Winter Annual Weeds Weaken Tomorrow’s Yield

Pests and soil conditions fade with out-of-control weeds. ( Darrell Smith )

While harvest might still be your priority, keep field prep for 2020 in mind and take notes from the combine. Specifically, which weeds you’re seeing and where.

“Fall burndown is something to consider if the season allows time,” says Lyndsie Kaehler, U.S. corn herbicides product manager for Corteva Agriscience. “It’s going to be a tight window, but if you have the opportunity, get out and control weeds early because it’s going to be a lot easier now.”

You’ll likely be seeing summer annual weeds from the combine, with a few winter annuals sneaking in here and there. Document the location of mature summer annual weeds as the weed seed bank will be what you battle next summer.

However, don’t discount the winter annual weeds.

“The impact of winter annual weeds in cropping systems is sometimes overlooked because these weeds typically complete most of their life cycle prior to or shortly after corn and soybean planting,” says Lowell Sandell, weed science Extension educator at University of Nebraska.

Winter annual weeds will impact future plantings though. They act as hosts for pests, affect soil temperature and can sequester moisture and nutrients from future crops.

  • Winter annual weeds give insects and parasites a leg up. Henbit, for example, acts as an alternate host for soybean cyst nematode, who’s populations are already challenging soybean growers in many parts of the country. In addition, black cutworm can use several weed species to boost populations prior to corn and soybean emergence, populations that could decimate crop stands.
  • Residue buildup keeps soils cooler, longer. According to research by the University of Missouri, when winter annual residue has been removed from fields it increases soil temperatures by up to 5° F in corn and as much as 8° F in soybeans. In tight planting windows this could be the difference between timely and late planting.
  • Weeds steal moisture and nutrients from crops. Winter annuals can reduce soil moisture by up to 13% in corn and 6% in soybeans, according to Missouri research. The same weeds, or their residue, tie up or use valuable nutrients that could progress corn and soybean crops.

“It’s really important we go out with a plan for a burndown pass that’s economical and sound,” Kaehler says. “Be versatile, and when we get to spring have a pre- and post-pass with multiple modes of action; that’s a pretty solid plan. 

Manage Weeds on Prevent Plant

For fields that didn’t get planted this year, get ready for a battle against problem weeds.

“The old weed science adage ‘one year’s seedling equals seven years weeding’ reinforces the need to adequately manage weeds on prevented planting acres,” said Aaron Hager, University of Illinois Extension weed scientist.

Hager outlines four management techniques to help farmers gain control over problematic weeds. While it can be a financial strain to control weeds on unplanted acres, it is critical to set fields up for success in years to come.


Certain tillage can bury and destroy already germinated weeds. Hager warns vertical tillage equipment might not do the job, but tandem disks, field cultivators and other soil- moving tools should. Tillage might stimulate germination and emergence of other weed seeds, so it might require multiple passes to prevent summer annuals from going to seed.


This practice can help suppress weed growth but might not effectively stop seed production of all summer annual weeds. Some weeds will regrow or run tillers below cutting height. If you use this practice, run the mower as low as possible. Hager recommends mowing, followed by tillage.


Consider talking to a local agronomist to learn what resistance is in your area before selecting herbicides. Hager recommends combining glyphosate with 2,4-D or dicamba to control emerged waterhemp, marestail and giant ragweed. Because pigweed species have extended emergence periods, it will require two to three applications before frost.

Cover crops

Species such as rye, wheat, sudangrass or other well-established grass cover crops can limit weed emergence. Control weeds before planting cover crops to give them the best start possible.