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Rainbows in the Fields

May 15, 2003

The persistent rains across some areas of Illinois have delayed corn and soybean planting and the implementation of weed control practices on many acres. These delays have allowed existing vegetation to continue growing, and in many fields winter annuals are flowering, producing a colorful landscape (yes, "rainbow" might be a stretch, but you have to admit it caught your attention). In other areas of the state, emerged corn is being sprayed with postemergence herbicides. It's somewhat difficult to consider all possible weed control questions or scenarios for a crop so varied (some still in the bag, some receiving postemergence herbicides), but these are scenarios for consideration:

No herbicide applied, crop not planted. Existing vegetation should be controlled prior to planting. This can be accomplished by either preplant tillage or herbicide application. Tillage would provide the shortest interval between the weed control practice and planting, but if fields are tilled "on the wet side," larger weeds can sometimes survive the tillage operation and continue to be problems after the crop emerges. If you opt for a herbicide to control existing vegetation prior to planting, several factors should be considered:

  1. Some herbicides (2,4-D in particular) have a minimum interval between application and planting. Many times, this interval is put in place to decrease the likelihood of crop injury. Several (not all) 2,4-D formulations are labeled for preplant applications, but not all 2,4-D product labels have identical waiting intervals (if any) between application and corn planting, so it pays to check the respective product label.
  2. Even if no waiting period is specified on the herbicide label, burndown herbicides require time to work. Planting too soon after application can injure the weeds, potentially reducing the level of weed control. Contact herbicides (those that do not move much within the plant following absorption) generally require less time between application and planting than translocated herbicides. Translocated herbicides must have sufficient time to move within the target plant to provide good control.
  3. Adjust the herbicide rate to control the vegetation as it stands now. If you prepaid last fall for a particular burndown herbicide rate, that rate may or may not be sufficient to control the existing vegetation once you can make the application.

No herbicide applied, corn has been planted. We discussed the use of preplant or preemergence herbicides after corn emergence in a previous issue of the Bulletin (see issue no. 7, "Missed the Preemergence Application Window in Corn?"), but a couple of additional points deserve consideration:

  1. Closing the seed furrow can be difficult if planting occurs under wet soil conditions. This in itself can lead to establishment problems, but if a preemergence herbicide will be applied soon after planting, an open seed furrow provides an avenue for direct contact of the seed with the herbicide. Labels of many soil-applied corn herbicides warn that severe corn injury can result if the herbicide comes in direct contact with the seed.
  2. Be especially cautious about making preemergence applications to fields where the corn is within a day or two of emerging, especially with nonselective herbicides or soil-applied herbicides that should not be applied after crop emergence. Even if the crop hasn't fully emerged or isn't yet visible from the road, small cracks or other openings in the soil surface may allow the herbicide to come into direct contact with the emerging coleoptile. Do not use nitrogen fertilizer as the herbicide carrier if corn has begun to emerge.

Herbicide applied, crop not planted. These fields, especially fields in which the herbicide application was made several weeks ago, are excellent candidates for scouting prior to planting. If weeds are present, you should consider controlling them prior to planting. Why not just wait and spray after planting? That may be a feasible option, but the planting operation will likely injure some of the weeds, and they will need time to recover before being sprayed. Waiting to control the existing weeds after planting is also gambling that the weather will cooperate and allow you to make the application before the existing weeds begin to adversely impact the crop.

Herbicide applied, crop has been planted. Whether you initially planned to use a soil-applied herbicide program for weed control or a soil-applied followed by postemergence herbicide program, keep a close eye out for weed emergence. Heavy precipitation in many areas of the state may have moved some soil-applied herbicides deeper into the soil profile than is conducive for good weed control. Less-than-ideal growing conditions (especially excessive soil moisture) may also increase the likelihood of corn injury from some soil-applied herbicides. When applying a postemergence herbicide, remember that a corn crop under stress from adverse environmental conditions may be more prone to develop significant injury symptoms. Spray additives that enhance herbicide penetration into the weeds also help increase the rate of uptake into the corn crop. Rapid herbicide uptake coupled with slow corn growth because of adverse environmental conditions is a good recipe for corn injury.

One Other Consideration

If your initial plan was to plant a particular herbicide-resistant corn hybrid but you decide to switch to a different hybrid that does not have the resistance trait of your initial selection, make sure to note this and communicate the change to whoever will apply the postemergence herbicide. This may sound like common sense (actually it is), but each year we hear of an instance where an entire corn field was killed following a postemergence herbicide application because someone didn't remember or know that hybrid selection had changed.--Aaron Hager and Christy Sprague

Author: Aaron Hager Christy Sprague

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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