Persistence and Biodegradation of
Herbicides
Floyd M. Ashton
5.1. INTRODUCTION
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The destruction of herbicides applied to the environment by man is essential to
their sustained use. Without such a mechanism, they would ultimately accumulate
to levels that would be phytotoxic to most higher plants. Ideally a given herbicide
should persist just long enough to control the target weeds and then be rapidly
degraded to its constituent atoms. The accomplishment of this ideal situation
can only be a goal; however, it must be approximated for a truly successful
practice. If the period of persistence is too short, weed control will be inadequate
and other control measures must be utilized. If the period of peristence is too
long, injury to susceptible plants subsequently planted or long-term environmental pollution can occur. Based on our present knowledge, the latter situation
does not appear to be a problem with the herbicides currently used; however, it
must be an area of continuing investigation.
Herbicides are inactivated in the environment by biological, chemical, and
physical means. This inactivation usually involves degradation of the molecule;
however, certain intact herbicide molecules can also be made nonphytotoxic by
binding tightly to other substances. The means and rate of inactivation depend
upon the nature of the herbicide molecule and the environmental conditions. In
general, biodegradation is dominant, but certain herbicides undergo chemical
change or physical modification more readily. A limited number of herbicides
Floyd M. Ashton. Department of Botany, University of California, Davis, California 95616.
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