Post-harvest weed control helps minimize spring weed emergence

Fall weed control can pay big dividends as soon as next spring, says Greg Dahl, Weed Science Society of America (WSSA) president. He urges farmers and land managers to check with local weed control experts, Extension specialists and/or WSSA weed scientists to glean tips on post-harvest weed management that can minimize weed proliferation during 2025. 

″Certain residual herbicides applied in the fall can provide weed control in the early spring, allowing growers to start with clean fields,″ says Dahl. ″This can be especially important if the spring becomes wet and sowing is delayed.″  

Fall is a busy time for farmers, but it’s no time to forget about weeds, he emphasizes. ″In fact, post-harvest fall weed control might be the ideal time to eliminate your most troublesome weed species,″ says Dahl. 

Fall weed management can be vital, especially to control winter annual weeds, agrees Aaron Hager, Ph.D., WSSA member, and University of Illinois Weed Science Extension Specialist. ″Whenever managing any weeds, consider both your purpose and your timing,″ he says. ″Winter annual weeds like marestail or horseweed are easier to control in the fall while they are still small. If left untreated after harvest, most winter annual weeds that successfully overwinter often will be more difficult to control in the spring.″ 

However, he cautions that winter annual weeds do not always emerge immediately after harvest, so scouting and weed mapping are key first steps. ″I always encourage people to scout, no matter what time of year,″ says Hager. ″I’d also caution growers to avoid targeting summer annuals with fall applications of residual herbicides, because many of those species have prolonged emergence patterns that are better handled in spring.″ 

Still, compared to spring herbicide applications, fall applications of translocated herbicides on most perennial weeds can translocate more effectively to the roots for better control, he adds. In addition, Hager notes that fall weed control can be advantageous in cases where higher herbicide application rates are possible due to fewer crop damage concerns compared to spring applications. On the other hand, fall residual herbicide applications can limit which crops can be sown in the spring and should be planned carefully. 

To improve weed management outcomes, farmers should also take strategic steps to avoid weed seed dispersal during both harvest and fall tillage operations, says Rodrigo Werle, Ph.D., WSSA member, and University of Wisconsin-Madison Associate Professor & Extension Cropping Systems Weed Scientist. ″It’s better to start harvest and tillage operations from weed-free to low-weed infestation fields, and lastly, to weedy parts and fields,″ he advises. ″Cleaning your equipment – especially combines – after use in weedy areas is also a best management practice to minimize weed dispersal.″ 

Temperature is another key factor in weed control. ″Avoid spraying foliar herbicides such as glyphosate and synthetic auxins when temperatures are below 50 F,″ advises Werle. ″Warmer temperatures are necessary for proper herbicide uptake and translocation.″  

Cover crops are an additional fall weed management tactic to consider, particularly to obtain weed suppression benefits. ″Cover crops can suppress weeds through direct competition with emerging weeds, or suppress their emergence through allelopathic compounds, and/or by making the topsoil environment less conducive for weed seed proliferation,″ points out Werle. ″Assuming enough biomass accumulation before termination, cover crop mulches can suppress weed seed germination, emergence, and growth.″ 

Most importantly, safety needs to remain the top priority. ″Remember to stay safe at all times, especially when operating or moving around mechanized equipment,″ says Werle. ″When cleaning sprayers and combines, please assure that you’re using proper personal protective equipment.″