Advice on residual length|Syngenta US

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Syngenta offers advice on getting the most residual length out of herbicides

  • Apply full rates of both pre- and post-emergence herbicides
  • Apply post-emergence applications early to small weeds
  • Overlap residuals to provide the most control of weeds

GREENSBORO, N.C., USA, Dec. 12, 2018– As corn and soybean growers look for advantages over yield-robbing, tough-to-control weeds, Syngenta is providing key insights into factors that determine how long a herbicide’s residual lasts.

Joe Wuerffel, Ph.D., Global Technical Manager for Herbicides at Syngenta, said these factors are the key to keeping fields clean.

“It really boils down to six main points,” he said. “If growers take these into consideration when planning their weed-management programs, they’ll be much more likely to increase the mileage on their herbicides.”

Amount of moisture
In the 2019 challenge, participants are asked to develop a methodology that addresses how high-yielding corn hybrids will perform in diverse environments and under stresses that typically affect yield. These predictive models can help plant breeders make better decisions on placing hybrids in specific environments, and assisting farmers with the seed purchasing decisions.

Weed type and density
Growers should know what the primary weeds are in each field to make sure they are using the right products on the right fields at the right rate and at the right time in the right way.

Primary weeds, including large-seeded broadleaf weeds like giant ragweed, cocklebur and morningglory, are tough to control because they germinate from deep within the soil profile.

“This is what led us to seek out the unique active ingredient bicyclopyrone,” Wuerffel said. “It complements the other three active ingredients in Acuron® corn herbicide to deliver more effective and more consistent control of large-seeded broadleaf weeds. Herbicides without bicyclopyrone only provide partial control, if any, over these large-seeded broadleaf weeds.”

Weed and crop size
Growers should spray a post-emergence herbicide when both weeds and crops are small. The tallest weed in a field shouldn’t grow taller than what is stated on the herbicide label, which for most weeds is smaller than 4 inches. Keep in mind, it may only take a few days for weeds to reach those heights and grow too large for post-emergence herbicides to be completely effective. At the same time, post-emergence herbicides should be applied before corn or soybeans reach canopy, as this allows the herbicide to penetrate the soil and reach the weeds.

Type of herbicide used
Herbicides are created with different active ingredients, some of which are stronger against specific weed species than others, so it’s important to remember that not all herbicides are created equal.

“Many generic premixes contain metolachlor, an older and less potent version of S-metolachlor from Syngenta, which is included in many of our products. To obtain an equal level of weed control, the metolachlor rate has to be 1.5X higher than the S-metolachlor rate,” Wuerffel said. “Growers have to be careful that they’re aware of these seemingly subtle differences that could have a major impact on their weed control.”

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.

“Using a full rate is the best way to maximize the length of that residual herbicide,” Wuerffel said. “If growers start to take it down to a three-quarters rate or a half rate, they’re essentially planning on having to follow up with a post-emergence application.”

Syngenta Technical Product Lead Dane Bowers agrees. Use rate is a major factor in maximizing residual control.

“Always apply herbicides at the full labelled rate and at the correct growth stage,” Bowers said. “Anything other than a full use rate is a recipe for disaster and may cause yield loss or additional spending to get weeds under control. Use of reduced rates can also contribute to selection of resistant weeds.”

Bowers also recommends using a two-pass program of overlapping residuals to extend the length of herbicide performance.

Bowers also recommends using a two-pass program of overlapping residuals to extend the length of herbicide performance. “It’s very important to use products with multiple, effective sites of action,” Bowers said. “In soybeans, I recommend using a pre-emergence application of Boundary® 6.5 EC, BroadAxe® XC or Prefix® herbicides. These each contain two 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 two additional sites of action.”

Using multiple effective sites of action gives greater control of weeds. It also prevents yield-robbing weeds from going to seed, limiting their impact on the following year and minimizing uncontrolled weeds being harvested with the crop. Gordon Vail, technical product lead for corn herbicides at Syngenta, recommends a similar course of action in corn.

“If you’re looking for the longest-lasting residual weed control in corn, I’d recommend Acuron,” Vail said. “It contains four active ingredients, including the unique component bicyclopyrone, and three effective sites of action. It also has great timing flexibility, from 28 days preplant, including burndown, up to 12-inch corn. Acuron can also be applied in a split-shot application with a portion of the rate applied pre-emergence, followed by the remainder of the rate applied post-emergence.”

Vail said that not only have multi-year trials shown Acuron offers the most consistent performance on a broad spectrum of tough weeds, but with four effective active ingredients, it also offers the best solution for responsible weed-resistance management.

Learn more about how to get the most residual control out of herbicides in this video with Wuerffel.

For more information about how to fight weed resistance, visit Join the conversation online – connect with us at

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Media Contacts:

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Web Resources:
Iowa State University
Boundary 6.5 EC
BroadAxe XC
Flexstar GT 3.5
Resistance Fighter
Syngenta Newsroom
Thrive Magazine
Know More, Grow More
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