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Issue No. 5, Article 2/May 6, 2011

Weed Management Reminders for a Wet Spring

Much of the vegetation visible from the road is winter annual species, including the now obvious yellow-flowered cressleaf groundsel (butterweed) and yellow rocket. Certain summer annual species, such as common lambsquarters and smartweeds, also have begun to emerge. When field conditions become conducive for planting, several possible scenarios exist for managing existing weed vegetation. One is that planting will be done before any type of weed management program (tillage or herbicide application) is implemented to control existing vegetation. In other instances, herbicides applied earlier this spring often have done well at controlling winter annual species. The following reminders and suggestions might help farmers overcome some of the weed management problems already visited upon them by this season's challenging planting conditions.

  • As mentioned last week, the labels of most 2,4-D formulations specify intervals that must elapse between application and planting. If you plan to make a burndown herbicide application and plant corn or soybean less than 7 days later, leave out the 2,4-D and adjust the rate(s) of the other burndown herbicide(s), or include a different herbicide.
  • Contact herbicides, such as paraquat and glufosinate, may be less effective than translocated herbicides against larger weeds, but they can begin to desiccate existing vegetation much more quickly. Adding a triazine herbicide such as atrazine or metribuzin to paraquat or glufosinate often improves overall burndown performance. When the application of contact herbicides is followed by one or more cloudy days, expect symptoms to take a bit longer to develop.
  • Applying a burndown herbicide before preplant tillage can improve overall control of larger weeds in many situations, but when applying a translocated herbicide such as glyphosate, it is advisable to wait 24 to 48 hours between application and tillage to allow the herbicide to translocate within the target vegetation. Generally, the longer the interval, the more complete the control of existing vegetation ultimately will be.
  • Closing the seed furrow can be difficult when planting in wet soil conditions. This in itself can lead to crop 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 herbicide with the seed. Labels of many soil-applied herbicides warn that severe crop injury can result if the herbicide comes in direct contact with the seed.
  • Most soil-residual herbicides can be applied either before or immediately after planting. Additionally, some soil-residual products can be applied after crop emergence, whereas others must be applied before emergence. If you plan to use a soil-residual herbicide but are unsure when it will be applied, be sure to check the label to determine if it can be applied after crop planting and/or emergence, should the application be delayed.
  • Be especially cautious about making preemergence applications to fields where 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.
  • If you plan to include products containing saflufenacil (Sharpen, Optill, or Verdict) for burndown prior to soybean planting, remember that their labels prohibit tank-mixing or sequential applications of other soil-applied PPO-inhibiting herbicides within 30 days of planting.

Fields where a herbicide was applied several weeks ago are excellent candidates for scouting before planting. The 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. If weeds are present, you should consider controlling them before you plant. Why not just wait to spray after planting? That may be feasible, but the planting operation will likely injure some of the weeds, and they will need time to recover before being sprayed. To waiting 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. The less-than-ideal growing conditions may also increase the likelihood of corn injury from some soil-applied herbicides.--Aaron Hager

Aaron Hager

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