By Bob Hartzler, Extension weed management specialist Department of Agronomy, Iowa State University

With our current reliance on glyphosate for weed control in corn and soybean, a major concern is the appearance and spread of glyphosate resistant weeds. However, it is important to recognize that nature has other ways of getting around our efforts to eliminate weeds from agricultural fields.

At the recent Weed Science Society of America's Annual Meeting, Dale Shaner, USDA Plant Physiologist, described adaptations in soil microbes resulting in rapid degradation of atrazine.* The persistence of atrazine in soils from several states with different histories of triazine use was evaluated. The assay found that atrazine's half-life ranged from 8 to 11.5 days in soils with no history of triazine use, whereas in soils from fields with a long history of triazine use the half-life was less than one day.

Rapid degradation of atrazine would greatly reduce its performance as a preemergence herbicide, but atrazine would still function effectively as a post product. It is unknown whether this problem occurs in Iowa, but since enhanced degradation was found in several other states there is no reason not to expect it, especially in areas of the state where atrazine has been the foundation of corn weed control programs for more than 30 years.

This is another example of an adaptation to the management tactics used to sustain an artificial system (monocultures). A similar problem with enhanced herbicide degradation occurred in the late 1970s in fields where Eradicane (EPTC) had been used repeatedly to control shattercane. In the entomology world, microorganisms adapted to repeated use of the rootworm insecticide Furadan.

These examples emphasize the importance of utilizing integrated management systems that rely on a variety of control tactics.

*Shaner, D., B. Henry, B. Hanson and J. Krutz. 2006. A rapid assay to detect atrazine degradation in soil. WSSA Abstracts 47:115.

SOURCE: ISU Weed Science Online.