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Persistence of herbicide resistance
by Bob Hartzler
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June, 1997 -- One of the questions that farmers and weed managers frequently ask is how long will a herbicide resistant biotype remain in a field after it has been selected. For example, let's assume repeated applications of ALS-herbicides have selected ALS-resistant cocklebur in a field. How long would the farmer need to avoid using ALS-herbicides in the field for ALS-susceptible cocklebur to become the dominant biotype found in the field?

An article in the April-June issue of Weed Technology provides some insight to this issue (Andrews, T.S. and I.N. Morrison. 1997. The persistence of trifluralin resistance in green foxtail populations. Weed Technol. 11:369-372.). This work investigated the stability of resistance in green foxtail to trifluralin (Treflan) in southwestern Manitoba. The resistance was selected following repeated applications of Treflan in wheat fields.

Andrews and Morrison collected foxtail seed samples in 1995 from 17 fields that Treflan resistance was first reported in 1988. Since 1988, six of these fields had not been treated with DNA (dinitroaniline) herbicides, four had been treated once, and seven had been treated two or more times. The proportions of resistant (R) and susceptible (S) biotypes in the foxtail populations were determined and compared to the field history. All of the fields contained at least 90% R individuals when tested in 1988.

Two of the fields that had not been treated with Treflan or other DNA herbicides since 1988 contained > 99% R foxtail, the other fields ranged from 40 to 83% R biotypes. Among the four fields treated once with DNA herbicides, two fields had > 99% R foxtail, and the other two contained 59 and 67% R individuals. Five of the seven fields treated two or more times contained > 97% R, and the other two fields contained 89 and 68 % R biotypes.

The results clearly indicate that Treflan resistance can persist in a population for at least seven years in the absence of additional selection pressure. The authors concluded that the potential exists for resistance to render some chemicals useless for the forseeable future. They stated that the emphasis for managing resistant weeds should be on prevention, rather than cure, and the incorporation of integrated weed management techniques.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
http://www.weeds.iastate.edu
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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.