Many factors contribute to weeds developing resistance to herbicides. One common problem is applying herbicides at the wrong timing. To help growers, Syngenta is providing key insights into herbicide resistance management as well as optimal application timing.
“The key to preventing herbicide-resistance development is growers’ weed-management practices,” said Joe Wuerffel, Ph.D., research and development scientist at Syngenta. “Applying both pre- and post-emergence herbicides at the right time is one basic step growers can take in their management plans to give all of us a leg up on tough-to-control weeds.”
Spray close to planting
It’s important with pre-emergence herbicide applications to spray as close to planting as possible. This will help get the most out of pre-emergence herbicides as well as achieve the longest residual activity.
It’s also important to use a herbicide that will perform on each grower’s schedule.
“Mother Nature can throw a wrench into any planning,” said Wuerffel. “That’s why I recommend growers use a highly flexible herbicide that will maintain its effectiveness, whether you have to move up your applications weeks before planting or delay your application until your crops have already emerged.”
Don’t let weeds grow
For post-emergence applications, Wuerffel said it is crucial to consider weed height. Syngenta recommends growers spray weeds based on heights recommended on the label. For most weeds that’s before weeds reach 4 inches.
“When the label says spray before weeds are 4 inches, that’s for the tallest weed in your field,” he explained. “If you wait for an average height of 4 inches, but your tallest weed is 8 inches, you are essentially using a reduced rate application for most or part of your field.”
Wuerffel says spraying weeds when they’re too tall can lead to resistance development.
“This can lead to weed escapes, which will then lead to those weeds producing seed,” said Wuerffel. “The weeds that were exposed to a herbicide, but not killed by it, may be passing on herbicide resistance traits to the next generation.”
Plan post-emergence applications
He recommends growers plan a time for their post-emergence application with an overlapping residual, usually three to four weeks after the pre-emergence application, regardless of whether there are any weeds present in the field.
“Mark your calendar or set a reminder on your cell phone. Don’t wait until you see that you need it, because oftentimes that’s already too late,” said Wuerffel. “Also, follow weather forecasts closely for anything that could delay timely herbicide applications. Being proactive in planning your post-emergence application can help you plan around those curveballs.”
The global Herbicide Resistance Action Committee agrees. In their best management practices guide, they say the more weeds present in the field when the herbicide is applied, the higher the chance that resistance will develop.
Best application practices
Wuerffel says growers’ best option is to use a two-pass system of overlapping residual products with multiple effective sites of action.
“If you’re looking for performance, application flexibility and maximum yield protection, Acuron® herbicide is your best choice for weed management in corn,” he said. “It contains four active ingredients, including the unique component bicyclopyrone, and three effective sites of action for the most effective and consistent control of tough weeds, including resistant weeds.”
Acuron can 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 for overlapping residual control.
It is also highly flexible, with the ability to be applied up to 28 days before planting or until corn has reached 12 inches in height.
“For soybeans, I tell growers to start off with a pre-emergence application of Boundary® 6.5 EC, BroadAxe® XC or Prefix® herbicide,” he said. “Boundary 6.5 EC and BroadAxe XC can be followed by Flexstar® GT 3.5 herbicide as a post-emergence application for multiple effective sites of action.”
Wuerffel notes that Boundary 6.5 EC can also be followed by Prefix as an early post-emergence herbicide.
Learn more about the best time to apply herbicides in this video with Wuerffel.