Are those weeds resistant? Here are the warning signs
The 2012 drought caused a complex network of agronomic challenges for farmers. Now, they can add another one to their list: the drought has made it harder to spot herbicide-resistant weeds.
Fortunately, there are some resistance "red flags" that farmers can use to more accurately diagnose problems, says Mark Peterson, global biology leader for the Enlist Weed Control System at Dow AgroSciences.
"The most important thing is to walk fields and ask if weed escapes are occurring in any kind of pattern," he says. "A pattern could indicate a plugged nozzle or a sprayer that drove too wide. In the early stages of resistance, seeds from a resistant individual can also create patches."
Another red flag is observing a variety of weed sizes. If only big weeds escaped, there’s a better chance that the herbicide was sprayed too late or at an improper rate. The presence of large, medium and small weeds could indicate that resistance is at work.
Even the weed species itself can be a red flag, Peterson contends.
"The top species tend to be some type of amaranth, marestail or ragweed species," he says. "They also produce a lot of seeds—such as pigweed—or have seeds that are easily relocated—such as marestail."
This last point has resulted in Midwestern weed scientists warning farmers to be wary of equipment or even manure that comes from areas with established resistant weed populations or from cows that were fed cottonseed meal.
Resistant or not, Ohio State University Extension weed scientist Mark Loux says any weeds that survive a drought might be harder to kill. That’s because during extended dry conditions, weeds grow more slowly and develop thicker cuticles on leaf surfaces, which can reduce herbicide movement throughout the plant.
Therefore, weed specialists say, a more aggressive burndown program might be in order in advance of the 2013 planting season. Zemax herbicide from Syngenta joins a group of several effective burndown options that further diversify modes of action to stave off the spread of resistant weeds.
"Based on the same capsule-suspension formulation technology as Halex GT corn herbicide, Zemax combines two active ingredients—mesotrione and S-metolachlor—into one product with optimized handling and physical properties," explains Johnny Reynolds, senior group leader of herbicide formulation development for Syngenta.
Increasingly, zero-tolerance pigweed policies are the norm for farmers everywhere—whether or not the weeds are actually resistant to any herbicide.
"I treat them all as if they were resistant and kill them all," says John Everitt, Monsanto technology development representative. His advice to farmers: "Start clean and stay clean."
Three Resistance Options
University of Tennessee row crop weed specialist Larry Steckel has seen more resistant weeds than he’d care to admit.
He knows that control options can quickly become limited—especially when some resistant species, such as pigweed and waterhemp, can grow more than 2" per day.
When resistant weeds have spread from patches of a field to the entire field, Steckel says you have three options:
- "Till the field and replant it."
- "Hit the resistant weeds with a PPO over and over."
- "Let it go."
You can e-mail Ben Potter at email@example.com.