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Rescuing Fields with Waterhemp (or other) Weed Escapes (07/20/17)

I walked a field last week that was not scouted in May and June (image). The question was “what are my options?”

Rescuing Fields with Waterhemp (or other) Weed Escapes

I walked a field last week that was not scouted in May and June (image). The question was “what are my options?”

  • Rescue control with herbicide. Don’t even think about it. Herbicides are designed to control small, actively growing weeds. Herbicide must contact or translocate to the growing point(s) to effectively kill the weed. Failure of the herbicide to reach each the growing point will result in regrowth from that point on the plant. I counted nine growing points on a 3-inch waterhemp. A 6-inch waterhemp may have double that number. I can only imagine the number on waterhemp escapes, 24 to 30 inches tall.
  • Hand-pulling. A few waterhemp plants do not rob yield. Cordes et al., University of Missouri, found at densities of less than 10 plants per square foot, yields were reduced by only 1% when waterhemp was controlled at 6 inches height. However, waterhemp is a prolific producer of seed, capable of producing greater than 500,000 seeds per plant. Waterhemp robs yield following a herbicide failure and once it has become established in fields. Pulling and bagging waterhemp is challenging work but it is a way of preventing further seed production in a field. Harvest bigger plants first if you must make choices since bigger plants make more seed.
  • Mechanical mowing. It may be necessary under certain situations to mow crop and weeds to prevent seed production, especially in an area too large to hand-weed. Waterhemp is most problematic along ditches or in drowned-out areas of the field. Waterhemp must be mowed multiple times since it will attempt to produce a seed head from a growing point low on the stem, even after mowing in late July or August.
  • Harvest order. The combine is an effective way to distribute waterhemp seed across the field. The combine also may move seed from field-to-field. However, cleaning a combine can easily take 4 to 6 hours and it doesn’t ensure all weed seed will be removed. One way to reduce the impact of weed escapes is harvest order. That is, harvesting weed-free fields first and harvesting weedy fields last. In addition, be careful to harvest the cleaner portions of weedy fields first and leave heavily infested areas of fields for last.

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Tom Peters

Extension Sugarbeet Agronomist

NDSU & U of MN

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