Numerous herbicide options are available for postemergence weed control in soybean. The majority of these control either grass or broadleaf weed species, but some have substantial effects on both grasses and broadleaves. This fact lends support to the need for scouting fields before applying a postemergence herbicide, as not every herbicide will adequately control every weed species that may be present.|
Soybeans are usually tolerant of most postemergence herbicides at early developmental stages. Some postemergence herbicide labels do specify a minimum growth stage that the soybeans must have attained before an application is made. For example, the labels of Classic (chlori-
muron) and Pinnacle (thifensulfuron) specify that postemergence applications of these products may be made any time after the first trifoliolate has opened or has fully expanded, respectively. While it is a bit early in the season to say much, keep in mind also that most postemergence herbicides have some type of application cutoff timing on their labels (for example, prior to soybean bloom).
With the advent of herbicide resistant/tolerant soybean varieties, communication between who planted the soybean field and who will do the postemergence spraying of an herbicide requiring a resistant/tolerant soybean variety is absolutely essential.
Table 5 and Table 6 are reprinted from the 1999 Illinois Agricultural Pest Management Handbook and contain information on postemergence soybean herbicide efficacy ratings on grass and broadleaf weed species, respectively.--Aaron Hager, Marshal McGlamery, Christy Sprague