Get the Most from Residual Herbicides

April 21, 2011 08:37 AM
Get the Most from Residual Herbicides

The drum continues to beat to preserve herbicide and herbicide trait technology. Iowa State University weed scientist Micheal Owen says unless growers start considering stewardship, crops and growers alike will suffer.

“There are a number of ways to provide stewardship, and one way that does not,” says Owen. “Unfortunately, the one way that does not provide stewardship – recurrent use of the herbicide to which the crop is resistant – continues to prevail. I anticipate that we will have a breakout year for glyphosate-resistant weeds, particularly waterhemp, in 2011.”
Owen says one option is to select soil-applied residual herbicide(s) to control the most problematic weeds (i.e. waterhemp). However, appropriate application timing is also an important part of effective stewardship.
Here are Owen’s suggestions to get the most from residual herbicides:
The worst way to use a residual herbicide is to apply it post-emergence to the crop and weeds alone or in combination with a post product such as glyphosate. While this type of application is convenient and simple, it results in loss of most of the stewardship benefits (e.g. yield protection and better time management) accrued by residual herbicides. Applying soil-applied residual herbicides post-emergence allows weeds to compete with crop yield potential and does not provide any time management benefits relative to the other suggested herbicide application timings.  
The best and least risky application timing is early pre-plant (EPP) – which means now is the time to make applications. An EPP application results in the herbicide being in place to control weeds as they begin to germinate and precludes the loss of yield potential attributable to early-season weed interference. Importantly, the EPP timing does not interfere with planting, thus providing the greatest time management benefit.
The next best timing is pre-emergence (PRE) application timing. The PRE application timing also potentially provides similar weed control benefits as the EPP – but there is greater risk of insufficient rainfall to provide the appropriate environment for effective weed control. Also lost is the time management benefit provided by the EPP application timing.
Owen says data generated from a five-year field-scale on-farm project (Benchmark Study) conducted in Iowa clearly and consistently demonstrates the benefits of soil-applied residual herbicides when the application timing is correct. The benefits include:
  • greater yields compared to post treatments, regardless of whether the latter included a residual herbicide
  • more profitability; and
  • other stewardship benefits resulting from this tactic, such as mitigation of herbicide-resistant weed populations.
Keep in mind that recently, waterhemp populations have evolved resistance to HPPD herbicides (e.g. Callisto, Impact, and Laudis). “Unless appropriate stewardship of this class of chemistry is provided, it is inevitable that HPPD resistance will evolve too,” says Owen.
For more information on improving the success of residual herbicides read University of Illinois weed scientist Aaron Hager’s column on Revisting the realm of residuals.
Read more about resistant weeds:


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