What is herbicide resistance ?
A number of weed species that were once susceptible and easily managed by certain herbicides have evolved resistance. These weeds are no longer controlled by applications of previously effective herbicides. To date, at least 53 species of weeds are resistant to at least five different herbicide families. Some well-known herbicides and resistant species are presented in Table 1.
Plant populations respond to herbicide applications in one of three ways: • Plant populations are killed when the herbicide is applied at labeled use rates. These plants are called susceptible. • 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. • 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 to two or more herbicides due to the presence of a single resistance mechanism. For example,
