Are Herbicide mixtures useful for Delaying the Rapid Evolution of Resistance? a Case Study

Abstract

Mixtures of herbicides have been proposed as strategies to prevent or delay the evolution of resistance to the resistance-prone sulfonylurea and imidazolinone herbicides that inhibit acetolactate synthase. These herbicides have become or are becoming widely used in soybean, wheat, rice, and other major crops. For a mixture to be efficacious in preventing resistance, the less resistance-prone component(s) should have the following traits compared to the vulnerable herbicide: a) control the same spectra of weeds; b) have the same persistence; c) have a different target site; d) be degraded in a different manner; and e) preferably exert negative cross-resistance. We compared the proposed mixing partners for use with several widely used acetolactate synthase inhibiting herbicides to these criteria and found that: a) all have somewhat different weed spectra; e.g. none control common cocklebur as well as imazaquin or imazethapyr in soybean, or kochia as well as chlorsulfuron in winter wheat; b) all are far less persistent than these vulnerable herbicides. Less persistent sulfonylureas are now on the market but are in limited use. Late in the season, the mixing partner is not present while the vulnerable herbicide remains active; c) most have different target sites; d) in soybean most mixing partners are degraded differently than vulnerable herbicides. In wheat virtually all herbicides used without safeners are degraded by monooxygenases, thus it is impossible to meet this criterion in this crop; e) none of the mixing partners exert negative cross-resistance. The present mixtures may have superior or more cost-effective weed control properties than the acetolactate synthase inhibitors used alone, but they do not meet all the criteria for resistance management. Not meeting the key criteria of identical control spectra and equal persistence aggravates future resistance problems, as has happened with insecticides.