The objective of an early preplant (EPP) herbicide application is to apply the herbicide(s) prior to the germination of weed seeds. Thus, you must time the application, in part, to soil temperature. Most weeds will not germinate when soil temperature is 50 or less. When you consider an EPP application, evaluate the residual activity and relative mobility of the herbicide. If you apply an EPP herbicide and rainfall delays planting, the herbicide may degrade or move in the soil profile, compromising weed control. Also consider that no tillage planters typically remove some soil in the row, and the soil contains the EPP herbicide treatment. The more soil the planter moves, the less consistent the weed control in the row.
A split application of an EPP treatment followed by a preemergence treatment or a postemergence treatment provides more consistent weed control than a single EPP application. Further, if you use a split application, you can use a nonselective herbicide if the EPP application is not completely effective.
Concern about delayed planting relative to the EPP application is particularly valid for soybean treatments. EPP treatments are typically applied in late March or early April, thus providing a several week period before crops are planted. Generally, we recommend a split application for soybean EPP treatments.
Most preemergence herbicides registered for Iowa row crops have labels describing an EPP application. New herbicides such as Frontier, Harness Plus, and Surpass also have an EPP label. Give specific consideration to Command EPP; fields south of I-80 must be applied by April 1, but prior to field green-up, while fields north of I-80 must be applied by April 10, but prior to field green-up.
Environmental conditions this spring suggest that a number of weeds will germinate prior to typical EPP applications. EPP treatments may not provide any postemergence activity. Thus, you should carefully scout fields prior to EPP applications to determine if weeds have begun to germinate and emerge. Several soil-applied herbicides have postemergence activity, including the triazines, imidazolinones (i.e. Pursuit), sulfonylureas (i.e. Canopy), and sulfonamides (i.e. Broadstrike). If the intended EPP herbicide treatment has no postemergence activity, consider the use of a nonselective herbicide such as Gramoxone Extra or Roundup.