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Weed management continues at harvest

Waterhemp is the No. 1 weed escape in fields across southwest Minnesota. Waterhemp and other pigweed species can produce viable seed within 10 days of pollination, according to University of Illinois research.

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Lizabeth Stahl
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WORTHINGTON — Driving through the countryside close to harvest time, it is not too hard to find fields where weeds are poking through the soybean canopy. Although a few plants here and there may not have affected soybean yield much this year, weed escapes can have a dramatic impact on future weed problems, especially considering that one waterhemp plant can produce up to 250,000 seeds or more.

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

What can you do to help prevent weed escapes from giving you future headaches?

First, do some detective work

  • Was there an issue with the application?
  • Did you use the right herbicides, rates and proper 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 high when making postemergence applications) and were weeds actively growing?
  • Did you use the right carrier volume, droplet size and spray pressure?
  • Was your sprayer properly calibrated, or did you have any clogged nozzles?
  • Do you have weeds that are resistant to the herbicides applied?

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

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

Pull weed escapes

Waterhemp is the No. 1 weed escape in fields across southwest Minnesota. Waterhemp and other pigweed species can produce viable seed within 10 days of pollination, according to University of Illinois research. Once viable seed is produced, it is important to carry waterhemp plants out of the field rather than simply dropping them on the soil surface when hand-pulling weeds.

Scout, keep records and clean the combine

Make maps of weedy areas to help plan weed management strategies for next year. Segregate weedy areas of fields and harvest these spots last to help prevent the spread of weed seed into cleaner areas.

Combines are remarkably effective dispersal mechanisms for weed seeds, so check field entrances for new weeds. If you must run the combine through weedy areas (e.g. hand weeding is just not feasible), thoroughly clean out the combine afterwards to prevent the spreading of weed seeds to other fields.

See https://z.umn.edu/combinecleanout for combine cleanout strategies. Also, be on the lookout for Palmer amaranth ( https://bit.ly/3UekhCm ). Report any suspect plants through the MDA “Report the Pest” website at mda.state.mn.us/reportapest.

Impact on exports

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International customers for U.S. soybeans, including China, are rejecting soybean shipments that have more than 1% foreign material (FM). FM includes any material that is not soybean seed, so weed seed plays a significant role in this issue. See the University of Minnesota website on Managing FM at

to learn more.

Lizabeth Stahl is a University of Minnesota Extension crops educator.

Related Topics: WORTHINGTONNOBLES COUNTYAGRICULTURE RESEARCHAGRICULTUREFARMING
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