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Consider Your Herbicide’s Length of Residual

December 12, 2018
This image shows a machine spraying fields.

Overlapping residual herbicides helps keeps weeds at bay next season

As you look for advantages over yield-robbing, tough-to-control weeds, there are several factors to consider that will dictate how long a herbicide’s residual will last.

  1. Amount of moisture

Rainfall or irrigation is critical to activate pre-emergence herbicides. Most herbicides require at least 1/2” of water after application. Try to plan herbicide applications around rain forecasts to help ensure a good activating rainfall for pre-emergence herbicides. It’s important to scout fields after either rainfall or irrigation to ensure proper activation has occurred. If escaped weeds are present, make plans to treat these weeds as soon as possible to ensure they don’t continue to develop.

  1. Weed type and density

It’s critical to identify the problem weeds in your fields to ensure you’re using the right products on the right fields, at the right rate and the right time.

Large-seeded broadleaf weeds, like giant ragweed, cocklebur and morningglory, are tough to control because they germinate from deep within the soil profile. This makes it important to knock out these weeds early before they grow too large or multiply in your fields.

  1. Weed and crop size

We recommend spraying a post-emergence herbicide when both weeds and crops are small. The tallest weed in a field shouldn’t grow taller than 4”. Keep in mind, it may only take a few days for weeds to surpass this height and grow too large for post-emergence herbicides to be completely effective. Post-emergence herbicides should also be applied before corn or soybeans reach canopy, since canopy closure prevents herbicides from being able to penetrate the soil and reach the weeds.

  1. Type of herbicide used

Not all herbicides contain the sites of action (SOAs) that will be effective against the weeds present in your fields. Herbicides have different active ingredients that target different weed species.

For instance, many generic premixes contain an active ingredient called metolachlor, an older and less potent version of S-metolachlor – used in corn and soybean herbicide premixes from Syngenta. The metolachlor rate in these generics has to be 1.5x higher than the S-metolachlor rate in Syngenta premixes in order to provide a similar level of control. These subtle differences can have a major impact on weed control.

  1. Use rate

Experts recommend always using the full, labelled rate of a herbicide. An Iowa State University study found that reduced rates may be adequate against a flush of early-season weeds, but the residual control is greatly reduced – potentially creating problems later in the growing season.

Syngenta Research and Development Scientist Joe Wuerffel, Ph.D., says using a 3/4 or half-rate essentially guarantees having to follow up with a post-emergence application.

Use of reduced rates could also lead to yield loss, as well as contribute to the development of resistant weeds.

  1. Application

Experts also recommend using a 2-pass program of overlapping residuals to extend the length of herbicide performance.

Using multiple effective SOAs helps you to better manage weeds. It also prevents weeds from going to seed, limiting their impact the following year and minimizing unmanaged weeds being harvested with the crop.

In soybeans, we offer Boundary® 6.5 EC and BroadAxe® XC pre-emergence herbicides, each containing 2 effective sites of action. Both Boundary 6.5 EC and BroadAxe XC can be followed by a post-emergence application of Flexstar® GT 3.5 herbicide for 2 additional SOAs. Boundary 6.5 EC can also be followed by or Prefix® herbicide for management of ALS- and glyphosate-resistant weeds.

In corn, we recommend Acuron® or Acuron Flexi corn herbicides. Acuron has 4 active ingredients and 3 effective SOAs and Acuron Flexi, its atrazine-free counterpart, has 3 active ingredients and 2 effective SOAs. Both contain the active ingredient bicyclopyrone, which complements their other active ingredients to deliver more effective and more consistent control than competitive products – especially on large-seeded broadleaf weeds. For optimal control, these brands should be applied in a 2-pass system. In a 2-pass system, a pre-emergence foundation rate of Acuron or Acuron Flexi can also be followed by a post-emergence application of Halex® GT herbicide plus either an atrazine or dicamba product. Applying Halex GT post-emergence adds an additional SOA.

Learn more about how to get the most residual control out of your herbicides.

For more information on weed resistance management and local herbicide recommendations, visit ResistanceFighter.com.

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