The list of herbicide active ingredients available for pre-harvest applications in corn or soybeans is relatively short, says University of Illinois Extension weed specialist Aaron Hager.

"In corn, glyphosate, paraquat and some formulations of 2,4-D or premixes containing 2,4-D may be applied to provide suppression/control of weeds prior to harvest, while glyphosate, paraquat, dicamba (Clarity) and carfentrazone are labeled for pre-harvest applications in soybean," he said.

Hager encourages growers to consult the respective product label for specific application information. For example, not all formulations of 2,4-D are labeled for pre-harvest applications in corn. Specific application intervals, rates and restrictions can vary by product.

"Pre-harvest herbicide applications should be made soon enough before harvest to allow sufficient time to dry down treated weeds," he said. "Dry-down of weed vegetation may be slowed during periods of cool and wet weather conditions. All products labeled for these applications specify a period of time that must elapse between application and harvest, but additional time may be needed to dry down large weeds."

Hager said contact herbicides usually provide faster weed dry-down than translocated herbicides. Application practices that increase spray coverage of the target vegetation can improve control.

"Be very cautious not to make pre-harvest herbicide applications prior to the crop developmental stage indicated on the respective product label," Hager added. "Applying herbicides prior to these stated intervals may reduce crop seed production or viability."