"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.

 

Resistance and labels

Waterhemp plants with resistance to one or more herbicide sites-of-action challenge the effectiveness of many postemergence herbicides. The occurrence of herbicide-resistant weed biotypes and populations is likely to escalate across areas of central Illinois during the 2012 growing season. Depending on the resistance mechanism, these plants may not demonstrate much injury or a reduced rate of growth following a herbicide application.

"We anticipate that many of these herbicide-resistant populations will not be discovered until several days after the initial postemergence herbicide application," Hager says. A follow-up or "rescue" herbicide application to control resistant plants is more likely to be successful if the initial application is made when plants are 3 in. tall or smaller than it would be if they are 6 in. tall or larger.

The choice of foliar-applied corn herbicides could be affected by prior application of soil insecticides. Specifically, using an organophosphate (OP) insecticide at planting or after corn emergence could restrict the use of herbicides that inhibit either the ALS or HPPD enzymes. Be sure to consult the most current product labels.

Labels of most postemergence corn herbicides allow applications at various crop growth stages, but almost all product labels indicate a maximum growth stage after which broadcast applications should not be made. A few specify a minimum growth stage before which applications should not be made. These growth stages are usually indicated as a particular plant height or leaf stage; sometimes both are listed.

"For product labels that indicate a specific corn height and growth state, be sure to follow the more restrictive of the two," says Hager. Application restrictions exist for several reasons, but of particular importance is the increased likelihood of crop injury if applications are made outside a specified growth stage or range.