No. 18 Article 6/July 25, 2008

Considerations for Late-Season Herbicide Applications in Soybean

Many areas of Illinois experienced later-than-normal soybean planting due to the frequency and excessiveness of precipitation. The delays in soybean planting also have caused postemergence herbicide applications to be delayed later into the season. This, coupled with the seemingly ubiquitous occurrence of waterhemp in Illinois soybean fields, suggests that postemergence herbicide applications in soybean may continue throughout the remainder of July. However, late-season herbicide applications are not without some risks.

First, larger weeds can be expected to be more difficult to control than smaller weeds because they are older plants and because spray coverage can be limited. Application rate, volume, and spray additives are important factors to keep in mind, especially if you are attempting to achieve good spray coverage on larger weeds. The possibility of herbicide drift that can injure sensitive vegetation is ever-present.

Almost every postemergence soybean herbicide specifies on its label a preharvest interval, or a soybean developmental stage beyond which applications should not be made. Labels of some products, such as Pursuit and Extreme, indicate both a developmental stage (before soybean bloom) and a preharvest interval (85 days). Preharvest intervals indicate the time that must elapse between herbicide application and crop harvest. These intervals are established to allow sufficient time for the herbicide to be broken down or metabolized in the plant. Additionally, the preharvest interval reduces the likelihood of herbicide residues' remaining on the harvested portion of the crop. Failure to observe the preharvest interval may result in residue levels in the crop in excess of established limits. In addition to preharvest intervals, there are restrictions on many postemergence soybean herbicide labels about whether the soybean crop may be used for livestock feed or if treated fields may be grazed as forage. Table 1 details 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 and postemergence) specify the 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 likelihood that sufficient herbicide residues will persist in the soil, which could adversely affect the rotational crop. Some restrictions are based solely on time, while for other products additional factors, such as soil pH and the precipitation received after herbicide application, can influence the length of the crop rotational interval. Rotational intervals for many soybean herbicides can be found in Table 5b of the 2008 Illinois Agricultural Pest Management Handbook.

Another potential risk of late-season postemergence soybean herbicide applications is crop injury. Much of the soybean crop across Illinois is in bloom, and a small percentage has begun setting pods. Many postemergence soybean herbicide labels caution about making applications after the crop has begun to bloom, as crop injury during this time could potentially reduce soybean yield.--Aaron Hager

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