When one door closes, another one opens.
Most people figure that’s an optimistic way of looking at life. People say it to someone who has been kicked down to help them get back up. The ones getting up repeat it to themselves.
Problem is, the flip side of that is Ian Malcolm telling the foolish folks in “Jurassic Park” that “life finds a way.”
In his case, he’s talking about dinosaurs. In agriculture, the term “opportunistic weed” suddenly is relevant. It’s just a coincidence that one of the ALS herbicides is named “Raptor.”
This isn’t news: A weed will find a way to live.
• Manage herbicides to avoid cross-resistance.
• Write down a long-term herbicide management plan.
• Consider tillage or “extreme” cover crops.
“What would Charles Darwin have thought about herbicides being applied on weeds?” asks Steve Powles, director of the Western Australia Pesticide Resistance Initiative. “He would have said, ‘Nature will win.’ ”
In a Macon County, Ga., field in 2004, a Palmer amaranth found a way to live by resisting glyphosate.
In that same county — and in counties across a couple of states — Palmer pigweed now also resists ALS herbicides, such as Staple and Envoke.
All told, the International Survey of Herbicide Resistant Weeds now lists 346 resistant biotypes, 195 species, in more than 330,000 fields.
Read that again.
So now, even though everyone who grows row crops is a candidate for post-traumatic stress disorder from the war on pigweed, another front is open, and the soldiers must roll to protect their flank. The new fronts are: several different resistant weeds in one field and weeds that are resistant to more than one chemistry.
Though the focus in much of the Southeast is on glyphosate-resistant Palmer amaranth, 16 other weeds also are resistant to glyphosate.
University of Tennessee Extension weed scientist Larry Steckel spends most of his days focused on marestail, also called horseweed.
“It is the most widespread glyphosate-resistant weed,” Steckel points out. Resistant marestail started in Tennessee in 2001 and went from bad to worse in 2003. Now a weed that once was controlled at burndown, and emerged no later than June, shows up in a field at all times during a summer crop.
And some marestail is glyphosate- and ALS-resistant.
ALS-resistant weeds also are rampant. Cross-resistance is growing, and weed scientists fear resistance to PPOs is coming soon.
“In a four-year rotation we could spray 16 PPOs,” says University of Georgia Extension weed scientist Eric Prostko. “We’re overusing the PPOs.”
Survival, however, is possible for growers who plan their battle.
Write down a battle plan
The first step, Prostko says, is to write down a resistance management plan that covers the entire crop rotation. If the rotation is peanut, corn, cotton and cotton, then the plan should cover four years. As the plan is followed, notations should be made regarding exactly what was used and when.
While scientists throw around terms like “mode of action” and sometimes say “protoporphyrinogen oxidase” instead of PPO, all a grower has to know is which number is on the bag. Regardless of the company or the active ingredient, all commercially available chemicals have a number on the bag that identifies the mode of action. Growers don’t have to know that a 2 is an ALS inhibitor; they just need to make sure they don’t follow a 2 with another 2.
When applying chemicals, Powles says, use high rates.
“We should use herbicides at high rates infrequently,” he says. “Be very scared of this resistance mechanism [in plants]. ... We have plants that are resistant to herbicides that have not yet been discovered.”
In addition to rotating crops and chemicals, growers also must decide whether to use tillage or “extreme” cover crops, which means growing a cover such as rye to 6 or 7 feet tall, killing it in spring and stripping the summer crop into heavy residue. Both accomplish what’s needed to destroy Palmer pigweed seeds: Bury them 4 inches deep.
“We don’t do what we’ve always done: Just spray herbicides,” says University of Georgia Extension weed scientist Stanley Culpepper. “We’re using an integrated approach to battle a pest that is ‘whupping’ us.”
One of the most effective tools against herbicide-resistant weeds is tillage. Steckel remembers losing most of the no-till acres in his area in 2005-06. Using Dicamba, however, growers now are holding on to the financially and environmentally preferred no-till and reduced-tillage practices.
“The key is to get [resistant horseweed] before it gets to Coke can size,” Steckel says.
Using the size of the weed to determine when to spray is essential to control, notes Bayer CropScience technical representative Keith Rucker.
When making a herbicide application, calibrate the equipment and check all the nozzles.
“It’s all about the system,” Culpepper says. “It isn’t any one thing that we do.”
NOTHING LESS: When a herbicide achieves 90% control, weed scientists call that “excellent.” “That’s not enough with Palmer pigweed,” says Auburn University weed scientist Mike Patterson. “You’ve got to have 100% control.” Patterson suggests a three-year program that includes using a moldboard plow in the fall followed by heavy cover in the first year and then heavy cover the next two years.