Soil-residual herbicides are important components of integrated weed management programs.  Reducing the number of weeds exposed to foliar-applied herbicides helps reduce the selection intensity for weeds resistant to foliar-applied herbicides. Residual herbicides applied with postemergence soybean herbicides also can reduce the need for a second postemergence application. However, simply applying a soil-residual herbicide does not guarantee the product will provide the desired level or duration of weed control. 

Soil-residual herbicides applied with postemergence herbicides require precipitation to move them into the soil solution where they are available for uptake. Herbicide effectiveness is reduced when a soil-residual herbicide is sprayed on a dry soil surface with no precipitation for several days following application. Residual herbicides generally require 0.5-1.0 inch of precipitation within 7-10 days after application for optimal incorporation. Soil condition, soil moisture content, residue cover, and the chemical properties of the herbicide influence how much and how soon after application precipitation is needed for optimal activity.

Also, keep in mind the labels of soil-residual products that can be applied postemergence include a maximum soybean growth stage or time interval prior to harvest beyond which an application cannot be made.  A sample of these growth stages and pre-harvest intervals is presented below. Many of the products included can be tankmixed with dicamba- or 2,4-D- containing products labeled for use in dicamba-  or 2,4-D-resistant soybean varieties, but be sure to check the respective tankmix web pages prior to making the application.

Originally posted by University of Illinois.