"The smaller the weed, the better," says University of Illinois Associate Professor of Weed Science Aaron Hager. Proper timing of the application of postemergence herbicides provides the corn crop with the best opportunity to express its full genetic yield potential. Allowing weeds to compete with the crop for too long reduces its seed yield. Yield losses can accumulate very rapidly, and the associated costs can far exceed the cost of an integrated weed management program that includes a properly timed application of a postemergence herbicide.

The problem, Hager says, is "We know that the longer weeds are allowed to remain with the crop, the greater the likelihood of crop yield loss, but we don't know the specific day after planting or emergence when weed interference begins to reduce corn yield."

This critical time is influenced by many factors, including the weed spectrum, density of species, and available soil moisture. Weed scientists generally suggest an interval, based upon either weed size (in inches) or days after crop/weed emergence, during which postemergence herbicides should be applied to prevent weed interference from causing crop yield loss. They often recommend removing weeds in corn before they are more than 2 in. tall.

Another reason to apply postemergence herbicides to small weeds is that they are generally easier to control than larger weeds. Application rates of postemergence herbicides are often based on weed size, with higher rates often recommended to control larger weeds.

To be effective, the postemergence herbicide has to be taken into the plant (usually by absorption through the leaves) and then moved to its target site. Younger plant leaves often absorb herbicides more rapidly and completely than older leaves. High relative humidity, adequate soil moisture and moderate to warm air temperatures also favor enhanced herbicide absorption.