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Combat Common Enemies of Corn and Soybean Weed Control

February 3, 2020
common ragweed

Common ragweed that has grown well beyond the recommended 4” herbicide application height.

While some weeds are certainly plentiful these days, we fortunately have many proven ag herbicides available to help us manage them. But how does weed control work? For optimal results, it’s important to remember that the interaction between weeds, herbicides and the environment is complex. Knowing when to apply herbicides and in what conditions will help ensure you are set up for success and can keep fields clean.

Weeds compete with young crops for water and nutrients, and if allowed to grow, can take over your fields. Timing is critical when it comes to herbicide applications, and weeds may be harming crops sooner than you think.

The Iowa State University Extension notes that weeds can impact yield potential as soon as they begin to take light, water and nutrients from the crop. This time is known as the “critical period.” The job of herbicides is to protect crop yields from this early-season competition.

Weed height is a helpful and commonly used marker to determine when to apply herbicides. However, Iowa State University Extension cautions that herbicide label guidelines indicate only whether a herbicide can kill a weed at that height, not the implications of allowing the weed to reach that height. In other words, you may be able to achieve control of a 12” foxtail with a post-emergence herbicide application, but delaying the application until the foxtail is 12” is not necessarily a good management decision, as yields may be damaged. This is one reason why we recommend a max application height of 4”.

The rapid growth of weeds and potential delays due to weather conditions should also be considered. A grower might plan to target a weed at a certain height, but be kept out of their fields due to rain or wind, resulting in a much larger weed and significant yield losses.

Targeting weeds a little bit early is better than a little bit late.

Rainfall is also an important consideration for herbicide effectiveness and weed control. Michigan State University Extension says this rainfall serves 2 purposes:

  • It stimulates seed germination, which exposes the seedling to the herbicide.
  • It activates the herbicides, incorporating them into the soil and into weed roots or shoots.

While we can prepare as much as possible for planting, we simply can’t control every variable. Growers will be well-positioned for strong weed control by preparing for timely herbicide applications and having both a preemergence and post-emergence plan at the ready.

There are a few herbicides to consider as you’re planning for 2020. For soybeans, a burndown of Gramoxone® SL 3.0 herbicide followed by a preemergence application of Boundary® 6.5 EC, BroadAxe® XC or Prefix® herbicides can help keep weed growth in check at the start of the season. In dicamba-tolerant soybeans, Tavium® Plus VaporGrip® Technology herbicide can be applied early post-emergence. Gramoxone SL 3.0 has no plant-back restrictions, and Tavium is the market’s first premix dicamba herbicide that provides both contact and residual control of weeds.

For corn, Acuron® corn herbicide plus Gramoxone SL 3.0 provides fast burndown and residual all in one pass. As mentioned, weeds begin to damage yields as soon as they take resources from young crops. Acuron controls the tough weeds that other products miss, delivering 5 to 15 more bushels per acre when used preemergence at full label rates*. Acuron can be followed up with a post-emergence herbicide, such as Halex® GT. This approach also helps with weed resistance management, as it gives four total sites of action in fields.

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*Yield advantage range based on 2016 Syngenta and University trials comparing Acuron to Corvus®, Resicore®, SureStart® II and Verdict®. For more information on Acuron versus an individual product, ask your Syngenta representative.

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