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

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

A certain amount of moisture, from either rainfall or irrigation, in fields after a pre-emergence application is critical to activate the herbicide. Most herbicides require at least a half-inch of water after a pre-emergence herbicide has been sprayed. Growers should try to plan herbicide applications around forecasted rain to help ensure a good activating rainfall for their pre-emergence herbicide. Fields should then be scouted after either a rainfall or irrigation watering to ensure proper activation has occurred. If escapes are present, plans should be made to treat these weeds early for maximum control.

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."

Application

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.