As weeds continue to develop resistance to glyphosate and other herbicide chemistries, growers have to modify and diversify their weed management programs. Syngenta offers the Resistance Fighter program, which provides expertise, a comprehensive herbicide portfolio and local recommendations, that helps growers and retailers effectively control resistant weeds in their specific area.

Resistant weeds like marestail, Palmer amaranth, waterhemp, giant ragweed and common ragweed have very complex life cycles. The idea of a simple solution, regardless of how good a specific herbicide might be, won’t work for long. Resistant weeds are highly adaptable and have long periods of emergence and rapid growth, so they require a complex herbicide program. Any hope of managing resistant weeds requires employing diverse management practices.

Mark Loux

"If growers are trying to go with a simple approach – just loading up with herbicide – resistant weeds won’t respond. It takes the right systems approach, taking into account resistance to multiple modes of action and weed biology," said Mark Loux, professor at The Ohio State University. "New technology doesn’t short-cut the need for diverse management practices."

There is a hierarchy of decision-making that can help growers build diversity into their system.

  1. Crop rotation: Even though a corn/soybean rotation still battles the same set of weeds, at least there’s an opportunity to use different herbicide sites of action in corn that wouldn’t have to be used in soybeans.
  2. Seed technology: In a four-year corn/soybean rotation, the soybean years could include Roundup Ready one year and LibertyLink another year. New technology could offer additional options.
  3. Herbicide use: Are you optimizing diversity to make sure you’re not exposing those same weeds to the same site of action too many times? Are you using a fall herbicide and/or burndown, and pre- and post- emergence applications as dictated by the weed’s biology?

"There are certain weeds you spray multiple times, especially with foliar herbicides, and there are a lot of weeds that you don’t. The weeds being sprayed multiple times are the ones to pay attention to – what is the weed, what sites of action are being used and how many times are you spraying?" Loux said.

A diverse management program also needs to include cultural and mechanical practices.

  • Narrow row spacing for soybeans can help, especially with a weed like marestail that emerges midseason. The goal is to create a soybean canopy as fast as you can.
  • Cover crops like rye can also help keep early-season weeds such as Palmer amaranth and waterhemp at bay.
  • Tillage can reduce certain weeds, but this isn’t true for ragweed, which has adapted to all types of tillage. On the other hand, as soon as you bury marestail by tilling, it won’t emerge. Waterhemp and Palmer amaranth require deep tillage to reduce populations, which is an option every three or four years.
  • Walk soybean fields. Pull out those last resistant survivor weeds before they go to seed.

Battling Waterhemp and Marestail On-farm
Joe Humes, soybean, corn and wheat grower in Wyaconda, Missouri, said resistant weeds in his area include waterhemp and marestail. He battles resistant weeds with tillage, multiple modes of action and a residual herbicide program.

"If you don’t get marestail early in the spring with spray, you’ll need a tillage tool to uproot them or cut them out of the ground. Otherwise they are just there for the rest of the season," Humes said. "I’ve been combatting waterhemp on our bottom ground by plowing it every five years. We bury the seed at least 6 to 8 inches, and the following two years are relatively easy to keep the fields clean because the seed is buried so deep it just can’t germinate."

On Humes’ hill ground, he applies a pre-emergence herbicide every spring. He used to come back with Roundup, but when Roundup began to fail, he switched to Flexstar GT 3.5.

"We don’t cut rates. We want products with residual that will stay around after you spray," he said. "We’re going to a three-pass program, instead of a two-pass. That third pass will kill whatever is left and gives a residual through the fall once you get the beans cut off late in the year."

Humes said the key is to know what your application window is.

After he applies his pre-emergence application, he comes back within 21 days with Flexstar GT 3.5, even if the field is clean.

"It’s a different mindset to spray when you don’t have any weeds, but we’ve done it and had remarkable success," Humes said.

The per-acre cost is about $40-$50 to keep Humes’ soybean fields clean. He said Roundup spoiled growers, but the reality is that he’s got to have a diverse weed management program that can keep the weeds under control.

"The most important thing to stress to growers is to not cut rates and watch weed height because the bigger it gets, the harder it is to kill," he said."Some guys want to wait and do it all in one pass, and the weeds just get too big, and at a certain point, they can’t be controlled."

Giant ragweed
Palmer amaranth (pigweed)

Driver Weeds Require Diversity
The overuse of glyphosate has created "driver weeds" like resistant waterhemp and Palmer amaranth – they are the weeds that drive a grower’s weed management plan and decisions.

Don Porter

"A grower’s plan should be all about diversity – introducing multiple modes of action and overlapping modes of action to control these driver weeds," said Don Porter, Syngenta herbicide technical lead. "Growers need to understand what weeds they have in their fields, which modes of action are still effective on those particular weeds and weed emergence patterns. If growers don’t manage these driver weeds, the driver weeds will manage growers."

Driver weeds can destroy a crop, add a significant number of seeds to the seed bank and force growers to rotate to other crops that may offer alternative means of weed control. Driver weeds are not only resistant to glyphosate but can also be resistant to other MOAs such as ALS and PPO herbicides.

"Resistance is Mother Nature selecting those weeds that can survive different chemistries and cultural practices – they will figure it out," Porter said. "Overlapping residuals and using multiple effective modes of action are ways to preserve the chemistry for a longer period of time."

Another key aspect is to select a herbicide residual program with multiple effective modes of action.

"There are some concerns about using residual herbicides. You do need moisture – either irrigation or rainfall – to activate these herbicides," noted Porter. "With driver weeds, you’ve got to get beyond that concern and apply residual herbicides before you see any weeds with the hope that you can effectively reduce that population."

Make sure that growers use full rates at the appropriate application timing. It’s much easier for a herbicide to control a weed that is just germinating versus controlling weeds post-emergence.

"Growers should consider which weed management practices fit their farming operation and plan to introduce both chemical and cropping diversity into their programs at least three years into the future," he said. "It’s really hard for growers to accept that idea. What I encourage growers to try to reconcile is that if a program is working today, then you need to change it up because the weeds will evolve a defense and quickly become a problem in the future. Diversity is really key in trying to slow resistance down and maintain the sustainability of these tools, so they’re still effective for the future."