Source: Penn State Extension
Historically, early pre-plant herbicide (EPP) application has not been recommended in Pennsylvania with a couple exceptions, such as marestail control. In general, residual herbicides applied more than 2 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 2 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.
Regarding some of the popular PPO/Group 14 soybean herbicides (e.g., Valor products, Envive, Fierce, Trivence, Authority products, Sonic, Surveil and 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 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.