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Be Vigilant About Herbicide Persistence

October 4, 2002
Adverse growing conditions were common across much of Illinois during the 2002 growing season. Precipitation, at least in many areas, was not as plentiful or as timely as producers had hoped for. The dry soil conditions contributed to problems not only this season but may also impact rotational crops in 2003 due to persistence of herbicide residues in the soil. For producers intending to plant wheat this fall, please remember that many corn and soybean herbicide labels have rotational intervals for wheat as well. Table 2 and Table 3 are reproduced from the 2002 Illinois Agricultural Pest Management Handbook and contain information on corn and soybean herbicide rotational intervals.

Herbicide persistence is influenced by a number of factors. Keep in mind that some of these factors influence certain herbicide chemical families more than others. Within a given herbicide chemical family, member herbicides can vary in their persistence due to differences in their chemical structures. For example, Harmony GT (thifensulfuron) and Classic (chlorimuron) are both in the sulfonylurea herbicide family, but Classic is inherently more persistent in the soil than Harmony GT. Rotational crop-planting intervals range from 0 to 45 days after application of Harmony GT, while the range is 3 to 30 months for Classic.

Timing of application also can be an influence if rotational crop injury occurs. Planting delays frequently lead to delayed herbicide applications, and late-season "rescue" applications may cause delays in planting the rotational crop in order to satisfy the rotational interval on the respective herbicide label.

Soil factors, such as organic matter content, temperature, pH, texture, and moisture, are some of the most critical factors that influence herbicide persistence. The relative importance of each factor can vary by herbicide chemistry, and these factors often interact to influence herbicide degradation. Soil organic matter and fine-textured soil particles can tightly adsorb herbicide molecules, reducing the availability for the various degradation processes and thus increasing persistence. Generally, herbicide persistence is favored in higher organic matter soils (2.5% or greater) compared with low organic matter soils. Soil pH above 6.8 to 7.0 can reduce the degradation of many sulfonylurea and triazine herbicides, while very low pH can reduce the degradation of imidazolinone herbicides, increasing the potential for carryover.

Soil moisture conditions deserve additional consideration. In general, dry soil conditions (much like what many areas of Illinois have experienced this year) reduce the rate of herbicide degradation. In particular, herbicides that are degraded by microbial processes are potential candidates for enhanced persistence during dry conditions since the microbial degradation process requires moisture. Dry soil conditions also can increase the amount of herbicide adsorbed to soil colloids, reducing its availability for degradation. Some herbicide rotational intervals are increased if a specified amount of precipitation is not received by a certain calendar date. Rotational intervals of imazaquin, prosulfuron, and clopyralid-containing products can be increased under low- precipitation environments.

What do you do if you are concerned about herbicide carryover and its potential adverse effects on next year's crop? Bioassays and laboratory analysis are methods that can assist you in determining if herbicide residues are high enough to potentially injure the rotational crop you intend to plant. For more information on herbicide persistence and a description of how to conduct these types of analyses, please refer to Chapters 14 and 15 in the 2002 Illinois Agricultural Pest Management Handbook.--Aaron Hager and Christy Sprague

Author: Aaron Hager Christy Sprague

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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