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Considerations for Late-Season Soybean Herbicide Applications

July 14, 2000
With mid-July upon us, many areas of Illinois have completed their postemergence soybean herbicide applications. However, there are still applications being made to late-planted fields, double-crop soybeans, and fields needing a "cleanup" due to weed escapes. With a number of fields yet to be treated for the first time and some to be resprayed, there are a number of considerations when choosing a postemergence herbicide for that final application.

Almost all postemergence soybean herbicides have a preharvest interval specified on their respective labels. Preharvest intervals indicate the amount of time that must elapse between herbicide application and crop harvest. These intervals are established to allow sufficient time for the herbicide to break down or metabolize in the plant. Failure to observe the preharvest interval may result in herbicide residue levels in the crop in excess of established limits. In addition to preharvest intervals, there are also restrictions on many postemergence soybean herbicide labels regarding the use of the soybean crop for feed or grazed as forage. Table 1 contains information regarding preharvest intervals and grazing restrictions for a number of postemergence soybean herbicides.

Another interval that is important to observe is the rotational crop interval. 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. This becomes particularly important with late-season herbicide applications. These intervals are established to reduce the possibility that sufficient herbicide residues will persist in the soil that could 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, influence the length of the crop rotational interval. For example, the Classic label indicates that field corn may be planted 9 months after application; however, the interval is extended 2 additional months if applications containing chlorimuron are made after August 1. Other examples include a 10-month rotational interval for field corn and a 4-month rotational interval for wheat following applications of Authority, Flexstar, and Reflex. Table 2 contains rotational crop intervals for soybean herbicides.--Christy Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague

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

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