Late-Season Herbicide Applications in Soybean

August 6, 1999
This time of year is generally well past the time when most postemergence soybean herbicides have been applied. Recently, however, we have had several phone calls with questions about spraying postemergence soybean herbicides to try and "clean up" weeds that have emerged above the soybean canopy. In particular, many have recently asked about spraying soybeans to attempt to control waterhemp.

Late-season herbicide applications can pose some significant risks. Obviously, larger weeds can be expected to be more difficult to control because they are older plants and also because spray coverage can be limited. Additionally, many postemergence soybean herbicides (including the postemergence grass control herbicides) have preharvest intervals on their labels that specify the amount of time that must elapse between application and soybean harvest. As the second week of August approaches, the amount of time remaining before soybean harvest may not be sufficient to satisfy certain soybean herbicide preharvest intervals. Preharvest intervals for postemergence soybean herbicides can be found in Table 26, Chapter 2, of the 1999 Illinois Agricultural Pest Management Handbook.

Another potential risk of late-season postemergence soybean herbicide applications is crop injury. Most of the soybean crop across Illinois is in full bloom, and a significant percentage has begun setting pods. Many postemergence soybean herbicide labels caution against making applications after the crop has begun to bloom because crop injury during this time could potentially reduce soybean yield.

Rotational crop restrictions should also be taken into consideration when thinking about making a late-season postemergence soybean herbicide application. Most herbicide labels contain rotational crop restrictions, usually indicating the time that must elapse between the herbicide application and planting the rotational crop. Herbicide applications late in the season may cause a delay in planting the rotational crop next season. Rotational crop intervals for soybean herbicides can be found in Table 2b, Chapter 2, of the 1999 Illinois Agricultural Pest Management Handbook.

We generally do not recommend making late-season postemergence soybean herbicide applications to attempt to control waterhemp. Our data have indicated that herbicide applications made to large waterhemp usually do not provide a satisfactory level of waterhemp control and may cause severe soybean injury.--Aaron Hager, Marshal McGlamery, Christy Sprague

Author: Aaron Hager Christy Sprague Marshall McGlamery