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Know Your Crop’s Opponents

August 21, 2018
This agronomic image shows giant ragweed.

August 10, 2018, Columbia, MO: Agronomist Brett Craigmyle shows just how large uncontrolled giant ragweed plants can grow.

Athletes spend time studying their opponents, and the greatest athletes maximize those learnings, turning their behind-the-scenes work into measurable on-field results. At a recent Grow MoreTM Experience event in Columbia, MO, learning about driver weeds (opponents) was a point of emphasis. Each weed is unique and knowing the differences – strengths vs. weaknesses – can translate into higher yields.

Giant ragweed’s strength is its immense size. And this size makes it a formidable opponent and substantial barrier to high-yielding corn and soybean crops. As its name implies, giant ragweed can grow upward of 6-7’ tall, consuming nearly 3x as much water to produce 1 lb. of dry matter as corn.

Those numbers mean just a few giant ragweed plants per acre can drastically reduce yields while costing additional money in wasted nitrogen and water. Lost water is especially problematic in years where drought is a concern. Worse yet, early-season competition from any tough weeds drastically alters the way corn grows – permanently reducing yield from the start even if weeds are cleaned up via a post-emergence herbicide application.

While giant ragweed relies on its size, waterhemp is a strength-in-numbers weed. By itself, waterhemp’s impact is far less than giant ragweed. However, just a few waterhemp plants, producing up to 1 million seeds per plant, can quickly multiply – presenting an immense potential for yield loss in subsequent seasons.

To control tough weeds like giant ragweed and waterhemp, it’s important to use the right tools – herbicides with multiple effective sites of action (SOAs) that your driver weeds have weak or non-existent defenses against:

  • In soybeans, giant ragweed is best controlled with a pre-emergence herbicide applied prior to the weed reaching 4’’ in height followed by a post-emergence herbicide, such as Flexstar® GT 3.5, which delivers 2 different sites of action (SOAs). For waterhemp the use of pre-emergence, residual herbicides like Boundary® 6.5 EC or BroadAxe® XC  is key. In PPO-susceptible populations, Flexstar® GT 3.5 is a strong post-emergence option following a residual pre-emergence product for effective management.
  • To combat weeds like giant ragweed and waterhemp in corn, we recommend a 2-pass system of Acuron® or Acuron Flexi pre-emergence followed by the remaining rate post-emergence, or a pre-emergence application of Acuron or Acuron Flexi followed by Halex® GT post-emergence for an additional SOA. Each of these corn herbicide premixes contains multiple SOAs for built-in resistance management and provides long-lasting residual control, especially when layered in a 2-pass system.

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All photos are either the property of Syngenta or are used with permission.