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.