Nearly all herbicide labels (soil-applied or postemergence) have rotational crop intervals that specify the amount of time that must elapse between herbicide application and planting a rotational crop. Adhering to these intervals is always important, but becomes particularly important with late-season herbicide applications or when soil moisture is limited. These intervals are established to reduce the likelihood that herbicide residues will persist in sufficient quantities to adversely affect the rotational crop. Some herbicide rotational restrictions are based solely on time, while other factors, such as soil pH and the amount of precipitation received after herbicide application, can influence the length of the crop rotational intervals.
Soil moisture is often the most critical factor governing the efficacy and persistence of soil-residual herbicides. Many herbicides are degraded in soil by the activity of soil microorganisms, and populations of these microorganisms can be greatly depressed when soil moisture is limited. Additionally, dry soils can enhance herbicide adsorption to soil colloids, thus rendering the herbicide unavailable for plant uptake and degradation by soil microbial populations. Some herbicide rotational intervals are increased if a specified amount of precipitation is not received by a certain calendar date.
Diphenylether herbicides (Flexstar, Cobra and Ultra Blazer, for example) are routinely applied to soybean for control of waterhemp. In non-GMO and glyphosate-resistant soybean varieties these herbicide active ingredients can provide good to excellent control of small, susceptible waterhemp, but control is often poor when waterhemp plants exceed 5–6 inches or are resistant to PPO-inhibiting herbicides.
Fomesafen has the longest soil residual activity among the three foliar-applied diphenylether herbicides. Soil half-life values (the time required for half of the applied herbicide to degrade) for fomesafen have been reported to range from100 days to 6 to 12 months. The range is dependent upon several factors, including soil type and soil moisture. For example, the soil half-life of fomesafen under anerobic conditions (flooded soil) is only 3 weeks, but persistence is extended as soil moisture becomes more limited. Labels of fomesafen-containing products (Table 1) specify 10 months must elapse between application and planting corn. Applying a fomesafen-containing product at this time of the 2014 growing season would preclude planting corn before May 2015.
Table 1. Fomesafen-containing products labeled in Illinois.
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