The list of herbicide active ingredients available for preharvest applications in corn or soybean is relatively short. In corn, glyphosate, paraquat, and some formulations of 2,4-D or premixes containing 2,4-D may be applied to suppress/control weeds before harvest, while glyphosate, paraquat, dicamba (Clarity), and carfentrazone are labeled for preharvest applications in soybean. Be sure to consult product labels for specific application details. For example, not every formulation of 2,4-D is labeled for preharvest application in corn, and specific application intervals, rates, and restrictions can vary by product. See Table 1 for a compilation of general guidelines by product.
Preharvest herbicide applications should be made soon enough before harvest to allow sufficient time for dry-down of treated weeds. Dry-down of weed vegetation may be slowed during periods of cool and wet weather. All products labeled for these applications specify a period that must elapse between application and harvest, but additional time may be needed for dry-down of large weeds. Contact herbicides usually provide faster dry-down than translocated herbicides. Application practices that increase spray coverage of the target vegetation can improve control.
Also, be very cautious not to make a preharvest herbicide application before the crop development stage indicated on the product label. Applying herbicide before the stated interval may reduce crop seed production or viability.