Source: Ohio State University

A dry weather period across most of the Midwest has allowed for an early and rapid start to corn and soybean planting, but continued dry conditions could lead some pre-emergence herbicides to partially fail, said Mark Loux, Ohio State University. In normal years, enough rain is received throughout the spring to ensure pre-emerge herbicides remain effective.

Preemergence herbicides reach maximum effectiveness only if rain moves them down into soil before weed shoots emerge. Once the shoots have emerged, herbicides that act strictly through residual activity have lost most of their effectiveness, unless you are a big believer in "reachback activity."

Original press release