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Preharvest Restrictions for Postemergence Soybean Herbicide Applications

August 9, 2002
In certain parts of the state, herbicide applications are still being made to later-planted fields, double-cropped soybeans, and areas that may need a little cleanup from weed escapes. Knowing that a number of these fields will undoubtedly be treated for the first time or re-treated, some considerations need to be made when choosing a herbicide for that final application.

Almost all postemergence soybean herbicides have what is known as a preharvest interval. Preharvest intervals specify 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 residue levels of the herbicide in the crop in excess of the legal limits that were established at the time the herbicide received its label. Along with preharvest intervals are also restrictions on many of the postemer-gence soybean herbicides on whether the soybean crop may be used for feed or graze forage. Table 1 contains information about the preharvest intervals and grazing restrictions for a number of the postemergence soybean herbicides.

Another time interval that is important to observe is the rotational crop interval. Almost all herbicide labels have rotational crop tables. These rotational intervals specify the amount of time that must elapse between herbicide application and the planting of a rotational crop. This is particularly important with late-season herbicide applications. These intervals are established to reduce the possibility that sufficient herbicide residues may persist in the soil that could adversely affect the following 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 states that field corn can be planted 9 months after application. However, this interval is extended 2 additional months if applications containing chlorimuron are made after August 1. Other examples include the 10-month rotational interval for field corn and the 4-month rotational interval for wheat from 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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