Maximum Weed Sizes for Post-emergence Soybean Broadleaf Herbicides

June 23, 2000
Surplus rainfall, hot temperatures, and some delayed soybean planting have all combined to create significant broadleaf weed problems in many soybean fields. Fields that received a soil-applied herbicide before or immediately after planting have, in general, had less weed pressure than those fields where no herbicide has yet been applied. In many of these "total post" fields, broadleaf weeds have become quite large, and control with post-emergence herbicides may not be as complete as desired.

One "good" point with respect to past and current environmental conditions is that the weeds have not been under moisture stress conditions and are relatively "soft." This may translate into better activity from many post-emergence broadleaf herbicides. Unfortunately, the soybeans are also "soft," which increases the likelihood for crop response following the postemergence herbicide application.

Table 2, reproduced from the Illinois Agricultural Pest Management Handbook, contains maximum broadleaf weed sizes as indicated on several postemergence soybean herbicide labels. The table lists only annual broadleaf weed species, but many of these herbicides also afford some suppression or control of certain perennial broadleaf species. Additionally, several of these herbicide labels indicate suppression of certain larger annual broadleaf species that are not included in this table. Tank mixes may broaden the control spectrum of a given herbicide, but consult the respective labels for spray additive recommendations and special precautions when tank-mixing herbicides.--Aaron Hager and Christy Sprague

Author: Aaron Hager Christy Sprague