No. 17 Article 2/July 29, 2011

Be Cautious With Late-Season Herbicide Applications in Soybean

Recent reports indicate that over two-thirds of the Illinois soybean crop has reached the bloom stage, and a glance at the office calendar indicates July will soon give way to August. Many remember an era when weed control at this time of year consisted of nothing more than a hoe or weed hook, but contemporary practices often include late-season herbicide applications. Herbicides applied at this time of year typically target large weeds; control is often less than desired, and application increases the potential for other problems.

Perhaps the most common scenario is applying diphenylether herbicides (Flexstar, Cobra, and Ultra Blazer, for example) to target waterhemp plants not controlled by a previous herbicide application. In non-GMO and glyphosate-resistant soybean varieties, these herbicide active ingredients can provide good to excellent control of small waterhemp, but control is often poor with plants exceeding 5 to 6 inches. These herbicides do not translocate following absorption into the plant, and the contact leaf burn they cause can vary with product, spray additive, and environmental conditions. Soybean injury following application of these herbicides can be severe and should be avoided at this point of the growing season. Consistent control of large waterhemp plants at this time can be achieved only through physical removal.

Remember that nearly all herbicide labels (soil-applied and postemergence) specify rotational crop intervals, or the amount of time that must elapse between herbicide application and planting a rotational crop. This becomes particularly important with late-season applications. These intervals are established to reduce the likelihood that herbicide residues will persist in sufficient quantities to adversely affect the rotational crop. Though some restrictions are based solely on time, 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. Dry soils also 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.

Fomesafen has the longest soil residual activity among the three postemergence diphenylether herbicides. Soil half-life values (the time required for half of the applied herbicide to degrade) for fomesafen have been reported to range from 100 days to 6 to 12 months. The range depends on 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. The Flexstar label has a 3-month rotational interval for small grains, a 10-month interval for corn, and an 18-month interval for sorghum, alfalfa, and sunflowers.--Aaron Hager

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