes no injury, as these crops are naturally tolerant to this particular herbicide.
  3. Plant populations that once were susceptible to a particular herbicide applied at its labeled use rate, are not injured, sometimes even with rates up to 50X the labeled use rate (Figure 1). These plants are called resistant. They have developed as a result of selection pressure for a genetically different biotype.
  • Cross-resistance is the result of resistance mechanism. For example, one herbicide selected for the biotype, but this biotype is also resistant to other herbicides in the same mode of action (Figure 2).
  • Multiple resistance occurs when resistance to several herbicides results from two or more distinct resistant mechanisms in the same plant. For example, resistance to both triazine and ALS-inhibiting herbicides, which are two different modes of action (Figure 3).

Fig. 1: Herbicide resistant plants can often resist high doses of a herbicide. This shattercane biotype is not affected by a dose 64X that recommended for Beacon (primisulfuron; ALS-inhibitor). Photo courtesy of Dan Anderson.

Fig. 2: Weed mode of action (ALS-inhibitor). This is known as cross-resistance. Photo courtesy of Dallas Peterson.

Fig. 3: Common waterhemp seedlings were identified as having multiple resistance, that is, resistance to both triazine and ALS-inhibiting herbicides, which are two different modes of action with different mechanisms conferring resistance. Photo courtesy of Dallas Peterson.

For the remainder of this lesson, susceptible and resistant biotypes, plants, populations, etc, of a given weed species will be designated as S- and R-, respectively.

The first herbicide resistant biotype was identified in Washington State in 1968. Common groundsel, Senecio vulgaris, R-biotype was found to be resistant to simazine. Since then, numerous cases of herbicide R-biotypes have been identified. At this time, approximately 165 weed species have R-biotypes to more than 16 different herbicide families, equaling 275 different R-biotypes. International Survey of Herbicide Resistant Weeds

The time required for a weed population to be identified as resistant varies. Resistance to a herbicide may occur in as little as 2-5 years or it may never develop. Resistance occurs with a wide variety of herbicide chemistries, weed species, and agricultural systems. For example, resistance to chlorsulfuron (Glean) developed after five years of continuous use in wheat. Triazine-resistant weeds occurred after 10 years of continuous use. Resistance to alachlor (Lasso) has never been reported despite 20 years of use in corn/soybean rotations.

We will examine the diversity of factors that influence the rate at which herbicide resistance appears in a weed population.