Source: University of Illinois
Given this season's frequent and excessive precipitation, many Illinois soybean fields have experienced later-than-normal applications of post-emergence herbicides. Coupled with the seemingly ubiquitous growth of waterhemp in Illinois soybean fields, this reality suggests that applications may continue throughout July. But producers should be aware that late-season herbicide applications are not without some risks.
First, larger weeds can be expected to be more difficult to control than smaller ones because the plants are older and spray coverage can be limited. Application rate, volume, and spray additives are important factors to keep in mind, especially if you are trying to achieve good coverage on larger weeds. The possibility of herbicide drift that can injure sensitive vegetation is ever-present.
Almost all post-emergence soybean herbicides specify on their labels a preharvest interval or a soybean developmental stage beyond which applications cannot be made (after bloom, for example). Labels of some products may indicate both. Preharvest intervals indicate the amount of time that must elapse between herbicide application and crop harvest. Failure to observe the preharvest interval may result in herbicide residue levels in the crop that exceed established limits.