Author
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Helling, Charles |
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Submitted to: Meeting Abstract
Publication Type: Monograph Publication Acceptance Date: 7/2/2005 Publication Date: 11/24/2005 Citation: Helling, C.S. 2005. The science of soil residual herbicides. Meeting Abstract. P. 3-22 in R.C. Van Acker, ed. Topics in Canadian Weed Science, Vol. 3, Sainte-Anne-de-Bellevue, Quebec: Canadian Weed Science Society. 125 pp. Interpretive Summary: Residual herbicides are expected to provide season-long weed control due to their soil persistence, but when carryover occurs, the herbicide residue may injure susceptible rotational crops and increase risk of environmental contamination. This review explores the underlying science of how soil, weather, field management, and herbicide chemistry interact to affect herbicide persistence and carryover in temperate region soils. A table of 49 herbicides with moderate to long persistence was developed, together with their estimated half-lives (40 or more days), estimated soil adsorption, and tendency to exhibit carryover. Especially for extension agents, researchers, and regulators in Canada, northern United States, and Europe, this review provides a clear basis for understanding soil residual herbicides and predicting those which might persist into the next growing season. Technical Abstract: Residual herbicides are those for which season-long weed control is expected due to their persistence in soil. The economic advantage of residual soil activity can be partially off-set by two problems: carryover of herbicide residue that may injure susceptible rotational crops, and increased risk of transport of herbicide to surface water or groundwater. Herbicide persistence is determined by complex interactions between the pesticide and the soil environment. Among the most important parameters and processes are (a) herbicide chemistry, (b) intrinsic soil properties (e.g., texture, organic matter content, pH), (c) extrinsic soil and meteorological factors (e.g., temperature, rainfall), and (d) other parameters (e.g., mode and rate of herbicide application, prior history of pesticide use, plant cover, topography). The major processes that reduce herbicide concentration in soil include transport (leaching, runoff, volatilization) and degradation, but adsorption to soil particles is extremely important in regulating herbicide concentration in the soil water, and therefore bioavailability. In general, low soil moisture, organic matter, and temperature are associated with slower microbial degradation; herbicide carryover is thus more likely in drier, seasonally cooler-climates, which characterize much of western Canada. Herbicide persistence is usually expressed as half-life, i.e., the time for dissipation of 50% (DT50) of the applied herbicide from soil; the DT50 is inherently variable, due to the effects of climate and soil. Herbicides that have a DT50 > 40 d are considered to have moderate to long soil persistence; 49 such herbicides are listed within this review. |
