Issue No. 22, Article 3/September 4, 2009
Preharvest Herbicide Applications
Despite efforts to keep fields devoid of weeds, there are nearly always instances in which weeds somehow avert management practices and remain in fields as crop harvest draws closer. These weeds can cause problems during harvest operations and potentially return weed seed back to the soil seedbank. For these reasons, farmers sometimes consider making a preharvest herbicide application.
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 in soybean, glyphosate, paraquat, dicamba (Clarity), and carfentrazone are labeled for preharvest applications. Be sure to consult the respective product label for specific application information. For example, not all formulations of 2,4-D are labeled for preharvest applications in corn, and specific application intervals, rates, and restrictions can vary by product. (See Table 1 for a summary of general guidelines.)
Preharvest herbicide applications should be made soon enough before harvest to allow sufficient time to dry down 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 to dry down large weeds. Contact herbicides usually provide faster weed dry-down than translocated herbicides. Application practices that increase spray coverage of the target vegetation can improve control.
While preharvest herbicides are unlikely to prevent weed seed production, previous research has demonstrated that herbicides can impact seed production or viability; however, the effects often depend on several factors, including the herbicide used, weed species, and application timing in relation to the stage of weed seed development. For example, researchers Clay and Griffin (Weed Science 48:481-486) investigated cocklebur seed production as influenced by glyphosate application timing. Their results showed that cocklebur seed production was significantly reduced when glyphosate was applied at the earliest timing (initial seed set), but little impact occurred when application was delayed until the latest timing (physiological maturity). Given the current developmental stage of weeds in Illinois, preharvest herbicides may not do much to limit weed seed production.
Also, be very cautious not to apply preharvest herbicide before the crop development stage indicated on the product label. Such application may reduce crop seed production or viability.
While preharvest herbicide applications can provide some benefits, it might be helpful to temper your expectations. Large/mature weeds are more difficult to control than small/immature weeds. Crop yield loss caused by weed interference has already occurred by the time preharvest applications are made, and such losses are not "restored" even if the application successfully dries down all weed vegetation. Finally, be very cautious about physical and/or vapor drift from preharvest herbicide applications.--Aaron Hager