Crop & Pest Report


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Mechanical Weed Control: Mowing, Hoeing and Pulling (07/26/18)

Rows have closed in most soybean, dry bean and sugarbeet fields.

Mechanical Weed Control: Mowing, Hoeing and Pulling

Rows have closed in most soybean, dry bean and sugarbeet fields. Weed control looks mostly good, but there are places where weed escapes are pushing through the canopy. Schedule time to mow, hoe or pull weeds to prevent them from making seed and continue to actively manage weed populations in ditches and along field edges. Weeds along field edges, especially in areas with drown out, have less competition from crop and are actively growing and will produce seed to add to the weed seed bank.

Why mow weeds in ditches and along field perimeters? There are several good reasons. First weeds along field perimeters might introduce species diversity into cultivated fields since new or different weeds often migrate from undisturbed ditches into cultivated fields. Weeds are bad enough, but weeds representing different species are even worse. Second, weeds especially along the edges of fields potentially are biotypes selected for resistance to herbicides since they often receive only a partial herbicide dose when they are sprayed. Finally, mowing along the field perimeter reduces the number of weed seeds that may eventually find their way into cultivated fields.

Manage weeds along the outside of the field perimeter by mowing weeds shortly after they begin to flower and by mowing regularly in August and September to ensure there is no regrowth. Mow weeds as close to the crop as possible or perhaps one row into the crop if necessary to eliminate weeds. Don’t forget about areas of the field with reduced stand. Managing weeds in wet spots, salt pockets, or other areas with reduced crop stand is important at preventing weed seed production and the addition of new seeds to the seedbank.

Why are weed escapes important to manage, especially if there are only a few weeds in fields? A few weed escapes such as waterhemp in soybean or sugarbeet probably will not rob yield. However, escapes produce a tremendous amount of seed. Waterhemp growing in areas of the field without crop competition can produce greater than 500,000 seeds per plant (fewer under competitive environments) and seed remains viable from 4 to 6 years. Applying herbicide to waterhemp near flowering is not recommended because this practice does not significantly reduce the amount of seed waterhemp plants produce.

Finally, there is great urgency to remove weeds once they begin to flower. Researchers at the University of Illinois examined the number of days for female waterhemp plants to produce viable seed after the flowers were pollinated. Female waterhemp plants were pollinated for 24 hours and then separated from the male plants In the Illinois study. Branches from female plants were harvested at various intervals after pollination and placed under either warm (86 degrees Fahrenheit) or cold (-4 degrees Fahrenheit) conditions for 48 hours, then stored at room temperature. Researchers then measured germination to determine how soon after pollination seeds were viable. They found seeds stored for 48 hours under warm conditions were viable 7 to 9 days after pollination; seeds stored under cold conditions were viable 11 days after pollination.

Carry a plastic garbage bag to physically remove weeds from the field. Why go through the effort to pull weeds if they still can produce viable seed? Remove weeds from fields and either allow them to dry and then burn or ensile to eliminate opportunity for viable seed.


Tom Peters

 Extension Sugarbeet Agronomist

NDSU & U of MN

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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