Larry Steckel’s PowerPoint photos send an uneasy murmur through the crowd. The University of Tennessee Extension weed specialist has returned to his native state of Illinois to explain how Southern growers are managing glyphosate-resistant weeds.
Most of the farmers, crop consultants and custom applicators in the room are familiar with the topic. Still, Steckel’s photos of wagons heaped high with hand-plucked Palmer amaranth are an attention grabber. They resemble those gag postcards you find in gas stations that brag of giant potatoes or monster carrots.
Weed resistance is no joke, however, and weed-choked fields have become all too common the past few years, Steckel maintains. "Palmer pigweed is so bad in some areas that growers have resorted to hand-weeding at a cost of $50 to $100 per acre. Some cotton fields have been completely abandoned," he says.
Perhaps more disturbing is Steckel’s observation that the waterhemp outbreaks in southern Illinois this past summer remind him of Tennessee only four years ago, before resistant weeds went wild.
"The first year you have glyphosate resistance on your farm is when it costs you the most because it is usually too late to do anything by the time you figure it out. There’s nothing that will control 10" to 12" Palmer or waterhemp if glyphosate fails," he says.
University of Illinois Extension weed specialist Aaron Hager explains that Palmer amaranth and common waterhemp are two different species that are closely related. Palmer is more aggressive than waterhemp, setting a taproot that can extend as long as 5'. Originally a desert plant, it is happy when conditions turn hot and dry.
To date, Palmer amaranth has mostly been a Southern problem, although it has been observed in the Midwest. "I keep waiting for the other shoe to drop on Palmer amaranth in this state," Hager says. "We’re not really sure why it hasn’t moved farther north."
Studies to uncover the secrets of Palmer amaranth movement are currently being conducted by Adam Davis, a University of Illinois and Agricultural Research Service invasive weed specialist. Changing climate conditions, human transport and pollen drift all play a role in the increasing establishment of the pesky intruder.
Waterhemp is a widespread problem throughout the Midwest and is now confirmed resistant to five different classes of chemistry.
Choose a herbicide system. Steckel says the first defensive step is to recognize that glyphosate resistance is real. "The total postemergence era is over and it is never coming back," he says. "A pre-emergence product is a necessity, and in many cases we also have to put down an early post application that provides residual control and is followed by another post application, or we have a mess." Depending on the summer, Tennessee can experience three generations of Palmer amaranth in one season.
Steckel says growers in his state are rapidly embracing the only alternative nonselective postemergence program: LibertyLink and Ignite. "It’s the only way we can grow dryland cotton and manage Palmer," he says. "There has been a wholesale switch to this technology in cotton and LibertyLink soybeans are in high demand.
"I’m not worried about glyphosate resistance anymore. That horse is out of the barn. My concern now is the long-term viability of Ignite and other tools like PPO inhibitors if we continue to use them at current levels," he says. "Growers have to plan to rotate chemistries and modes of action to keep the weeds confused."
Part of the challenge for many Southern growers is that they must also manage glyphosate-resistant marestail. "Once marestail gets bigger than the size of a Coke can, it’s hard to do anything with it," Steckel notes.
He says a dicamba-based system is still a very consistent horseweed burndown program. Another option, the new herbicide Sharpen (part of the Kixor family), falls into the protoporphyrinogen oxidase (PPO)–inhibiting class of herbicides.
"Sharpen translocates more effectively through the leaf material than Valor and as a result can provide post control of weeds like horseweed," Steckel says. "However, at the typical use rate of 1 oz. per acre, it will provide only seven to 10 days of residual control compared to four to five weeks of residual control with Valor."
For burndown 30 to 14 days prior to planting, he recommends dicamba plus glyphosate or Gramoxone Inteon.
Another option is to tank-mix Sharpen with glyphosate or Gramoxone Inteon.
His preferred pre-emergence strategy is to tank-mix Gramoxone Inteon with the farmer’s choice of Valor, Valor XLT, Envive, Authority MTZ, Boundary or Prefix. The Gramoxone Inteon will eliminate newly emerged glyphosate-resistant horseweed or Palmer amaranth, and the other herbicides provide residual control.
"Keep in mind that if you use Sharpen as a burndown within 30 days of the pre-emerge, no PPO-based products can be applied," Steckel cautions.
Scout repeatedly. "In order to manage glyphosate-resistant Palmer amaranth, you have to know, not assume, the pre-emerge worked," Steckel says.
"If the pre was activated, the field will still need to be scouted to determine when the pre is starting to break," he says. See the chart below for Steckel’s postemergence strategies based on pre-emergence success or failure.
If the pre was not activated by rain, he suggests an early postemergence application of Flexstar/Prefix in conventional or Roundup Ready soybeans, or Ignite in LibertyLink systems.
"Remember, once glyphosate Palmer amaranth grows larger than 4" tall, there are no herbicides that will control it in a conventional or Roundup Ready soybean system. A very high rate of Ignite [36 oz. per acre] can control Palmer up to about 7" tall, but it will provide very inconsistent control of anything beyond that," Steckel says.
It’s a battle. What growers often fail to grasp is how many things change as they head toward using more complex herbicide programs.
"We’re so used to simply spraying glyphosate," Steckel notes. "However, 120' booms at 18 mph don’t work as well with Sharpen, Gramoxone and Flexstar," he says. Flat fan nozzles need to replace air induction tips. Think retro when it comes to water volume too—15 gal. is a minimum requirement.
Timing may be the biggest hurdle. "We know that Palmer amaranth can grow 2.4" to 2.6" per day and waterhemp 1.5" to 2.1" per day. You have hours—not days—to get Palmer amaranth and waterhemp under control if glyphosate hasn’t done the job," he says.
Steckel says operating loans and cash rents are beginning to reflect the increased cost of weed management and added herbicides. "Conventional soybeans are picking up a bit," he says. "We experienced shortages in some herbicides last year. For the first time, I’m seeing growers back off on acres because they aren’t sure they can be timely with herbicide applications."
Glyphosate-Resistant Weed Management in Soybeans*
- 30 to 14 days before planting, apply:
- Sharpen (Sharpen premixes) + glyphosate or Gramoxone Inteon
- Dicamba + glyphosate or Gramoxone Inteon
- Gramoxone Inteon + a residual herbicide product such as: Valor products (Valor, Valor XLT, Envive), Dual Magnum products (Prefix, Boundary), Authority MTZ or Prowl
If pre-emergence options work:
Prior to pre-fading out, apply:
- Dual Magnum products (Sequence, Prefix, Ignite + Dual)
- Scout to determine when pre begins to break and apply before Palmer is 4" tall
- Flexstar products (Flexstar GT, Prefix, Reflex) or Ignite
If pre-emergence options don’t work:
Spray as soon as possible.
- Early post: Flexstar products (Flexstar GT, Prefix, Reflex, Flexstar) or Ignite
- Followed by: Cobra, Ultra Blazer or Ignite before Palmer is 4" tall