Zero Tolerance Weed Control
Waterhemp and Palmer amaranth. Their reputations as tough-to-control weeds only increased as weed shifts evolved and they developed increasing resistance to glyphosate.
According to Penn State Extension, “A weed shift is the change in the composition or relative frequencies of weeds in a weed population…or community…in response to natural or human-made environmental changes in a system.”
Bob Hartzler, Extension agronomist with Iowa State University, explained what kinds of circumstances cause these weed shifts.
“Each member of the weed community has different optimum growing conditions and responds differently to control practices,” Hartzler said. “The community within a field is a direct result of current and past management practices.”
To control these weeds takes into account these practices and builds upon them to ensure season-long control from weeds, especially resistant ones.
A Million Weed Seeds Against You
Waterhemp and Palmer amaranth are two of the most commonly discussed weeds when it comes to weed shifts and resistance because they reproduce rapidly and prolifically. A single plant can produce nearly a million seeds.
“These are two very aggressive weed species in terms of growth habitat and seed production,” said Mark Waddington, product development manager for Bayer. “If left alone and untreated, a single Palmer amaranth plant can produce up to a million seeds. The seeds germinate from early spring all the way through the first killing frost, so you’ve got a very long germination window.”
That long germination window means no chemistry will last long enough to control all the flushes of waterhemp and Palmer amaranth.
“You really need to use multiple products and rely on multiple herbicide application timings,” Waddington said. “You need to have zero tolerance for these weeds –- they can’t get big enough to reproduce.”
Not only are herbicide choices one way to manage tough-to-control weeds, but stewardship and tillage practices come into play too.
“Even equipment use and practices can contribute to the spread of weeds and weed shifts,” Waddington said. “For example, people can run the combine through one field in which waterhemp or Palmer amaranth were present during the season. The seeds are still in the soil that clings to the combine tires, and are transferred to the next field.
That next field may not have had waterhemp or Palmer amaranth previously, but one or both will likely show up in those fields the next year.”
Kill Weeds at Their Smallest
Some growers might get overwhelmed by the number of factors that could play into the spread and management of these weeds. But Jason Manz, selective corn herbicides manager for Bayer, said the best place to start comes down to two words: zero tolerance.
“A zero tolerance policy means that you don’t even give these weeds a chance to go to seed,” Manz explained. “You just can’t let these plants go to seed and produce new weeds because once that gets started, it’s very difficult to slow down.”
“We recommend weeds be taken care of before they reach about 3 inches tall. That gives you the best chance at stopping escapes and having a real problem on your hands, this year or next.”
Fight Weed Resistance Now
“Growers need to exhibit meticulous stewardship when it comes to their herbicide programs,” said Manz. “A lot of growers have the expectation that companies will come out with new chemistries, as they did through the 80s and even into the early 90s. What they don’t realize is that the newest class of herbicides is at least 20 years old.”
“We are all looking, but even if a new mode of action was discovered today it would take eight to 10 years before it became commercially available to growers,” Manz said.
“That’s exactly why zero tolerance is needed. We need to be judicious in our use of the currently available herbicides.”
Since its introduction in 2009, Corvus® typically has been very successful at providing true one-pass control of weeds in corn. Today, Manz said, there is no such thing as a herbicide that can provide weed control in a single pass if waterhemp and Palmer amaranth are present.
“Corvus is just as effective as it always has been. It’s just that the weed spectrum has changed –- and is continuing to change –- very quickly,” Manz said.
Strategies for an Effective Two Pass System
The issue of glyphosate-resistant weeds and other herbicide-resistant weed populations is very significant to today’s growers. Because of this, the importance of using pre-emergence herbicides as part of an integrated weed management is growing each year.
Get a clean start for the crop with burndown and an early-control pre-emergence pass. “Use a minimum of 10 gallons of spray mixture per acre,” says Waddington. “Uniform, thorough spray coverage is important to achieve consistent weed control. Coverage is determined by spray volume and pressure.”
After years of using glyphosate, the standard post application has been 5 gal/A water applied in large droplets to minimize drift. “To maximize the foliar activity of Laudis herbicide, growers need to use 10 to 20 gallons of water per acre delivered in medium spray droplets, at a minimum,” Waddington points out. “Use the best adjuvant and the highest recommended rate for increased efficacy.”
An added benefit of applying Laudis as the second pass of a two-pass program is the opportunity to tankmix the postemergence herbicide spray with an early season application of Stratego YLD fungicide. Laudis can be tankmixed with Stratego YLD fungicide and applied from the V4 to V7 growth stages, optimizing weed and disease control with fewer passes.
This is a product recommendation for a two-pass system in no-till and conventional- tillage corn:
| Tankmix Partner
||3.3 fl. oz./A, coarse soils, less than 2% organic matter or pH of 7.5 or greater
4.0 to 5.6 fl. oz./A, all other soils
|Atrazine, optional if permitted
||Pre-emergence, 10 gallons of water per acre|
||3 fl. oz./A
||Glyphosate +/- atrazine
||Postemergence up to the V9 growth stage, 10 to 20 gallons of water per acres in medium droplets|