Weed management continues at harvest – The Globe

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WORTHINGTON – Driving through the countryside as harvest approaches, it’s not too hard to find fields where weeds are growing through the canopy of soybeans. Although a few plants here and there may not have affected soybean yield much this year, weed leakage can have a dramatic impact on future weed problems, especially since a plant of aquatic hemp can produce up to 250,000 seeds or more.

The economic impacts of weed escapes can also be felt at the elevator and via export markets.

What can you do to prevent leaking weeds from giving you future headaches?

First, do some detective work

  • Was there a problem with the app?
  • Did you use the correct herbicides, doses and adjuvants?
  • If you used a tank mix, were there any antagonism issues between tank mix partners?
  • How big were the weeds at the time of application (remember the goal is to target weeds less than 4 inches in height during post-emergent applications) and were the weeds actively growing?
  • Did you use the correct carrier volume, droplet size and spray pressure?
  • Was your sprayer properly calibrated or did you have clogged nozzles?
  • Do you have weeds resistant to the herbicides applied?

Poor weed control can be a symptom of many different problems, so proper diagnosis of the problem(s) can help prevent problems in the future.
Manage fence rows and field edges

Weed infestations often start from the edges of fields and fence rows. If left untouched, weeds can be picked up by the combine, resulting in good weed seed spread in the field. Tillage can then spread the weed seeds even further into the field. Mowing these areas can help prevent or minimize seed production.

Pull weed leaks

Waterhemp is the #1 weed in southwestern Minnesota fields. According to research from the University of Illinois, acacia and other amaranth species can produce viable seeds within 10 days of pollination. Once viable seed is produced, it is important to transport water hemp plants out of the field rather than simply dropping them onto the soil surface when manually pulling weeds.

Scout, keep records and clean the combine

Make weed area maps to help plan weed management strategies for the next year. Separate weed areas from fields and harvest these areas last to help prevent the spread of weed seeds to cleaner areas.

Combines are remarkably effective dispersal mechanisms for weed seeds, so check field entrances for new weeds. If you must drive the combine through weedy areas (for example, hand weeding is simply not possible), clean the combine thoroughly afterwards to prevent the spread of weed seeds in other fields.

See https://z.umn.edu/combinecleanout for combining cleaning strategies. Also be on the lookout for Palmer’s pigweed (https://bit.ly/3UekhCm). Report any suspicious plants through MDA’s “Report the Pest” website at mda.state.mn.us/reportapest.

Impact on exports

International customers of U.S. soybeans, including China, reject shipments of soybeans containing more than 1% foreign material (FM). FM includes any material that is not soybeans, so weed seeds play a big part in this problem. See the University of Minnesota FM Management website at

to learn more.

Elizabeth Stahl is an extension crops educator from the University of Minnesota.

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