Midwest Soybeans: 4 Tips for Fall Weed Management

Delayed planting has dogged farmers in much of the nation’s midsection. As if the threat of delayed harvest and substantial yield reductions weren’t enough, wet weather has given weeds in many areas the upper hand.

Farmers in many areas were unable to spray herbicides at the optimum time, leaving many to deal with weedy fields that posed yet another threat to this year’s yields. Others didn’t use herbicides with residual control and were prevented from returning later with a post-emergence application.

Uncontrolled weeds can impact farmers beyond this growing season. Left unchecked, weeds can quickly populate the soil seed bank, giving farmers challenges for many seasons to come.

“The old adage is one seeding equals seven years of weeding,” says University of Illinois Extension weed scientist Aaron Hager, Ph.D.

Fortunately, it’s not too late to do something about them.

University of Missouri weed scientist Reid Smeda, Ph.D., says despite the late calendar date, it’s important to get weeds under control to keep as many seeds as possible from entering the soil. Late-emerging weeds won’t get as large as those that sprouted earlier and they’re less likely to impact soybean yield, but they can still pose problems.

“Waterhemp that emerges in late July can still produce as many as 30,000 seeds,” Smeda says. “Farmers need to think about keeping those seeds out of the soil seed bank.”

Late-Season Concerns

Smeda says that farmers who do spray later in the year need to keep in mind their herbicide’s residual effects. Next year’s planting intentions will be a factor in determining which herbicides to use.  Products with residual activity may not be a good option for farmers who intend to follow up their soybean crop with corn next year.

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In addition, some herbicides with postemergence activity may not be effective on larger weeds. Farmers need to read and consider the instructions on the herbicide label to ensure it will be effective.

Annual grasses, like giant foxtail, fall panicum and large crabgrass, thrive in wet conditions and many fields have dense populations. Smeda says the shallow, fibrous root systems of these grasses can rob valuable moisture and nutrients from crops, so it’s important for farmers to get rid of them now, before they go to seed.

“In fallow fields lacking a crop, farmers should also be concerned with the establishment of perennial weeds, such as johnsongrass and curly dock,” Smeda says.  “Herbicide-resistant weeds in fallow fields can also generate large numbers of seed that will impact future crops.”

While crops may not be growing in fallow fields and drowned-out areas this year, weeds certainly are. Whether it’s by herbicide or mechanical means, vegetative growth needs to be controlled.

Hager says farmers who are dealing with weedy fields should also consider adjusting their harvest schedule depending upon their type and stage.

“You may want to harvest fields with the worst weeds last to keep from spreading seeds because combines are a very efficient means to spread weeds,” Hager says.

Four tips for managing weeds this fall:

  1. Beware of late-season application of residual herbicides that can impact next year’s crops.
  2. Read herbicide labels and know the weed type and seed stage to select the proper product for your situation.
  3. Don’t ignore your fallow fields. Prevent establishment of troublesome perennials in fallow fields and drowned-out areas.
  4. Harvest your weediest fields last – weed seeds would love to spread to your other fields by hitchhiking a ride on your equipment.