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Corn+Soybean Digest

Analyze your weeds

September soybean fields in Nebraska, Iowa and Minnesota displayed large numbers of escaped waterhemp. Perhaps this was the case in other states, too. While your combine journey across all your acres is fresh in your mind (and your partners’ minds), take time to evaluate your weed control program to enhance future effectiveness. If weed control was less than optimal this year, questions to ask yourself include:

Did I use a preemergence herbicide?

PRE herbicides can greatly enhance control of weeds like waterhemp that emerge over a long period of time. Use of a PRE herbicide helps provide a wider window in which to make a timely postemergence application because early-emerging weeds are controlled. A PRE herbicide can also increase the diversity of chemistries used. Using multiple sites of action is a key, long-term strategy to combat weed resistance. Be sure, however, to use a PRE herbicide that is rated highly in control of the problem weeds in your fields.

How large were weeds at the time of application? Repeated use of glyphosate over time has selected for weed populations that are tougher to control or even resistant to glyphosate. As a general guideline, smaller weeds are easier to control. Target 2- to 3-inch weeds at application. Using a PRE herbicide (see point above) makes this goal easier.

What herbicide(s) did I use postemergence?

Although not all weed escapes are due to resistance issues, resistance appears to be expanding throughout the region. Waterhemp is a prolific seed producer, with the potential to produce over 500,000 seeds per plant, and up to 40% of seeds shed in a year can emerge the following year. Considering these biological factors, one resistant plant can lead to an out-of-hand population within three years if the same herbicide continues to be applied. Diversify the sites of action used postemergence, and select products rated highly in control of your problem weeds. For example, Liberty herbicide used on LibertyLink corn or soybeans will control small waterhemp and provides a site of action different from glyphosate in the Roundup Ready system.

What rate of herbicide did I use?

Full-labeled rates are recommended from a herbicide resistance management perspective, as well as to help provide maximum control.

Would nonherbicide tactics help my weed control?

Timely cultivation can enhance weed control and help manage resistant weeds. Hand-pulling may be a viable option, too, particularly if weed escapes are in patches. Note which fields had significant weed escapes this year and thus the highest contributions to the weed seed bank. Target these fields first when making herbicide applications, and utilize a diversified weed management strategy. For resources to aid in weed management decisions, visit the University of Minnesota Extension Crops website. And check out effective use of multiple sites of action to manage resistance information by Iowa State University.

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