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Issue No. 9, Article 7/May 22, 2009

Some Additional Thoughts About Managing Weeds Before Planting

While many species of winter annual weeds are nearing completion of their life cycle, summer annual weed species appear to be thriving in fields where no weed management operation has occurred. Giant ragweed plants taller than 12 inches populate numerous fields, very dense populations of common lambsquarters are common, and waterhemp is enjoying the ample precipitation and soon-to-be-increasing air temperatures forecast for later this week.

In no-till situations, planting herbicide-resistant hybrids and varieties lets farmers plant first and spray weeds later (perhaps even as late as after crop emergence) with a nonselective herbicide such as glyphosate or glufosinate. Prior research has demonstrated that delayed burn-downs can sometimes provide satisfactory weed control, but this practice introduces significantly more risk for loss of crop yield potential than if weeds are adequately controlled prior to planting. Following are a few additional thoughts about managing weeds before planting.

  • Glyphosate-resistant populations of horseweed (aka marestail) and waterhemp occur across many areas of Illinois. Both species can be (and currently are) present before corn or soybean is planted. Failure to adequately control these glyphosate-resistant populations before planting could lead to significant challenges after the crop has emerged, especially in soybean, where very few alternative postemergence herbicide options exist. Tank-mix partners with glyphosate or alternative herbicides will be needed to control glyphosate-resistant weeds prior to crop planting. More tank-mix partners or alternative herbicide options are possible before planting than after planting.
  • Growth regulator herbicides used prior to planting generally have labeled waiting intervals that must elapse between application and planting. If you intend to plant before the labeled interval will have elapsed, omit the growth regulator from the burn-down application and replace it with another herbicide and/or increase the rate of the nonselective herbicide, if possible. Be cautious about which herbicide alternative you include with glyphosate. Herbicides that are more contact in activity may sometimes antagonize glyphosate, especially on large weeds. Alternatively, nonselective contact herbicides used for burn-down, such as paraquat or glufosinate, often provide improved burn-down of existing vegetation when other contact herbicides, such as metribuzin or atrazine, are tank-mixed with them.
  • Large weeds generally require a higher herbicide application rate for adequate control to be achieved. A 12-inch giant ragweed plant, for example, will be more difficult to control that the 2-inch giant ragweed growing adjacent to it. Delaying a burn-down application until after crop planting might necessitate an even higher application rate.
  • For glyphosate-based burn-downs, it is advisable under the current challenging conditions to add the full recommended rate of AMS. Be cautious about including "replacement" additives or blends that do not provide sufficient AMS. While most glyphosate products are formulated with a surfactant, some products require the addition of NIS. Be sure to check the respective product label for additive recommendations or requirements.

--Aaron Hager

Author:
Aaron Hager

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