Careful management limits chemical resistance in weeds
STARKVILLE, Miss. -- An ongoing challenge in farming is finding a way to manage weeds without creating populations that are chemical-resistant.
The scale of modern farming requires the careful use of chemicals as part of the overall management plan to harvest good yields and make a profit. Although an array of government agencies regulate and oversee the use of these chemicals, farmers themselves are among the most cautious with their use.
Jason Bond, weed scientist with the Mississippi State University Extension Service, said farmers do not make unnecessary pesticide applications, and this is not only dictated by economics.
“Profitability is the top priority, but profitability is not possible without stewardship of the land and environment,” Bond said. “These are also of the utmost importance to farmers in Mississippi.”
At the Row Crop Short Course held in December, sessions addressed resistance issues in various row crops. Topics included suspected causes of resistance, management tools, how to operate in the absence of a product long used for treatment and thresholds for using chemicals.
Justin Calhoun, MSU Extension soybean specialist, talked about suspected herbicide resistance cases in research he did at the University of Missouri. He looked at difficult populations of barnyard grass and found a 90% or greater control possible with normal field-use rates of typical control herbicides.
“The question then becomes: Is resistance really our issue? Is there something else that can be impacting our efficacy? Is it resistance?” he said. “Our growers certainly think it is, but we have a narrowed view -- where we just think about products and the weeds. If we don’t have the results we hope for, then we think we have resistance.”
The situation is not so easily decided, Calhoun said.
“There is a whole complex system where each molecule of that herbicide must go through many processes to enter the plant,” he said. “That means there is a whole system we have to concern ourselves with between the product in the jug and the plant that may affect our efficacy.”
The first variable Calhoun considered was nozzle type and the resulting droplet size.
“We began to ask the question, ‘Is there a difference in how we are putting out herbicides in small-plot research compared to how our growers are putting out herbicides?’” Calhoun said.
They also looked at carrier volumes, or the amount of water used to distribute the chemical, as well as plant size when it was treated and what happens in the tank when different chemicals are mixed for spraying.
“Our research showed that it is important to use appropriate spray nozzles, spray at small weed sizes, overlap residuals and follow label instructions,” Calhoun said. “But just because the label says you can mix two products doesn’t mean you should.
“Antagonism within labeled tank mixtures can cause losses in efficacy. Talk through those tank mixture options with your local weed scientists,” he said.
The research focused on suspected herbicide-resistant weed populations, and Calhoun said these types seem to be less common than sometimes thought.
“Not to downplay actual resistance, but what we see a lot is the snowball effect,” he said.
The problem can start with no preemergence chemicals, the field receiving poor coverage or preemergence applications not being activated. Chemicals mixed in the tank for spraying can react with each other and reduce chemical efficacy; then hot and dry weather might set in, which often slows down weed growth but reduces herbicide activity.
“As we work our way through this, the snowball gets bigger and bigger with nothing working and weeds continuing to get bigger, and all of the sudden, we have a big mess on our hands,” Calhoun said. “What we typically find is there was something in the system where we have some lapses in control rather than resistance.
“When we begin to shout resistance or say we can’t use this product or that product prematurely, we might be taking a tool out of the toolbox that we maybe could have retained if we had changed something in our system,” he said.
Find research updates and information on all the state’s major row crops at .
