Historically, we have not recommended early preplant herbicide (EPP) application in our region with a couple of exceptions such as for marestail control. In general, residual herbicides applied more than two weeks ahead of planting almost always require additional weed control often in the form of a post application. With the adoption of Roundup Ready soybean and corn, there may be more opportunity to apply a residual burndown program two or more weeks ahead of planting with the idea of coming back with a well-timed post treatment. A number of herbicide labels specify in days what is allowed EPP. Herbicide rates may increase for EPP, so consult a current herbicide label for specific use directions. The table below provides some spring EPP application intervals for some common corn and soybean products.
Early Preplant (EPP) Herbicide Applications...How Early Can They be Sprayed — Crops and Soils — Penn State Extension
Regarding some of the popular PPO/Group 14 soybean herbicides (e.g., Valor products, Envive, Fierce, Trivence, Authority products, Sonic, Surveil, others), if using these in combination with Sharpen/Verdict/OpTill for control of marestail, there is a 14 day restriction before planting. So, if you tank mix Sharpen-containing products with another PPO/Group 14 residual herbicide (with the exception of Prefix) you must wait 14 days to plant soybeans. Or if Sharpen is applied EPP in a burndown mix without the PPO residual product, then you must wait 2 weeks before applying the Group 14 residual herbicide at planting.