Farm Progress is part of the Informa Markets Division of Informa PLC

This site is operated by a business or businesses owned by Informa PLC and all copyright resides with them. Informa PLC's registered office is 5 Howick Place, London SW1P 1WG. Registered in England and Wales. Number 8860726.

Serving: IA
Weeds among corn plants. Rod Swoboda
YIELD ROBBERS: The specific time at which weeds begin to reduce yields varies widely. But the longer a postemergence application is delayed, the greater the risk of a yield penalty.

Scouting, using postemergents critical to weed control

Cropping Systems: Weeds can begin to affect yield as early as two weeks after crop emergence.

While scouting fields before applying postemergence herbicides is always important, it will be even more important this year due to dry conditions in April and the likelihood that weeds may have escaped weed management tactics.

Weeds initially grow slowly and can be difficult to identify when small. But once weeds are more visible at about an inch, they begin to grow more rapidly and become difficult to control.

Scouting regularly, identifying problems when they begin to develop and making quick decisions for postemergence herbicide applications are critical steps to managing any weed issues and avoiding escapes this time of year.

Scout and identify issues

As soon as crops emerge, regular scouting will help identify issues in a timely manner. Scouts will want to check areas with known weed issues or high populations from the prior year like field margins, areas prone to drowning out or sprayer skips from 2019. If preemergence herbicides were sprayed before planting this year, look for weeds emerging within the row where planter row units may have moved soil.

Early identification of small plants can be difficult, but weed identification resources like the Iowa State University Extension Weed ID Guide, Second Edition, are useful tools. Once you’ve identified weeds emerging, the clock starts to determine appropriate timing and products for postemergence herbicide applications.

Why are weeds there?

While waterhemp is the most common weed issue across the state, other weeds may be more prominent than normal this year due to escapes from early herbicide applications. Recognizing why weeds are present is important in developing the best management tactics for them. Some key questions to consider:

Was this weed the result of a large input of seed from escaped plants last year? When a large amount of weed seed is produced, we expect larger populations of that species the following year which can reduce herbicide performance.

Should earlier herbicide applications have been effective against the weed species? Possible reasons for escape include:

  • weed not being susceptible to the herbicides applied
  • lack of rain to activate preemergence herbicides

Is it possible that herbicide resistance is developing in this population? Resistance usually begins from single resistant individuals dropping seed around the mother plant, so resistant weeds occur in patches that grow larger over several years.

Post-herbicide applications

After scouting in a timely manner and identifying problem weeds and why they are present, it’s time to make an effective postemergence herbicide application. Choose a post-herbicide based on:

  • crop traits
  • known or suspected resistances to herbicides in the field
  • weed species present and ability of the herbicide to control them
  • weed size

While these are all important factors to base herbicide decisions on, timely application may be the most important step that will be more of a struggle this year. Check herbicide labels to confirm weed size restrictions for complete control. While one herbicide may be labeled to control 4-inch waterhemp, other products may only control 1- to 2-inch waterhemp.

Scouting doesn’t stop there

After postemergence herbicide applications are complete, fields should be scouted within three to seven days and again about two weeks after the application to evaluate weed control. Scouting soon after applications in this manner lets you see whether weeds survived the applications in a much more effective way than waiting until the end of the growing season. Notes regarding effectiveness and weed escapes are invaluable for management the following year.

While weeds are a constant battle in Iowa crop fields, regular scouting and thorough evaluation of current strategies will aid in developing an effective, integrated weed management program that provides effective long-term control.

Anderson is the ISU Extension field agronomist for central Iowa. Contact




Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.